Chapter 4: OpenTK.Graphics (OpenGL and ES)

In order to use OpenGL functions, your System requires appropriate drivers for hardware acceleration.

The OpenGL Programming Guide is a book written by Silicon Graphics engineers and will introduce the reader into graphics programming. It is highly recommended you take a look at this resource to learn about the essential concepts in OpenGL.

The GraphicsContext class


The OpenTK.Graphics.GraphicsContext is a cross-platform wrapper around an OpenGL context. The context routes your OpenGL commands to the hardware driver for execution - which means you cannot use any OpenGL commands without a valid GraphicsContext.


OpenTK creates a GraphicsContext automatically as part of the GameWindow and GLControl classes:

  • You can specify the desired GraphicsMode of the context using the mode parameter. Use GraphicsMode.Default to set a default, compatible mode or specify the color, depth, stencil and anti-aliasing level manually.
  • You can specify the OpenGL version you wish to use through the major and minor parameters. As per the OpenGL specs, the context will use the highest version that is compatible with the version you specified. The default values are 1 and 0 respectively, resulting in a 2.1 context.
  • You can request an embedded (ES) context by specifying GraphicsContextFlags.Embedded to the flags parameter. The default value will construct a desktop (regular OpenGL) context.
  • If you are creating the context manually, you must specify a valid IWindowInfo instance to the window parameter (see below). This is the default window the GraphicsContext will draw on and can be modified later using the MakeCurrent method.


// Creates a 1.0-compatible GraphicsContext with GraphicsMode.Default
GameWindow window = new GameWindow();
// Creates a 3.0-compatible GraphicsContext with 32bpp color, 24bpp depth
// 8bpp stencil and 4x anti-aliasing.
GLControl control = new GLControl(new GraphicsMode(32, 24, 8, 4), 3, 0);

Sometimes, you might wish to create a second context for your application. A typical use case is for background loading of resources. This is very simple to achieve:

// The new context must be created on a new thread
// (see remarks section, below)
// We need to create a new window for the new context.
// Note 1: new windows remain invisible unless you call
//         INativeWindow.Visible = true or IGameWindow.Run()
// Note 2: invisible windows fail the pixel ownership test.
//         This means that the contents of the front buffer are undefined, i.e.
//         you cannot use an invisible window for offscreen rendering.
//         You will need to use a framebuffer object, instead.
// Note 3: context sharing will fail if the main context is in use.
// Note 4: this code can be used as-is in a GLControl or GameWindow.
EventWaitHandle context_ready = new EventWaitHandle(false, EventResetMode.AutoReset);
Thread thread = new Thread(() =>
    INativeWindow window = new NativeWindow();
    IGraphicsContext context = new GraphicsContext(GraphicsMode.Default, window.WindowInfo);
    while (window.Exists)
         // Perform your processing here
         Thread.Sleep(1); // Limit CPU usage, if necessary
thread.IsBackground = true;

If necessary, you can also instantiate a GraphicsContext manually. For this, you will need to provide an amount of platform-specific information, as indicated below:

using OpenTK.Graphics;
using OpenTK.Platform;
using Config = OpenTK.Configuration;
using Utilities = OpenTK.Platform.Utilities;
// Create an IWindowInfo for the window we wish to render on.
// handle - a Win32, X11 or Carbon window handle
// display - the X11 display connection
// screen - the X11 screen id
// root - the X11 root window
// visual - the desired X11 visual for the window
IWindowInfo wi = null;
if (Config.RunningOnWindows)
    wi = Utilities.CreateWindowsWindowInfo(handle);
else if (Config.RunningOnX11)
    wi = Utilities.CreateX11WindowInfo(display, screen, handle, root, visual);
else if (Config.RunningOnMacOS)
    wi = Utilities.CreateMacOSCarbonWindowInfo(handle, false, false);
// Construct a new IGraphicsContext using the IWindowInfo from above.
IGraphicsContext context = new GraphicsContext(GraphicsMode.Default, wi);

Finally, it is possible to instantiate a 'dummy' GraphicsContext for any OpenGL context created outside of OpenTK. This allows you to use OpenTK.Graphics with windows created through SDL or other libraries:

IntPtr external_context = ...; // create or retrieve existing third-party context handle
GraphicsContext opentk_context = new GraphicsContext(
    new ContextHandle(external_context),

You can now use OpenTK GL functions normally.

If external_context is not a handle to a WGL, GLX or AGL/NSOpenGL/CGL context, you will have to specify a custom GetAddress and GetCurrentContext implementation in terms of the toolkit you are using. For instance, when using SDL2, the context handle returned by SDL_GL_CreateContext points to a SDL-specific structure. In this case:

IntPtr external_context = SDL_GL_CreateContext(...);
GraphicsContext opentk_context = new GraphicsContext(
    new ContextHandle(external_context),
    (name) => { return SDL_GL_GetProcAddress(name); }, // implement GetAddress via SDL
    () => { return SDL_GL_GetCurrentContext(); }); // implement GetCurrentContext via SDL

Note that GraphicsContext functions like context.SwapBuffers() will have no effect on external contexts. You *must* use the third-party toolkit to manage the external context.

A common use-case is integration of OpenGL 3.x through OpenTK.Graphics into an existing application.


A single GraphicsContext may be current on a single thread at a time. All OpenGL commands are routed to the context which is current on the calling thread - issuing OpenGL commands from a thread without a current context may result in a GraphicsContextMissingException. This is a safeguard placed by OpenTK: under normal circumstances, you'd get an abrupt and unexplained crash.


  • MakeCurrent

    You can use the MakeCurrent() instance method to make a context current on the calling thread. If a context is already current on this thread, it will be replaced and made non-current. A single context may be current on a single thread at any moment - trying to make it current on two or more threads will result in an error. You can make a context non-current by calling MakeCurrent(null) on the correct thread.

    To retrieve the current context use the GraphicsContext.CurrentContext static property.

    If you wish to use OpenGL on multiple threads, you can either:

    • create one OpenGL context for each thread, or
    • use MakeCurrent() to make move a single context between threads.

    Both alternatives can be quite complicated to implement correctly. For this reason, it is usually advisable to restrict all OpenGL commands to a single thread, typically your main application thread, and use asynchronous method calls to invoke OpenGL from different threads. The GLControl provides the GLControl.BeginInvoke() method to simplify asynchronous method calls from secondary threads to the main System.Windows.Forms.Application thread. The GameWindow does not provide a similar API.

    To use multiple contexts on a single thread, call MakeCurrent to select the correct context prior to any OpenGL commands. For example, if you have two GLControls on a single form, you must call MakeCurrent() on the correct GLControl for each Load, Resize or Paint event.

    GLControl.MakeCurrent() and GameWindow.MakeCurrent() are instance methods that simplify the handling of contexts.

  • SwapBuffers

    OpenTK creates double-buffered contexts by default. Single-buffered contexts are considered deprecated, since they do not work correctly with compositors found on modern operating systems.

    A double-buffered context offers two color buffers: a "back" buffer, where all rendering takes place, and a "front" buffer which is displayed to the user. The SwapBuffers() method "swaps" the front and back buffers and displays the result of the rendering commands to the user. The contents of the new back buffer are undefined after the call to SwapBuffers().

    The typical rendering process looks similar to this:

    // Clear the back buffer.
    GL.Clear(ClearBufferMask.ColorBufferBit | ClearBufferMask.DepthBufferBit);
    // Issue rendering commands, like GL.DrawElements
    // Display the final rendering to the user

    Note that caching the current context will be more efficient than retrieving it through GraphicsContext.CurrentContext. For this reason, both GameWindow and GLControl use a cached GraphicsContext for efficiency.

[Stereoscopic rendering]

You can create a GraphicsContext that supports stereoscopic rendering (also known as "quad-buffer stereo"), by setting the stereo parameter to true in the context constructor. GameWindow and GLControl also offer this parameter in their constructors.

Contexts that support quad-buffer stereo distinguish the back and front buffers between "left" and "right" buffer. In other words, the context contains four distinct color buffers (hence the name): back-left, back-right, front-left and front-right. Check out the stereoscopic rendering page for more information ([Todo: add article and link]).

Please note that quad-buffer stereo is typically not supported on consumer video cards. You will need a workstation-class video card, like Ati's FireGL/FirePro or Nvidia's Quadro series, to enable stereo rendering. Trying to enable stereo rendering on consumer video cards will typically result in a context without stereo capabilities.

[Accessing internal information]

GraphicsContext hides an amount of low-level, implementation-specific information behind the IGraphicsContextInternal interface. This information includes the raw context handle, the platform-specific IGraphicsContext implementation and methods to initialize OpenGL entry points (GetAddress() and LoadAll()).

To access this information, cast your GraphicsContext to IGraphicsContextInternal:

IntPtr raw_context_handle = (my_context as IGraphicsContextInternal).Context.Handle;
IntPtr function_address = (my_context as IGraphicsContextInternal).GetAddress("glGenFramebufferEXT");

Using an external OpenGL context with OpenTK

Starting with version 0.9.1, OpenTK requires the existence of an OpenGL context prior to the initialization of the OpenGL subsystem. In other words, you cannot use any OpenGL methods prior to the creation of a GraphicsContext.

If you create the OpenGL context through an external library (for example SDL or GTK#), you will need to inform OpenTK of the context's existence using the GraphicsContext.CreateDummyContext() static method. This method will return a new GraphicsContext instance for the context that is current on the calling thread. Optionally, you can pass the handle (IntPtr) of a specific external context to CreateDummyContext; in this case, the external context need not be current on the calling thread.

You will typically call this method as soon as the external context is created. For example, using Tao.Glfw:

Tao.Glfw.glfwOpenWindow(640, 480, 8, 8, 8, 8, 16, 0, Tao.Glfw.GLFW_WINDOW);
// You may now use OpenTK.Graphics methods normally.

Please note that it is an error to call CreateDummyContext() multiple times for the same external context.


The following pages will describe the concepts of OpenGL Textures, Frame Buffer Objects and Pixel Buffer Objects. These concepts apply equally to OpenGL and OpenGL|ES - differences between the two will be noted in the text or in the example source code.

Loading a texture from disk

Before going into technical details about textures in the graphics pipeline, it is useful to know how to actually load a texture into OpenGL.

A simple way to achieve this is to use the System.Drawing.Bitmap class (MSDN documentation). This class can decode BMP, GIF, EXIG, JPG, PNG and TIFF images into system memory, so the only thing we have to do is send the decoded data to OpenGL. Here is how:

using System.Drawing;
using System.Drawing.Imaging;
using OpenTK.Graphics.OpenGL;
static int LoadTexture(string filename)
    if (String.IsNullOrEmpty(filename))
        throw new ArgumentException(filename);
    int id = GL.GenTexture();
    GL.BindTexture(TextureTarget.Texture2D, id);
    // We will not upload mipmaps, so disable mipmapping (otherwise the texture will not appear).
    // We can use GL.GenerateMipmaps() or GL.Ext.GenerateMipmaps() to create
    // mipmaps automatically. In that case, use TextureMinFilter.LinearMipmapLinear to enable them.
    GL.TexParameter(TextureTarget.Texture2D, TextureParameterName.TextureMinFilter, (int)TextureMinFilter.Linear);
    GL.TexParameter(TextureTarget.Texture2D, TextureParameterName.TextureMagFilter, (int)TextureMagFilter.Linear);
    Bitmap bmp = new Bitmap(filename);
    BitmapData bmp_data = bmp.LockBits(new Rectangle(0, 0, bmp.Width, bmp.Height), ImageLockMode.ReadOnly, System.Drawing.Imaging.PixelFormat.Format32bppArgb);
    GL.TexImage2D(TextureTarget.Texture2D, 0, PixelInternalFormat.Rgba, bmp_data.Width, bmp_data.Height, 0,
        OpenTK.Graphics.OpenGL.PixelFormat.Bgra, PixelType.UnsignedByte, bmp_data.Scan0);
    return id;

Now you can bind this texture id to a sampler (with GL.Uniform1) and use it in your shaders. If you are not using shaders, you should enable texturing (with GL.Enable) and bind the texture (GL.BindTexture) prior to rendering.

2D Texture differences

The most commonly used textures are 2-dimensional. There exist 3 kinds of 2D textures:

  1. Texture2D
    Power of two sized (POTS) E.g: 1024²
    These are supported on all OpenGL 1.2 drivers.

    • MipMaps are allowed.
    • All filter modes are allowed.
    • Texture Coordinates are addressed parametrically: [0.0f ... 1.0f]x[0.0f ... 1.0f]
    • All wrap modes are allowed.
    • Borders are supported. (Exception: S3TC Texture Compression does not allow borders)
  2. Texture2D
    Non power of two sized (NPOTS) E.g: 640*480.
    GL.SupportsExtension( "ARB_texture_non_power_of_two" ) must evaluate to true.

    • MipMaps are allowed.
    • All filter modes are allowed.
    • Texture Coordinates are addressed parametrically: [0.0f ... 1.0f]x[0.0f ... 1.0f]
    • All wrap modes are allowed.
    • Borders are supported. (Exception: S3TC Texture Compression does not allow borders)
  3. TextureRectangle
    Arbitrary size. E.g: 640*480.
    GL.SupportsExtension( "ARB_texture_rectangle" ) must evaluate to true.

    • MipMaps are not allowed.
    • Only Nearest and Linear filter modes are allowed. (default is Linear)
    • Texture Coordinates are addressed non-parametrically: [0..width]x[0..height]
    • Only Clamp and ClampToEdge wrap modes are allowed. (default is ClampToEdge)
    • Borders are not supported.

Note that 1 and 2 both use the same tokens. The only difference between them is the size.

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BCn Texture Compression

A widely available texture compression comes from S3, mostly due to Microsoft licensing it and including it into DirectX 7. It uses the file format .dds (DirectDraw Surface), which is basically a copy of the texture in video memory. Every graphics accelerator compatible with DirectX 7 or higher supports this texture compression.

The BCn Formats are:
BC1 = DXT1 = 8 Bytes per Block, Accuracy: R5G6B5 or R5G5B5A1 (0.5 Byte/Texel)
BC2 = DXT3 = 16 Bytes per Block, Accuracy: R5G6B5A8 (1 Byte/Texel)
BC3 = DXT5 = 16 Bytes per Block, Accuracy: R5G6B5A8 (1 Byte/Texel)
BC4 = Basically this is only the alpha channel from DXT5 without the color channel. CompressedRedRgtc1 (0.5 Byte/Texel)
BC5 = This is 2 channels of BC4, twice the size. CompressedRgRgtc1 (1 Byte/Texel)

BC1, 2 and 3 from EXT_texture_compression_s3tc (DirectX7+ Hardware)
BC4 and 5 from ARB_texture_compression_rtgc. (~DirectX10 Hardware)

The formats DXT2 and DXT4 do exist, but they include pre-multiplied Alpha which is problematic when blending with images with explicit Alpha (RGBA, DXT3/5, etc). That's why those formats have barely been used, and are partially not supported by hardware and export/import tools and they have no BCn number.

Compressed vs. Uncompressed

Texture compression encodes the whole Image into blocks of 4x4 Texel, instead of storing every single Texel of the Image. Thus the ideal compressed Texture dimension is a multiple of 4, like 640x480 or a power of 2, which can be nicely fit into these Blocks. This is the ideal and not a restriction, the specification allows any non-power-of-two dimension, but will internally use a 4x4 Block for a Texture with the size of 2x1 (the other Texels in the Block are undefined).

You probably guessed it already, there is a catch involved when reliably shrinking an image to 25% of it's uncompressed size: A lossy compression technique. This quality loss involved, which can be altered by tweaking the Filter options when compressing the image, is different to the one used in JPG compression. Although both formats - .dds and .jpg - are designed to compress an Image, the S3TC format was developed with graphics hardware in mind.

A bilinear Texture lookup usually reads 2x2 Texels from the Texture and interpolates those 4 Texels to get the final Color. Since a Block consists of 4x4 Texels, there is a good chance that all 4 Texels - which must be examined for the bilinear lookup - are in the same Block. This means that the worst case scenario involves reading 4 Blocks, but usually only 1-2 Blocks are used to achieve the bilinear lookup. When using uncompressed Textures, every bilinear lookup requires reading 4 Texels.

If you do the maths now you will notice that the compressed image actually needs 16 Bytes for 1 Block of RGBA Color, but the uncompressed 4 Texels of RGBA need 16 Bytes too. And yes, if you would only draw a single Pixel on the screen all this would not bring any noticable performance gains, actually it would be slower if multiple Blocks must be read to do the lookup.

However in OpenGL you typically draw more than a single Pixel, at least a Triangle. When the Triangle is rasterized, alot of Pixels will be very close to each other, which means their 2x2 lookup is very likely in the same 4x4 Block used by the last lookup, or a close neighbour. Graphic cards usually support this locality by using a small amount of memory in the chip for a dedicated Texture Cache. If a Cache hit is made, the cost for reading the Texels is very low, compared to reading from Video Memory.

That's why S3TC does decrease render times: the earlier mentioned 16 Bytes of a DXTn Block contain 16 Texels (1 Byte per Texel), while 16 Bytes of uncompressed Texture only contain 4 Texels (4 Bytes per Texel). Alot more data is stored in the 16 Bytes of DXTn, and alot of lookups will be able to use the fast Texture Cache. The game Quake 3 Arena's Framerate increases by ~20% when using compressed Textures, compared to using uncompressed Textures.

Although you might be convinced now that Texture Compression is something worth looking into, do handle it with care. After all, it's a lossy compression Technique which introduces compression Artifacts into the Texture. For Textures that are close to the Viewer this will be noticed, that's why 2D Elements which are drawn very close to the near Plane - like the Mouse Cursor, Fonts or User Interface Elements like the Health display - are usually done with uncompressed Textures, which do not suffer from Artifacts.
As a rule of thumb, do not use Texture Compression where 1 Texel in the Texture will map to 1 Pixel on the Screen.

Using OpenTK.Utilities .dds loader
At the time of writing, the .dds loader included with OpenTK can handle compressed 2D Textures and compressed Cube Maps. Keep in mind that the loader expects a valid OpenGL Context to be present. It will only read the file from disk and upload all MipMap levels to OpenGL. It will NOT set minification/magnification filter or wrapping mode, because it cannot guess how you intent to use it.

void LoadFromDisk( string filename, bool flip, out int texturehandle, out TextureTarget dimension)

Input Parameter: filename
A string used to locate the DDS file on the harddisk, note that escape-sequences like "\n" are NOT stripped from the string.

Input Parameter: flip
The DDS format is designed to be used with DirectX, and that defines GL.TexCoord2(0.0, 0.0) at top-left, while OpenGL uses bottom-left. If you wish to use the default OpenGL Texture Matrix, the Image must be flipped before loading it as Texture into OpenGL.

Output Parameter: texturehandle
If there occured any error while loading, the loader will return "0" in this parameter. If >0 it's a valid Texture that can be used with GL.BindTexture.

Output Parameter: dimension
This parameter is used to identify what was loaded, currently it can return "Invalid", "Texture2D" or "TextureCube".

Example Usage

TextureTarget ImageTextureTarget;
int ImageTextureHandle;
ImageDDS.LoadFromDisk( @"", true, out ImageTextureHandle, out ImageTextureTarget );
if ( ImageTextureHandle == 0 || ImageTextureTarget == TextureTarget.Invalid )
   // loading failed
// load succeeded, Texture can be used.
GL.BindTexture( ImageTextureTarget, ImageTextureHandle );
GL.TexParameter( ImageTextureTarget, TextureParameterName.TextureMagFilter, (int) TextureMagFilter.Linear );
int[] MipMapCount = new int[1];
GL.GetTexParameter( ImageTextureTarget, GetTextureParameter.TextureMaxLevel, out MipMapCount[0] );
if ( MipMapCount == 0 ) // if no MipMaps are present, use linear Filter
  GL.TexParameter( ImageTextureTarget, TextureParameterName.TextureMinFilter, (int) TextureMinFilter.Linear );
else // MipMaps are present, use trilinear Filter
  GL.TexParameter( ImageTextureTarget, TextureParameterName.TextureMinFilter, (int) TextureMinFilter.LinearMipmapLinear );

Remember that you must first GL.Enable the states Texture2D or TextureCube, before using the Texture in drawing.

Useful links:

ATi Compressonator:

nVidia's Photoshop Plugin:

nVidia's GPU-accelerated Texture Tools:

Detailed comparison of uncompressed vs. compressed Images:

OpenGL Extension Specification:

Microsoft's .dds file format specification (was used to build the OpenTK .dds loader)

DXT Compression using CUDA

Real-Time YCoCg-DXT Compression

Last Update of the Links: January 2008

Frame Buffer Objects (FBO)

Every OpenGL application has at least one framebuffer. You can think about it as a digital copy of what you see on your screen. But this also implies a restriction, you can only see 1 framebuffer at a time on-screen, but it might be desireable to have multiple off-screen framebuffers at your disposal. That's where Frame Buffer Object (FBO) comes into play.

Typical usage for FBO is High Dynamic Range Rendering, Shadow Mapping and other Render-To-Texture effects. Assuming the buzzwords tell you nothing, here's a quick example scenario. We have a Texture2D of a sign that has some wooden texture and reads "Blacksmith". However you intend to localize that sign, so the german version of your game reads "Schmiede" or the spanish version "herrería". What are the options? Manually create a new Texture for every sign in the game with a paint program? No. All you need is the wooden texture of the sign, without any letters. The texture can be used as target for Render-To-Texture, and OpenTK.Fonts provides you a way to write any text you like ontop of that texture.

The traditional approach to achieve that was rendering into the visible framebuffer, read the information back with GL.ReadPixels() or GL.CopyTexSubImage(), then clear the screen and proceed with rendering as usual. With FBO the copy can be avoided, since it allows to render directly into a texture.

Framebuffer Layout

A framebuffer consists of at least one of these buffers:

  • A depth buffer, with or without stencil mask. Typical depth buffer formats are 16, 24, 32 Bit integer or 32 Bit floating point. Stencil buffers can only be 8 Bits in size, a good mixed depth and stencil format is depth 24 Bit with stencil 8 Bit.
  • Color buffer(s) have 1-4 components, namely Red, Green, Blue and Alpha. Typical color buffer formats are RGBA8 (8 Bit per component, total 32 Bit) or RGBA16f (16 Bit floating point per component, total 64 Bit). This list is far from complete, there exist dozens of formats with different amount of components and precision per component.

Please note that there is no requirement to use both. It's perfectly valid to create a FBO which has only a color attachment but no depth attachment. Or the other way around.

When you use more than one buffer, some restrictions apply: All attachments to the FBO must have the same width and height. All color buffers must use the same format. For example, you cannot attach a RGBA8 and a RGBA16f Texture to the same FBO, even if they have the same width and height. OpenGL 3.0 does relax this restriction, by allowing attachments of different sizes to be attached. But only the smallest area covered by all attachments can be written to. The Extension EXTX_mixed_framebuffer_formats allows attaching different formats to the framebuffer, however this is reported to be very slow so far.


FBO allows 2 different types of targets to be attached to it. The already known textures 1D, 2D, Rectangle, 3D or Cube map, and a new type: the renderbuffer. They are not restricted to depth or stencil like the name might suggest, they can be used for color formats aswell.


  • May support formats which are not available as texture.
  • Allows multisampling through Extensions.


  • Does not allow MipMaps, filter or wrapping mode to be specified.
  • Cannot be bound as sampler for shaders.
  • Restricted to be a 2-dimensional image.


  • Allows MipMaps, filter and wrapping modes, just like every other texture.
  • Can be bound as sampler to a shader.


  • Might be slower than a renderbuffer, depending on hardware.

As a rule of thumb, do not use a renderbuffer if you plan to use the FBO attachments as textures at some later stage. The copy from renderbuffer into a texture will perform worse than rendering directly to the texture.

Let's take the wooden "Blacksmith" sign example from earlier again. The required end result must be a Texture2D, which can be bound when drawing the geometry of the sign. To give an overview about the options, here are some brief summaries how to accomplish obtaining the desired Texture2D:

  1. Using a visible framebuffer.
    The wooden texture is drawn into the framebuffer. Text is drawn. The final Texture is copied into a Texture2D. The screen must be cleared when done.
  2. Using a renderbuffer.
    The renderbuffer must be attached and the FBO bound. The wooden texture is drawn. Text is drawn. The final Texture is copied into a Texture2D. Either the renderbuffer is redundant now, or the screen must be cleared.
  3. Using a Texture2D.
    The texture must be attached and the FBO bound. Only Text is drawn. Done.

Example Setup

To give a concrete example how all this theory looks in practice: let's create a color texture, a depth renderbuffer and a FBO, then attach the texture and renderbuffer to the FBO. I'm assuming you read the VBO tutorial before this, so I'm not going through the purpose of handles, GL.Gen*, GL.Bind* and GL.Delete* functions again. Note that this is a C API and the same rule of binding 0 to disable or detach something is valid here too. E.g. GL.Ext.BindFramebuffer( FramebufferTarget.FramebufferExt, 0 ); will disable the last bound FBO and return rendering back to the visible window-system provided framebuffer.

const int FboWidth = 512;
const int FboHeight = 512;
uint FboHandle;
uint ColorTexture;
uint DepthRenderbuffer;
// Create Color Texture
GL.GenTextures( 1, out ColorTexture );
GL.BindTexture( TextureTarget.Texture2D, ColorTexture );
GL.TexParameter( TextureTarget.Texture2D, TextureParameterName.TextureMinFilter, (int) TextureMinFilter.Nearest );
GL.TexParameter( TextureTarget.Texture2D, TextureParameterName.TextureMagFilter, (int) TextureMagFilter.Nearest );
GL.TexParameter( TextureTarget.Texture2D, TextureParameterName.TextureWrapS, (int) TextureWrapMode.Clamp );
GL.TexParameter( TextureTarget.Texture2D, TextureParameterName.TextureWrapT, (int) TextureWrapMode.Clamp );
GL.TexImage2D( TextureTarget.Texture2D, 0, PixelInternalFormat.Rgba8, FboWidth, FboHeight, 0, PixelFormat.Rgba, PixelType.UnsignedByte, IntPtr.Zero );
// test for GL Error here (might be unsupported format)
GL.BindTexture( TextureTarget.Texture2D, 0 ); // prevent feedback, reading and writing to the same image is a bad idea
// Create Depth Renderbuffer
GL.Ext.GenRenderbuffers( 1, out DepthRenderbuffer );
GL.Ext.BindRenderbuffer( RenderbufferTarget.RenderbufferExt, DepthRenderbuffer );
GL.Ext.RenderbufferStorage(RenderbufferTarget.RenderbufferExt, (RenderbufferStorage)All.DepthComponent32, FboWidth, FboHeight);
// test for GL Error here (might be unsupported format)
// Create a FBO and attach the textures
GL.Ext.GenFramebuffers( 1, out FboHandle );
GL.Ext.BindFramebuffer( FramebufferTarget.FramebufferExt, FboHandle );
GL.Ext.FramebufferTexture2D( FramebufferTarget.FramebufferExt, FramebufferAttachment.ColorAttachment0Ext, TextureTarget.Texture2D, ColorTexture, 0 );
GL.Ext.FramebufferRenderbuffer( FramebufferTarget.FramebufferExt, FramebufferAttachment.DepthAttachmentExt, RenderbufferTarget.RenderbufferExt, DepthRenderbuffer );
// now GL.Ext.CheckFramebufferStatus( FramebufferTarget.FramebufferExt ) can be called, check the end of this page for a snippet.
// since there's only 1 Color buffer attached this is not explicitly required
GL.DrawBuffer( (DrawBufferMode)FramebufferAttachment.ColorAttachment0Ext );
GL.PushAttrib( AttribMask.ViewportBit ); // stores GL.Viewport() parameters
GL.Viewport( 0, 0, FboWidth, FboHeight );
// render whatever your heart desires, when done ...
GL.PopAttrib( ); // restores GL.Viewport() parameters
GL.Ext.BindFramebuffer( FramebufferTarget.FramebufferExt, 0 ); // return to visible framebuffer
GL.DrawBuffer( DrawBufferMode.Back );

At this point you may bind the ColorTexture as source for drawing into the visible framebuffer, but be aware that it is still attached as target to the created FBO. That is only a problem if the FBO is bound again and the texture is used at the same time for being a FBO attachment target and the source of a texturing operation. This will cause feedback effects and is most likely not what you intended.
You may detach the ColorTexture from the FBO - the texture contents itself is not affected - by calling GL.Ext.FramebufferTexture2D() and attach a different target than ColorTexture to the ColorAttachment0 slot, for example simply 0. However the FBO would then be incomplete due to the missing color attachment, the best course of action is to detach the DepthRenderbuffer too and delete the renderbuffer and the FBO. Do not repeatedly attach and detach the same Texture if you want to update it every frame - just keep it attached to the FBO and make sure no feedback situation arises.

It is valid to attach the same texture or renderbuffer to multiple FBO at the same time. Example: you can avoid copies and save memory by attaching the same depth buffer to the FBOs, instead of creating multiple depth buffers and copy between them.

Special care has to be taken about 2 states that are always affected by FBOs: GL.Viewport() and GL.DrawBuffer(s). When switching from the visible framebuffer to a FBO, you should always set a proper viewport and drawbuffer. Switching framebuffer targets is such an expensive operation that the cost of the 2 extra calls to set up drawbuffers and viewport can be ignored. In the example setup above, the Viewport was stored and restored using GL.PushAttrib() and GL.PopAttrib(), but you may ofcourse specify it manually using GL.Viewport().


A FBO supports multiple color buffer attachments, if they have the same dimension and the same format. It is allowed to attach multiple color buffers - but only draw to one of them - by using the GL.DrawBuffer() command. Selecting multiple color buffers to write to is done with the GL.DrawBuffers() command, which expects an array like this:

DrawBuffersEnum[] bufs = new DrawBuffersEnum[2] { (DrawBuffersEnum)FramebufferAttachment.ColorAttachment0Ext, (DrawBuffersEnum)FramebufferAttachment.ColorAttachment1Ext }; // fugly, will be addressed in 0.9.2
GL.DrawBuffers( bufs.Length, bufs );

This code declares the color attachments 0 and 1 as buffers that can be written to. In practice this makes only sense if you're writing shaders with GLSL. (Look up "gl_FragData" for further info)

The exact number how many attachments are supported by the hardware must be queried through GL.GetInteger( GetPName.MaxColorAttachmentsExt, ... ) and the number of allowed Drawbuffers at the same time through GL.GetInteger( GetPName.MaxDrawBuffers, ... )

To select which buffer is affected by GL.ReadPixels() or GL.CopyTex*() calls, use GL.ReadBuffer().


For the sake of simplicity, the window-system provided framebuffer was called "visible framebuffer". In reality this is only true if you requested a single-buffer context from OpenGL, but the more likely case is that you requested a double-buffered context. When using double buffers, the 'back' buffer is the one used for drawing and never visible on screen, the 'front' buffer is the one that is visible on screen. The two buffers are swapped with each other when you call this.SwapBuffers(), to avoid that slow computers show unfinished images on screen. FBO are not designed to be double buffered, because they are off-screen at all times.

The wooden "Blacksmith" sign example has some hidden complexities that are ignored for the sake of simplicity, such as that you may not want to print with standard fonts on the sign, that words in different languages can have different length or that it might be desireable to add an additional mask when writing the text to simulate the paint peeling off the sign.

This page does not cover all commands exposed by FBO. For a more detailed description you'll have to dig through the official specification.

These Extensions were merged into ARB_framebuffer_objects with OpenGL 3.0:

EXT_framebuffer_multisample allows the creation of renderbuffers with n samples per image.

EXT_framebuffer_blit allows to bind 2 FBO at the same time. One for reading and one for writing. Without this Extension the active framebuffer is used for both: reading and writing.

Snippet how to interpret the possible results from GL.CheckFramebufferStatus

        private bool CheckFboStatus( )
            switch ( GL.Ext.CheckFramebufferStatus( FramebufferTarget.FramebufferExt ) )
            case FramebufferErrorCode.FramebufferCompleteExt:
                    Trace.WriteLine( "FBO: The framebuffer is complete and valid for rendering." );
                    return true;
            case FramebufferErrorCode.FramebufferIncompleteAttachmentExt:
                    Trace.WriteLine( "FBO: One or more attachment points are not framebuffer attachment complete. This could mean there’s no texture attached or the format isn’t renderable. For color textures this means the base format must be RGB or RGBA and for depth textures it must be a DEPTH_COMPONENT format. Other causes of this error are that the width or height is zero or the z-offset is out of range in case of render to volume." );
            case FramebufferErrorCode.FramebufferIncompleteMissingAttachmentExt:
                    Trace.WriteLine( "FBO: There are no attachments." );
                     Trace.WriteLine("FBO: An object has been attached to more than one attachment point.");
            case FramebufferErrorCode.FramebufferIncompleteDimensionsExt:
                    Trace.WriteLine( "FBO: Attachments are of different size. All attachments must have the same width and height." );
            case FramebufferErrorCode.FramebufferIncompleteFormatsExt:
                    Trace.WriteLine( "FBO: The color attachments have different format. All color attachments must have the same format." );
            case FramebufferErrorCode.FramebufferIncompleteDrawBufferExt:
                    Trace.WriteLine( "FBO: An attachment point referenced by GL.DrawBuffers() doesn’t have an attachment." );
            case FramebufferErrorCode.FramebufferIncompleteReadBufferExt:
                    Trace.WriteLine( "FBO: The attachment point referenced by GL.ReadBuffers() doesn’t have an attachment." );
            case FramebufferErrorCode.FramebufferUnsupportedExt:
                    Trace.WriteLine( "FBO: This particular FBO configuration is not supported by the implementation." );
                    Trace.WriteLine( "FBO: Status unknown. (yes, this is really bad.)" );
            return false;


These pages of the book discuss how to define, reference and draw geometric Objects using OpenGL.

Focus is on storing the Geometry directly in Vertex Buffer Objects (VBO), for using Immediate Mode please refer to the red book.

1. The Vertex

A Vertex (pl. Vertices) specifies a number of Attributes associated with a single Point in space. In the fixed-function environment a Vertex commonly includes Position, Normal, Color and/or Texture Coordinates. The only Attribute that is not optional and must be specified is the Vertex's Position, usually consisting of 3 float.
In Shader Program driven rendering it is also possible to specify custom Vertex Attributes which are previously unknown to OpenGL, such as Radius or Bone Index and Weight for Skeletal Animation. For the sake of simplicity we'll re-create one of the Vertex formats OpenGL already knows, namely InterleavedArrayFormat.T2fN3fV3f. This format contains 2 float for Texture Coordinates, 3 float for the Normal direction and 3 float to specify the Position.

Thanks to the included Math-Library in OpenTK, we're allowed to specify an arbitrary Vertex struct for our requirements, which is much more elegant to handle than a float[] array.

struct Vertex
{ // mimic InterleavedArrayFormat.T2fN3fV3f
  public Vector2 TexCoord;
  public Vector3 Normal;
  public Vector3 Position;

This leads to a Vertex consisting of 8 float, or 32 byte. We can now declare an Array of Vertices to describe multiple Points and allow easy indexing/referencing them.

Vertex[] Vertices;

The Vertex-Array Vertices can now be created and filled with data. Addressing elements is as convenient as in the following example:

Vertices = new Vertex[ n ]; // -1 < i < n  (Remember that arrays start at Index 0 and end at Index n-1.)
// examples how to assign values to the Vector's components:
Vertices[ i ].Position = new Vector3( 2f, -3f, .4f ); // create a new Vector and copy it to Position.
Vertices[ i ].Normal = Vector3.UnitX; // this will copy Vector3.UnitX into the Normal Vector.
Vertices[ i ].TexCoord.X = 0.5f; // the Vectors are structs, so the new keyword is not required.
Vertices[ i ].TexCoord.Y = 1f;
// Ofcourse this also works the other way around, using the Vectors as the source.
Vector2 UV = Vertices[ i ].TexCoord;

An Index is simply a byte, ushort or uint, referencing an element in the Vertices Array. So if we decide to draw a single Vertex 100 times at the same spot, instead of storing 100 times the same Vertex in Vertices, we can reference it 100 times from the Indices Array:

uint[] Indices;

Basically the Indices Array is used to describe the primitives and the Vertex Array is used to declare the corner points.

We can also use collections to store our Vertices, but it's recommended you stick with a simple Array to make sure your Indices are valid at all times.
Now the Vertices and Indices Arrays can be used to describe the edges of any Geometric Pritimitve Type.

Once the Arrays are filled with data it can be drawn in Immediate Mode, as Vertex Array or sent into a Vertex Buffer Object.

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2. Geometric Primitive Types

OpenGL requires you to specify the Geometric Primitive Type of the Vertices you wish to draw. This is usually expected when you begin drawing in either Immediate Mode (GL.Begin), GL.DrawArrays or GL.DrawElements.

Geometric Primitive Types in OpenTK.OpenGL

Fig. 1: In the above graphic all valid Geometric Primitive Types are shown, their winding is Clockwise (irrelevant for Points and Lines).

This is important, because drawing a set of Vertices as Triangles, which are internally set up to be used with Quads, will result only in garbage being displayed.
Examine Figure 1, you will see that v3 in a Quad is used to finish the shape, while Triangles uses v3 to start the next shape. The next drawn Triangle will be v3, v4, v5 which isn't something that belongs to any surface, if the Vertices were originally intended to be drawn as Quads.

However Points and Lines are an Exception here. You can draw every other Geometric Primitive Type as Points, in order to visualize the Vertices of the Object. Some more possibilities are:

  • QuadStrip, TriangleStrip and LineStrip can be interchanged, if the source data isn't a LineStrip.
  • Quads can be drawn as Lines with the restriction that there are no lines between v1, v2 and v3, v0.
  • Polygon can be drawn as LineLoop
  • TriangleFan can be drawn as Polygon or LineLoop

The smallest common denominator for all filled surfaces (i.e. no Points or Lines) is the Triangle. This Geometric Primitive Type has the special attribute of always being planar and is currently the best way to describe a 3D Object to GPU hardware.
While OpenGL allows to draw Quads or Polygons aswell, it is quite easy to run into lighting problems if the surface is not perfectly planar. Internally, OpenGL breaks Quads and Polygons into Triangles, in order to rasterize them.

  1. Points
    Specifies 1 Point per Vertex v, thus this is usually only used with GL.DrawArrays().
    n Points = Vertex * (1n);
  2. Lines
    Two Vertices form a Line.
    n Lines = Vertex * (2n);
  3. LineStrip
    The first Vertex issued begins the LineStrip, every consecutive issued Vertex marks a joint in the Line.
    n Line Segments in the Strip = Vertex * (1+1n)
  4. LineLoop
    Same as LineStrip, but the very first and last issued Vertex are automatically connected by an extra Line segment.
    n Line Segments in the Loop = Vertex * (1n);
  5. Polygon
    Note that the first and the last Vertex will be connected automatically, just like LineLoop.
    Polygon with n Edges = Vertex * (1n);
    Note: This primitive type should really be avoided whenever possible, basically the Polygon will be split to Triangles in the end anyways. Like Quads, polygons must be planar or be displayed incorrectly. Another Problem is that there is only 1 single Polygon in a begin-end block, which leads to multiple draw calls when drawing a mesh, or using the Extensions GL.MultiDrawElements or GL.MultiDrawArrays.
  6. Quads
    Quads are especially useful to work in 2D with bitmap Images, since those are typically rectangular aswell. Care has to be taken that the surface is planar, otherwise the split into Triangles will become visible.
    n Quads = Vertex * (4n);
  7. QuadStrip
    Like the Triangle-strip, the QuadStrip is a more compact representation of a sequence of connected Quads.
    n Quads in Quadstrip = Vertex * (2+2n);
  8. Triangles
    This way to represent a mesh offers the most control over how the Triangles are sorted, a Triangle always consists of 3 Vertex.
    n Triangles = Vertex * (3n);
    Note: It might look like an inefficient brute force approach at first, but it has it's advantages over TriangleStrip. Most of all, since you are not required to supply Triangles in sequenced strips, it is possible to arrange Triangles in a way that makes good use of the Vertex Caches. If the Triangle you currently want to draw shares an edge with one of the Triangles that have been recently drawn, you get 2 Vertices, that are stored in the Vertex Cache, almost for free. This is basically the same what stripification does, but you are not restricted to a certain Direction and forced to insert degenerated Triangles.
  9. TriangleStrip
    The idea behind this way of drawing is that if you want to represent a solid and closed Object, most neighbour Triangles will share 2 Vertices (an edge). You start by defining the initial Triangle (3 Vertices) and after that every new Triangle will only require a single new Vertex for a new Triangle.
    n Triangles in Strip = Vertex * (2+1n);
    Note: While this primitive type is very useful for storing huge meshes (2+1n Vertices per strip as opposed to 3n for BeginMode.Triangles), the big disadvantage of TriangleStrip is that there is no command to tell OpenGL that you wish to start a new strip while inside the glBegin/glEnd block. Ofcourse you can glEnd(); and start a new strip, but that costs API calls. A workaround to avoid exiting the begin/end block is to create 2 or more degenerate Triangles (you can imagine them as Lines) at the end of a strip and then start the next one, but this also comes at the cost of processing Triangles that will inevitably be culled and aren't visible. Especially when optimizing an Object to be in a Vertex Cache friendly layout, it is essential to start new strips in order to reuse Vertices from previous draws.
  10. TriangleFan
    A fan is defined by a center Vertex, which will be reused for all Triangles in the Fan, followed by border Vertices. It is very useful to represent convex n-gons consisting of more than 4 vertices and disc shapes, like the caps of a cylinder.

When looking at the graphic, Triangle- and Quad-strips might look quite appealing due to their low memory usage. They are beneficial for certain tasks, but Triangles are the best primitive type to represent an arbitrary mesh, because it's not restricting locality and allows further optimizations. It's just not realistic that you can have all your 3D Objects in Quads and OpenGL will split them internally into Triangles anyway. 3 ushort per Triangle isn't much memory, and still allows to index 64k unique Vertex in a mesh, the number of Triangles can be much higher. Don't hardwire BeginMode.Triangles into your programs though, for example Quads are very commonly used in orthographic drawing of UI Elements such as Buttons, Text or Sprites.

Should TriangleStrip get an core/ARB command to start a new strip within the begin/end block (only nVidia driver has such an Extension to restart the primitive) this might change, but currently the smaller data structure of the strip does not make up for the performance gains a Triangle List gets from Vertex Cache optimization. Ofcourse you can experiment with the GL.MultiDraw Extension mentioned above, but using it will break using other Extensions such as DirectX 10 instancing.

3.a Vertex Buffer Objects

The advantage of VBO (Vertex Buffer Objects) is that we can tell OpenGL to store information used for drawing - like Position, Colors, Texture Coordinates and Normals - directly in the Video-card's Memory, rather than storing it in System Memory and pass it to the graphics Hardware every time we wish to draw it. While this has been already doable with Display Lists before, VBO has the advantage that we're able to retrieve a Pointer to the data in Video Memory and read/write directly to it, if necessary. This can be a huge performance boost for dynamic meshes and is for years the best overall solution for storing - both, static and dynamic - Meshes.

Handling VBOs is very similar to handling Texture objects, we can generate&delete handles, bind them or fill them with data. For this tutorial we will need 2 objects, one VBO containing all Vertex information (Texture, Normal and Position in this example case) and an IBO (Index Buffer Object) referencing Vertices from the VBO to form Triangles. This has the advantage that, when we have uploaded the data to the VBO/IBO later on, we can draw the whole mesh with a single GL.DrawElements call.

First we acquire two Objects to use:

uint[] VBOid = new uint[ 2 ];
GL.GenBuffers( 2, out VBOid );

Although it is unlikely, OpenGL could complain that it ran out of memory or that the extension is not supported, it should be checked with GL.GetError. If everything went smooth we have 2 objects to work with available now.

The OpenGL driver should clean up all our mess when it deletes the render context, it's always a good idea to clean up on your own where you can. We remove the objects we reserved at the buffer creation by calling:

GL.DeleteBuffers( 2, ref VBOid );

To select which object you currently want to work with, simply bind the handle to either BufferTarget.ArrayBuffer or BufferTarget.ElementArrayBuffer. The first is used to store position, uv, normals, etc. (named VBO) and the later is pointing at those vertices to define geometry (named IBO).

GL.BindBuffer( BufferTarget.ArrayBuffer, VBOid[ 0 ] );
GL.BindBuffer( BufferTarget.ElementArrayBuffer, VBOid[ 1 ] );

It is not required to bind a buffer to both targets, for example you could store only the vertices in the VBO and keep the indices in system memory. Also, the two objects are not tied together in any way, for example you could build different triangle lists for BufferTarget.ElementArrayBuffer to implement LOD on the same set of vertices, simply by binding the desired element array.

Theres two important things to keep in mind though:

1) While working with VBOs, GL.EnableClientState(EnableCap.VertexArray); must be enabled. if using Normals, GL.EnableClientState(EnableCap.NormalArray), just like classic Vertex Arrays.

2) All Vertex Array related commands will be used on the currently bound objects until you explicitly bind zero '0' to disable hardware VBO.

GL.BindBuffer( BufferTarget.ArrayBuffer, 0 );
GL.BindBuffer( BufferTarget.ElementArrayBuffer, 0 );

Passing Data
There are several ways to fill the object's data, we will focus on using GL.BufferData and directly writing to video memory. The third option would be GL.BufferSubData which is quite straightforward to use once you are familiar with GL.BufferData.

  1. GL.BufferData
    We will start by preparing the IBO, it would not make a difference if we set up the VBO first, we simply start with the shorter one.

    We make sure the correct object is bound (it is not required to do this, if the buffer is already bound. Just here to clarify on which object we currently work on)

    GL.BindBuffer( BufferTarget.ElementArrayBuffer, VBOid[ 1 ] );

    In the example application ushort has been used for Indices, because 16 Bits [0..65535] are more available Vertices than used by most real-time rendered meshes, however the mesh could index way more Vertices using a type like uint. Using ushort, OpenGL will store this data as 2 Bytes per index, saving memory compared to a 4 Bytes UInt32 per index.

    The function GL.BufferData's first parameter is the target we want to use, the second is the amount of memory (in bytes) we need allocated to hold all our data. The third parameter is pointing at the data we wish to send to the graphics card, this can be IntPtr.Zero and you may send the data at a later stage with GL.MapBuffer (more about this later). The last parameter is an optimization hint for the driver, it will place your data in the best suited place for your purposes.

    GL.BufferData( BufferTarget.ElementArrayBuffer, (IntPtr) ( Indices.Length * sizeof( ushort ) ), Indices, BufferUsageHint.StaticDraw );

    That's all, OpenGL now has a copy of Indices available and we could dispose the array, assuming we have the Index Count of the array stored in a variable for the draw call later on.

    Now that we've stored the indices in an IBO, the Vertices are next. Again, we make sure the binding is correct, give a pointer to the Vertex count, and finally the usage hint.

    GL.BindBuffer( BufferTarget.ArrayBuffer, VBOid[ 0 ] );
    GL.BufferData( BufferTarget.ArrayBuffer, (IntPtr) ( Vertices.Length * 8 * sizeof( float ) ), Vertices, BufferUsageHint.StaticDraw );

    There's a table at the bottom of this page, explaining the options in the enum BufferUsageHint in more detail.

  2. GL.MapBuffer / GL.UnmapBuffer

    While the first described technique to pass data into the objects required a copy of the data in system memory, this alternative will give us a pointer to the video memory reserved by the object. This is useful for dynamic models that have no copy in client memory that could be used by GL.BufferData, since you wish to rebuild it every single frame (e.g. fully procedural objects, particle system).

    First we make sure that we got the desired object bound and reserve memory, the pointer towards the Indices is actually IntPtr.Zero, because we only need an empty buffer.

    GL.BindBuffer( BufferTarget.ElementArrayBuffer, VBOid[ 0 ] );
    GL.BufferData( BufferTarget.ElementArrayBuffer, (IntPtr) ( Indices.Length * sizeof( ushort ) ), IntPtr.Zero, BufferUsageHint.StaticDraw );

    Note that you should change BufferUsageHint.StaticDraw properly according to what you intend to do with the Data, there's a table at the bottom of this page. Now we're able to request a pointer to the video memory.

    IntPtr VideoMemoryIntPtr = GL.MapBuffer(BufferTarget.ElementArrayBuffer, BufferAccess.WriteOnly);

    Valid access flags for the pointer are BufferAccess.ReadOnly, BufferAccess.WriteOnly or BufferAccess.ReadWrite, which help the driver understand what you're going to do with the data. Note that the data's object is locked until we unmap it, so we want to keep the timespan over which we use the pointer as short as possible. We may now write some data into the buffer, once we're done we must release the lock.

      fixed ( ushort* SystemMemory = &Indices[0] )
        ushort* VideoMemory = (ushort*) VideoMemoryIntPtr.ToPointer();
        for ( int i = 0; i < Indices.Length; i++ ) 
          VideoMemory[ i ] = SystemMemory[ i ]; // simulate what GL.BufferData would do
    GL.UnmapBuffer( BufferTarget.ElementArrayBuffer );

    The pointer is now invalid and may not be stored for future use, if we wish to modify the object again, we have to call GL.MapBuffer again.

Further reading
Visit this link in order to tell OpenGL about the composition of your Vertex data, and this link for drawing the data.


One hint from the nVidia whitepaper was regarding the situation, if we want to update all data in the buffer object by using GL.MapBuffer and not retrieve any of the old data. Although this is a bad idea, because mapping the buffer is a more expensive operation than just calling GL.BufferData, it might be necessary in cases where you have no copy of the data in system memory, but build it on the fly. The solution to making this somewhat efficient is first calling GL.BufferData with a IntPtr.Zero again, which tells the driver that the old data isn't valid anymore. Calling GL.MapBuffer will return a new pointer to a valid memory location of the requested size to write to, while the old data will be cleaned up once it's not used in any draw operations anymore.

Also note that either reading from a VBO or wrapping it into a Display List is very slow and should both be avoided.

Table 1:
BufferUsageHint.Static... Assumed to be a 1-to-n update-to-draw. Means the data is specified once (during initialization).
BufferUsageHint.Dynamic... Assumed to be a n-to-n update-to-draw. Means the data is drawn multiple times before it changes.
BufferUsageHint.Stream... Assumed to be a 1-to-1 update-to-draw. Means the data is very volatile and will change every frame.

...Draw Means the buffer will be used to sending data to GPU. video memory (Static|StreamDraw) or AGP (DynamicDraw)
...Read Means the data must be easy to access, will most likely be system or AGP memory.
...Copy Means we are about to do some ..Read and ..Draw operations.

3.b Attribute Offsets and Strides

Setting Strides and Offsets for Vertex Arrays and VBO

There are 2 ways to tell OpenGL in which layout the Vertices are stored:

  1. GL.InterleavedArrays()
    What GL.InterleavedArrays does is enable/disable the required client states for OpenGL to interpret our passed data, the first parameter tells that we have 2 floats for Texture Coordinates (T2f), 3 floats for Normal (N3F) and 3 floats for position (V3F). The second parameter is the stride that will be jumped to find the second, third, etc. set of texcoord/normal/position values. Since our Vertices are tighly packed, no stride (zero) is correct. The last parameter should point at Indices, but we already sent them to the VBO in video memory, no need to point at them again:

    GL.InterleavedArrays( InterleavedArrayFormat.T2fN3fV3f, 0, null );

    This command has the advantage that it's very obvious to the OpenGL driver what layout of data we have supplied, and it may be possible for the driver to optimize the memory. Remember that GL.InterleavedArrays will change states, if you manually disable EnableCap.VertexArray, EnableCap.NormalArray, EnableCap.TextureCoordArray or changing GL.VertexPointer, GL.NormalPointer or GL.TexCoordPointer (after calling GL.InterleavedArrays and before calling GL.DrawElements) make sure to enable them again or you won't see anything.

  2. Setting the offsets/strides manually
    For the Vertex format InterleavedArrayFormat.T2fN3fV3f, the correct pointer setup is:

    GL.TexCoordPointer( 2, TexCoordPointerType.Float, 8 * sizeof( float ), (IntPtr) ( 0 ) );
    GL.NormalPointer( NormalPointerType.Float, 8 * sizeof( float ), (IntPtr) ( 2 * sizeof( float ) ) );
    GL.VertexPointer( 3, VertexPointerType.Float, 8 * sizeof( float ), (IntPtr) ( 5 * sizeof( float ) ) );
    1. The first parameter is the number of components to describe that attribute. For GL.NormalPointer this is always 3 components, but is variable for Texture coordinates and Position (and Color).
    2. The second parameter is the type of the components.
    3. The third parameter is the number of bytes of the Vertex struct. This stride is used to define at which offset the next Vertex begins.
    4. The last parameter indicates the byte offset of the first appearance of the attribute, this makes perfect sense if you recall the layout of our Vertex struct.
    5. Byte 0-7 are used for the Texture Coordinates, Byte 8-19 for the Normal and Byte 20-31 for the Vertex Position.

3.c Vertex Arrays

Vertex Arrays were removed from OpenGL in version 3.1. Vertex Buffer Objects are the recommended alternative, since they are both faster and safer.

This is the correct way to use Vertex Arrays in .Net (pseudocode):

struct Vertex
    public Vector3 Position;
    public Vector2 TexCoord;
Vertex[] vertices = new Vertex[100];
    fixed (float* pvertices = vertices)
        GL.VertexPointer(3, VertexPointerType.Float, BlittableValueType.StrideOf(vertices), pvertices);
        GL.TexCoordPointer(2, VertexPointerType.Float, BlittableValueType.StrideOf(vertices), pvertices + sizeof(Vector3));
        GL.DrawArrays(BeginMode.Triangles, 0, vertices.Length);
        GL.Finish();    // Force OpenGL to finish rendering while the arrays are still pinned.

Vertex Arrays use client storage, because they are stored in system memory (not video memory). Since .Net is a Garbage Collected environment, the arrays must remain pinned until the GL.DrawArrays() or GL.DrawElements() call is complete.

If the arrays are unpinned prematurely, they may be moved or collected by the Garbage Collector before the draw call finishes. This will lead to random access violation exceptions and corrupted rendering, issues which can be difficult to trace.

Due to the asynchronous nature of OpenGL, GL.Finish() must be used to ensure that rendering is complete before the arrays are unpinned. However, this call introduces a sync point between the CPU and GPU, which can significantly degrade performance.

Vertex Buffer Objects and Display Lists use server storage (video memory) which does not suffer from this issue. Given the improved performance and safety of server storage, it is recommended to avoid Vertex Arrays completely.

Please visit this discussion in the forum for more information.

4. Vertex Array Objects

Vertex Array Objects (abbreviation: VAO) are storing vertex attribute setup and VBO related state. This allows to reduce the number of OpenGL calls when drawing from VBOs, because all attribute declaration and pointer setup only needs to be done once (ideally at initialization time) and is afterwards stored in the VAO for later re-use.

But before a VAO can be bound, a handle must be generated:

void GL.GenVertexArrays( uint[] );
void GL.DeleteVertexArrays( uint[] );
bool GL.IsVertexArray( uint );

Binding a VAO is as simple as

void GL.BindVertexArray( uint );

The currently bound VAO records state set by the following commands:


Indirectly it also saves the state set by GL.BindBuffer() at the point of time when GL.VertexAttribPointer() was called. A more technical description can be found at the OpenGL Wiki.

An example usage:


uint VboHandle;
uint VaoHandle;
GL.GenVertexArrays(1, out VaoHandle);
GL.GenBuffers(1, out VboHandle);
GL.BindVertexArray(VaoHandle);                     //Make sure to call BindVertexArray() before BindBuffer()
GL.BindBuffer(BufferTarget.ArrayBuffer, VboHandle);


///Drawing code here

5. Drawing

In order to tell OpenGL to draw primitives for us, there's basically two ways to go:

1. Immediate Mode, as in specifying every single Vertex manually.
2. Vertex Buffers (or Vertex Arrays), drawing a whole Mesh with a single Command.

In order for all GL.Draw*-Functions to output geometric Primitives, EnableCap.VertexArray must be enabled first.

  1. Immediate Mode
    Hereby every single Vertex we wish to draw has to be issued manually.

    // GL.DrawArrays behaviour
    GL.Begin( BeginMode.Points );
    for ( uint i = 0; i < Vertices.Length; i++ )
       GL.TexCoord2( Vertices[ i ].TexCoord );
       GL.Normal3( Vertices[ i ].Normal );
       GL.Vertex3( Vertices[ i ].Position );
    GL.End( );
    // GL.DrawElements behaviour
    GL.Begin( BeginMode.Points );
    for ( uint i = 0; i < Indices.Length; i++ )
       GL.TexCoord2( Vertices[ Indices[ i ] ].TexCoord );
       GL.Normal3( Vertices[ Indices[ i ] ].Normal );
       GL.Vertex3( Vertices[ Indices[ i ] ].Position );
    GL.End( );
  2. GL.DrawArrays( BeginMode, int First, int Length )
    This Command is used together with Vertex Arrays or Vertex Buffer Objects, see setting Attribute Pointers. This line will automatically draw all Vertex contained in Vertices in order of appearance in the Array.

    GL.DrawArrays( BeginMode.Points, 0, Vertices.Length );
  3. GL.DrawElements( BeginMode, int Length, DrawElementsType, object )
    Like DrawArrays this Command is used together with Vertex Arrays or VBO. It uses an unsigned Array (byte, ushort, uint) to Index the Vertex Array. This is particularly useful for 3D Models where the Triangles describing the surface share Edges and Vertices.

    GL.DrawElements( BeginMode.TriangleStrip, Indices.Length, DrawElementsType.UnsignedInt, Indices );
  4. GL.DrawRangeElements( BeginMode, int Start, int End, int Count, DrawElementsType, object )
    This function behaves largely like GL.DrawElements, with the change that you may specify a starting index rather than starting at 0.
    Start and End are the smallest and largest Array-Index into Indices. Count is the number of Elements to render.

    // behaviour equal to GL.DrawElements
    GL.DrawRangeElements( BeginMode.TriangleStrip, 0, Indices.Length-1, Indices.Length, DrawElementsType.UnsignedInt, Indices );
  5. GL.DrawArraysInstanced( BeginMode, int First, int Length, int primcount )
    This function is only available on DX10 hardware and basically behaves like this:

    for (int gl_InstanceID=0; gl_InstanceID < primcount; gl_InstanceID++ )
       GL.DrawArrays( BeginMode, First, Length );

    gl_InstanceID is a uniform variable available to the Vertex Shader, which (in conjunction with GL.UniformMatrix) can be used to assign each Instance drawn it's own unique Orientation Matrix.

  6. GL.DrawElementsInstanced( BeginMode, int Length, DrawElementsType, object, int primcount )
    This function is only available on DX10 hardware and internally unrolls into:

    for (int gl_InstanceID=0; gl_InstanceID < primcount; gl_InstanceID++ )
       GL.DrawElements( BeginMode, Length, DrawElementsType, Object );

    gl_InstanceID is a uniform variable available to the Vertex Shader, which (in conjunction with GL.UniformMatrix) can be used to assign each Instance drawn it's own unique orientation Matrix.

Extension References

5.b Drawing Optimizations

This page is just giving a starting point for optimizations, the links below provide more in-depth information.

  • Make sure there are no OpenGL errors. Any error usually kills the framerate.
  • Enable backface culling with GL.Enable(EnableCap.CullFace) rather than relying only on the Z-Buffer.
  • If you're drawing a scene that covers the whole screen each frame, only clear depth buffer but not the color buffer.
  • Organize drawing in a way that requires as few state changes as possible. Don't enable states that are not needed.
  • Use GL.Hint(...) wherever applicable.
  • Avoid Immediate Mode or Vertex Arrays in favor of Display Lists or Vertex Buffer Objects.
  • Use GL.DrawRangeElements instead of GL.DrawElements for a slight performance gain.
  • Take advantage of S3TC Texture compression and Vertex Cache optimizing algorithms.


Last edit of Links: March 2008

6. OpenTK's procedural objects


OpenGL rendering pipeline

Rendering works by projecting 3-dimensional objects to a 2-dimensional plane, so they can be displayed on a screen. In modern OpenGL the 3D objects are read from Vertex Buffer Objects (VBO) and the resulting image is written to a framebuffer. This page will cover the pipeline operations involved between input and output.

Some of the steps involved are fully programmable (namely: Vertex, Geometry and Fragment Shader) while the rest is hardwired. However even the hardwired logic can be manipulated by setting OpenGL's state machine's toggles, which are shown in the diagram and described in detail in the OpenGL specification.

In order to begin drawing, A Vertex and Fragment Shader are required and OpenGL must know about the 3D object, which is done by using VBO (and optionally VAO).

The Vertex Shader is responsible for transforming each Vertex from Object Coordinates into Clip Coordinates.
The primitive assembly will use the resulting Clip Coordinates to create geometric primitives, which are then divided by the vertex' w-component (perspective divide) and clipped against the [-1.0..+1.0] range of normalized device-coordinate space (NDC).
As a final step, the viewport application will offset&scale the normalized device-coordinates to window coordinates.

The resulting transformed geometric primitive types can now be rasterized into fragments. Each fragment receives interpolated vertex shader data from the primitive it belongs to, which is at least position and depth. The fragment shader's output must be either gl_FragColor, gl_FragData[] or set by GL.BindFragDataLocation(). This output is called a "fragment", which is a candidate to become a pixel in the framebuffer. Before this can happen, the fragment muss pass a series of tests called the Fragment Operations.

There is one noteworthy special case found in some modern hardware. The functionality is called "Early-Z" or "HyperZ". After rasterization of the primitive, the resulting Z is used to discard fragments even before the fragment shader is executed. This functionality is not exposed to OpenGL and works behind the scenes. In the diagram to the left, it would belong between the Triangle, Line or Point rasterization, and the fragment shader.

Also note: This diagram targets the OpenGL 3.2 pipeline, but contains a few commands which belong to the ARB_compatibility extension and may be unavailable.

Fragment Operations

A Fragment is a candidate to become a pixel in the framebuffer. For every fragment, OpenGL applies a series of tests in order to eliminate the fragment early to avoid updating the framebuffer.

Most of these tests can be toggled through GL.Enable/Disable, the pages below cover this in more detail. However the most in-depth description of the functionality can only be found in the official OpenGL specification.

The tests are executed from top to bottom, if a fragment did not pass an early test, later tests are ignored. I.e. If the fragment does not pass the Scissor Test, there would be no point in determining whether Depth Test passes or not.

01. Pixel Ownership Test

Citation with minor modifications. Cannot explain it better.

"GL 3.1 spec" wrote:

This test is used to determine if the pixel at the current location in the framebuffer is currently owned by the GL context. If it is not, the window system decides the fate the incoming fragment. Possible results are that the fragment is discarded or that some subset of the subsequent per-fragment operations are applied to the fragment. This test allows the window system to control the GL’s behavior, for instance, when a GL window is obscured.

If the draw framebuffer is a framebuffer object, the pixel ownership test always passes, since the pixels of framebuffer objects are owned by the GL, not the window system. If the draw framebuffer is the default framebuffer, the window system controls pixel ownership.



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02. Scissor Test

The Scissor Test is used to limit drawing to a rectangular region of the viewport. When enabled, only fragments inside the rectangle are passed to later pipeline stages.

The ScissorTest can be enabled or disabled using EnableCap.ScissorTest

GL.Enable( EnableCap.ScissorTest );
GL.Disable( EnableCap.ScissorTest ); // default

Only a single command is related to the ScissorTest, GL.Scissor( x, y, width, height ). By default the parameters are set to cover the whole window.

  • X and Y are used to specify the lower-left corner of the rectangle.
  • Width is used to specify the horizontal extension of the rectangle.
  • Height is used to specify the vertical extension of the rectangle.

State Queries

To determine whether ScissorTest is enabled or disabled, use Result = GL.IsEnabled( EnableCap.ScissorTest );

The values set by GL.Scissor() can be queried by GL.GetInteger( GetPName.ScissorBox, ... ); // returns an array

03. Multisample Fragment Operations (WIP)

Multisampling is designed to counter the effects of aliasing at the edges of a primitive, when it is rasterized into fragments. Multisampling can be also applied to transparent textures, like wire fences, blades of grass or the leaves of trees. In this case, it is called 'alpha-to-coverage' and replaces the legacy alpha test.

A multisample buffer contains multiple samples per pixel, with each sample having it's own color, depth and stencil values. The term 'coverage' refers to a bitmask that is used to determine which of these samples will be updated: a coverage value of 1 indicates that the relevant sample will be updated; a value of 0 indicates it will be left untouched.

However, when EnableCap.SampleAlphaToCoverage is used, the coverage is obtained by interpreting the alpha as a percentage: an alpha of 0.0 means that no samples are covered, while a value of 1.0 indicates that all samples are covered. For example, a multisample buffer with 4 samples per pixel and an Alpha value of 0.5 indicates that half of the samples are covered (their coverage bit is 1) and two are not covered (coverage bit is 0).

"figure out whether this is true" wrote:

The coverage bitmask of incoming fragments can be set in a Fragment Shader with the variable gl_Coverage.

To enable alpha-to-coverage, enable multisampling (GL.Enable(EnableCap.Multisample)) and make sure that GL.GetInteger(GetPName.SampleBuffers, out buffers) is 1. If EnableCap.Multisample is disabled but GetPName.SampleBuffers is 1, alpha-to-coverage will be disabled.

There are three OpenGL states related to alpha-to-coverage, they are controlled by GL.Enable() and GL.Disable()

  • EnableCap.SampleAlphaToCoverage
    For each sample at the current pixel, the Alpha value is read and used to generate a temporary coverage bitmask which is then combined through a bitwise AND with the fragment's coverage bitmask. Only samples who's bit is set to 1 after the bitwise AND are updated.
  • EnableCap.SampleCoverage
    Using GL.SampleCoverage( value, invert ) the temporary coverage bitmask is generated by the value parameter - and if the invert parameter is true it is bitwise inverted - before the bitwise AND with the fragment's coverage bitmask.
  • EnableCap.AlphaToOne
    Each Alpha value is replaced by 1.0.

The values set by the command GL.SampleCoverage( value, invert ) are only used when EnableCap.SampleCoverage is enabled.

  • value is a single-precision float used to specify the Alpha value used to create the coverage bitmask.
  • invert is a boolean toggle to control whether the bitmask is bitwise inverted before the AND operation.

State Queries

The states of EnableCap.Multisample, EnableCap.SampleAlphaToCoverage, EnableCap.SampleCoverage and EnableCap.AlphaToOne can be queried with Result = GL.IsEnabled( cap )

The value set by GL.SampleCoverage() can be queried with GL.GetFloat( GetPName.SampleCoverageValue, ... )

The boolean set by GL.SampleCoverage() can be queried with GL.GetBoolean( GetPName.SampleCoverageInvert, ... )

04. Stencil Test

The Stencil buffer's primary use is to apply a mask to the framebuffer. Simply put, you can think of it as a cardboard stencil where you cut out holes, so you may use a can of spraypaint to paint shapes. The paint will only pass the holes you had cut out and be blocked otherwise by the cardboard. OpenGL's Stencil testing allows you to layer several of these masks over each other.

A more OpenGL related example: In any vehicle simulation the interior of the cockpit is usually masked by a stencil buffer, because it does not have to be redrawn every frame. That way alot of fragments of the outside landscape can be skipped, as they would not contribute to the final image anyway.

In order to use the Stencil buffer, the window-system provided framebuffer - or the Stencil attachment of a FBO - must explicitly contain a logical stencil buffer. If there is no stencil buffer, the fragment is always passed to the next pipeline stage.

For the purpose of clarity in this article, the Stencil Buffer is assumed to be 8 Bit large and able to represent the values 0..255

StencilTest functionality is enabled and disabled with EnableCap.StencilTest

GL.Enable( EnableCap.StencilTest );
GL.Disable( EnableCap.StencilTest ); // default

The value used by GL.Clear() commands can be set through:

GL.ClearStencil( int ); // 0 is the default, Range [0..255]

If StencilTest is enabled, writing can be limited by a bitfield through a bitwise AND.

GL.StencilMask( bitmask ); // By default all bits are set to 1.

Note: The command GL.StencilMaskSeparate() behaves exactly like GL.StencilMask() but allows separate comparison functions for front- and back-facing polygons.


The command GL.StencilFunc( func, ref, mask ) is used to specify the conditions under which the StencilTest succeeds or fails. It sets the comparison function, reference value and mask for the Stencil Test.

  • ref is an integer value to compare against. By default this value is 0, range [0 .. 255]
  • mask is a bitfield which will be used in a bitwise AND. Only the bits which are set to 1 are considered. By default all bits are set to 1.
  • func of the test can have the following values, the default is StencilFunction.Always.

    • StencilFunction.Always - Test will always succeed.
    • StencilFunction.Never - Test will never succeed.
    • StencilFunction.Less - Test will succeed if ( ref & mask ) < ( pixel & mask )
    • StencilFunction.Lequal - Test will succeed if ( ref & mask ) <= ( pixel & mask )
    • StencilFunction.Equal - Test will succeed if ( ref & mask ) == ( pixel & mask )
    • StencilFunction.Notequal - Test will succeed if ( ref & mask ) != ( pixel & mask )
    • StencilFunction.Gequal - Test will succeed if ( ref & mask ) >= ( pixel & mask )
    • StencilFunction.Greater - Test will succeed if ( ref & mask ) > ( pixel & mask )
  • The word 'pixel' means in this case: The value in the Stencil Buffer at the current pixel.

Note: The command GL.StencilFuncSeparate() behaves exactly like GL.StencilFunc() but allows separate comparison functions for front- and back-facing polygons.


Depending on the result determined by GL.StencilFunc(), the command GL.StencilOp( fail, zfail, zpass ) can be used to decide what action should be taken if the fragment passes the test.

  • fail - behavior when StencilTest fails, regardless of DepthTest.
  • zfail - behavior when StencilTest succeeds, but DepthTest fails.
  • zpass - behavior when both tests succeed, or if StencilTest succeeds and DepthTest is disabled.

The following values are allowed, the default for all operations is StencilOp.Keep

  • StencilOp.Zero - set Stencil Buffer to 0.
  • StencilOp.Keep - Do not modify the Stencil Buffer.
  • StencilOp.Replace - set Stencil Buffer to ref value as specified by last GL.StencilFunc() call.
  • StencilOp.Incr - increment Stencil Buffer by 1. It is clamped at 255.
  • StencilOp.IncrWrap - increment Stencil Buffer by 1. If the result is greater than 255, it becomes 0.
  • StencilOp.Decr - decrement Stencil Buffer by 1. It is clamped at 0.
  • StencilOp.DecrWrap - decrement Stencil Buffer by 1. If the result is less than 0, it becomes 255.
  • StencilOp.Invert - Bitwise invert. If the Stencil Buffer currently contains the bits 00111001, it is set to 11000110.

Note: The command GL.StencilOpSeparate() behaves exactly like GL.StencilOp() but allows separate comparison functions for front- and back-facing polygons.

State Queries

To determine whether StencilTest is enabled or disabled, use Result = GL.IsEnabled( EnableCap.StencilTest );

The bits available in the Stencil Buffer can be queried by GL.GetInteger( GetPName.StencilBits, ... );

The value set by GL.ClearStencil() can be queried by GL.GetInteger( GetPName.StencilClearValue, ... );

The bitfield set by GL.StencilMask() can be queried by GL.GetInteger( GetPName.StencilWritemask, ... );

The state of the Stencil comparison function can be queried with GL.GetInteger() and the following parameters:

GetPName.StencilFunc - GL.StencilFunc's parameter 'func'
GetPName.StencilRef - GL.StencilFunc's parameter 'ref'
GetPName.StencilValueMask - GL.StencilFunc's parameter 'mask'

The state of the Stencil operations can be queried with GL.GetInteger and the following parameters:

GetPName.StencilFail - GL.StencilOp's parameter 'fail'
GetPName.StencilPassDepthFail - GL.StencilOp's parameter 'zfail'
GetPName.StencilPassDepthPass - GL.StencilOp's parameter 'zpass'

If the GL.Stencil***Separate() functions have been used, the tokens GetPName.StencilBack*** become available to query the settings for back-facing polygons. With Intellisense you should not have any problems finding them.

Related Extensions for further reading (promoted to core in GL 2.0) (promoted to core in GL 2.0) (basically the same functionality as ATI_separate_stencil, not in core though)

05. Depth Test

A commonly used logical buffer in OpenGL is the Depth buffer, often called Z-Buffer. The name was chosen due to X and Y being used to describe horizontal and vertical displacement on the screen, so Z is used to measure the distance perpendicular to the screen.

The general purpose of this buffer is determining whether a fragment is occluded by a previously drawn pixel. I.e. If the fragment in question is further away from the eye than an already existing pixel, the fragment cannot be visible and is discarded.

In order to use the Depth buffer, the window-system provided framebuffer - or the Depth attachment of a FBO - must explicitly contain a logical Depth buffer. If there is no Depth buffer, the fragment is always passed to the next pipeline stage.

DepthTest functionality is enabled and disabled with EnableCap.DepthTest

GL.Enable( EnableCap.DepthTest );
GL.Disable( EnableCap.DepthTest ); // default

The value used by GL.Clear() commands can be set through:

GL.ClearDepth( double ); // 1.0 is the default, Range: [0.0 .. 1.0]

If DepthTest is enabled, writing to the Depth buffer can be toggled by a boolean flag.

GL.DepthMask( bool ); // true is the default

The command GL.DepthFunc( func ) is used to specify the comparison method used whether a fragment is closer to the eye than the existing pixel it is compared to.

Function of the test can have the following values, the default is DepthFunction.Less.

  • DepthFunction.Always - Test will always succeed.
  • DepthFunction.Never - Test will never succeed.
  • DepthFunction.Less - Test will succeed if ( fragment depth < pixel depth )
  • DepthFunction.Lequal - Test will succeed if ( fragment depth <= pixel depth )
  • DepthFunction.Equal - Test will succeed if ( fragment depth == pixel depth )
  • DepthFunction.Notequal - Test will succeed if ( fragment depth != pixel depth )
  • DepthFunction.Gequal - Test will succeed if ( fragment depth >= pixel depth )
  • DepthFunction.Greater - Test will succeed if ( fragment depth > pixel depth )

The command GL.DepthRange( near, far ) is used to define the minimum (near plane) and maximum (far plane) z-value that is stored in the Depth Buffer. Both parameters are expected to be of double-precision floating-point and must lie within the range [0.0 .. 1.0].

It is allowed to call GL.DepthRange( 1.0, 0.0 ), there is no rule that must satisfy ( near < far ).

For an in-depth explanation how the distribution of z-values in the Depth buffer works, please read Depth buffer - The gritty details.

State Queries

To determine whether DepthTest is enabled or disabled, use Result = GL.IsEnabled( EnableCap.DepthTest );

The bits available in the Stencil Buffer can be queried by GL.GetInteger( GetPName.DepthBits, ... );

The value set by GL.ClearDepth() can be queried by GL.GetFloat( GetPName.DepthClearValue, ... );

The boolean set by GL.DepthMask() can be queried by GL.GetBoolean( GetPName.DepthWritemask, ... );

The Depth comparison function can be queried with GL.GetInteger( GetPName.DepthFunc, ... );

The Depth range can be queried with GL.GetFloat( GetPName.DepthRange, ... ); // returns an array

06. Occlusion Query

Occlusion queries count the number of fragments (or samples) that pass the depth test, which is useful to determine visibility of objects.

If an object is drawn but 0 fragments passed the depth test, it is fully occluded by another object. In practice this means that a simplification of an object is drawn using an occlusion query (for example: A bounding box can be the occlusion substitute for a truck) and only if fragments of the simple object pass the depth test, the complex object is drawn. Please read Conditional Render for a convenient solution.

Note that the simplified object does not actually have to become visible, one can set GL.ColorMask and GL.DepthMask to false for the purpose of the occlusion query. The only GL.Enable/Disable state associated with it is the DepthTest. If DepthTest is disabled all fragments will automatically pass it and the occlusion test becomes pointless.

Occlusion Query handles are generated and deleted similar to other OpenGL handles:

uint MyOcclusionQuery;
GL.GenQueries( 1, out MyOcculsionQuery );
GL.DeleteQueries( 1, ref MyOcculsionQuery );

The draw commands which contribute to the count must be enclosed with GL.BeginQuery() and GL.EndQuery().

GL.BeginQuery( QueryTarget.SamplesPassed, MyOcculsionQuery );
// draw...
GL.EndQuery( QueryTarget.SamplesPassed );

It is very important to understand that this process is running asynchronous, by the time the CPU is querying the result of the count the GPU might not be done counting yet. OpenGL provides additional query commands to determine whether the occlusion query result is available, but before it is confirmed to be available any query of the count is not reliable. The following code will get a reliable result.

uint ResultReady=0;
while ( ResultReady == 0 )
  GL.GetQueryObject( MyOcculsionQuery, GetQueryObjectParam.QueryResultAvailable, out ResultReady );
uint MyOcclusionQueryResult=0;
GL.GetQueryObject( MyOcculsionQuery, GetQueryObjectParam.QueryResult, out MyOcclusionQueryResult );
// MyOcclusionQueryResult is now reliable.

However this is not very efficient to use because the CPU will spin in a loop until the GPU is done counting.

A better approach is to do the occlusion queries in the first frame and do not wait for a result. Instead continue drawing as normal and wait for the next frame, before you check the results of the query. In other words frame n executes the query and frame n + 1 reads back the results.

This approach hides the latency inherent in occlusion queries and improves performance, at the cost of slight visual glitches (an object may become visible one frame later than it should). You can read a very detailed description of this technique on Chapter 29 of GPU Gems 1, which also covers other caveats of occlusion queries.

Conditional Render

The Extension NV_conditional_render adds a major improvement to occlusion queries: it allows a simple if ( SamplesPassed > 0 ) conditional to decide whether an object should be drawn based on the result of an occlusion query.

This is probably best shown by a simple example, in the given scene there are 3 objects:

  • A huge cylinder which acts as occluder. Think of it as a pillar in the center of the "room".
  • A small cube which acts as ocludee. Think of it as a box that is anywhere in the "room" but not intersecting the pillar.
  • A small sphere which sits ontop of the cube. If the cube is fully occluded by the cylinder, drawing the sphere can be skipped.

Here is some pseudo-code how the implementation looks like.

uint MyOcculsionQuery;
public void OnLoad()
  GL.GenQueries(1, out MyOcculsionQuery);
  // etc...
  GL.Enable( EnableCap.DepthTest ); 
public void OnUnload()
  GL.DeleteQueries(1, ref MyOcculsionQuery);
  // etc...
public void OnRenderFrame()
  // The cylinder is drawn unconditionally and used as occluder for the Cube and Sphere
  // Next, the cube is drawn unconditionally, but the samples which passed the depth test are counted.
  GL.BeginQuery( QueryTarget.SamplesPassed, MyOcculsionQuery );
  GL.EndQuery( QueryTarget.SamplesPassed );
  // depending on whether any sample passed the depth test, the sphere is drawn.
  GL.NV.BeginConditionalRender( MyOcculsionQuery, NvConditionalRender.QueryWaitNv );

Although the running program might only show a single object on screen (the cylinder), the cube is always drawn too. Only drawing of the sphere might be skipped, depending on the outcome of the occlusion query used for the cube.

Please note that this is not the standard case how to use occlusion query. The most common way to use them is drawing a simple bounding volume (of a more complex object) to determine whether samples passed and only draw the complex object itself, if the bounding volume is not occluded. For example: Drawing a character with skeletal animation is usually expensive, to determine whether it should be drawn at all, a cylinder can be drawn using an occlusion query and the character is only drawn if the cylinder is not occluded.

07. Blending

Without blending, every fragment is either rejected or written to the framebuffer. That behaviour is desireable for opaque objects, but it does not allow rendering of translucent objects. The correct order of operation to draw a simple scene containing a solid table with a transparent glass ontop of it: draw the opaque table first, then enable blending (also set the desired blend equation and factors) and finally the glass is drawn.

Blending is an operation to mix the incoming fragment color (SourceColor) with the color that is currently in the color buffer (DestinationColor). This happens in two stages for each channel of the color buffer:

  1. The factors used in this stage can be controlled with GL.BlendFunc()
    The SourceColor is multiplied by the SourceFactor.
    The DestinationColor is multiplied by the DestinationFactor.
  2. The equation used in this stage can be controlled with GL.BlendEquation()
    The results of the above multiplications are then combined together to obtain the final result.

In order to use blending, the logical color buffer must have an Alpha channel. If there is no Alpha channel, or the color buffer uses color-index mode (8 Bit), no blending can occur and behaviour is the same as if blending was disabled.

Blending functionality for all draw buffers is enabled and disabled with EnableCap.Blend

GL.Enable( EnableCap.Blend );
GL.Disable( EnableCap.Blend ); // default

To enable or disable only a specific buffer if multiple color buffers are attached to the FBO, use

GL.Enable( IndexedEnableCap.Blend, index);
GL.Disable( IndexedEnableCap.Blend, index);

Where index is used to specify the draw buffer associated with the symbolic constant GL_DRAW_BUFFER(index).

The command GL.BlendEquation( mode ) specifies how the results from stage 1 are combined with each other. If you wanted to implement this with OpenTK.Math, it would look like this:

Color4 SourceColor, // incoming fragment
       DestinationColor,  // framebuffer contents
       SourceFactor, DestinationFactor, // specified by GL.BlendFunc()
       FinalColor; // the resulting color

Please note that for fixed-point color buffers both Colors are clamped to [0.0 .. 1.0] prior to computing the result. Floating-point color buffers are not clamped. Clamping into this range is left out in this code to improve legibility.

BlendEquationMode.Min: When using this parameter, SourceFactor and DestinationFactor are ignored.

FinalColor.Red = min( SourceColor.Red, DestinationColor.Red );
FinalColor.Green = min( SourceColor.Green, DestinationColor.Green );
FinalColor.Blue = min( SourceColor.Blue, DestinationColor.Blue );
FinalColor.Alpha = min( SourceColor.Alpha, DestinationColor.Alpha );

BlendEquationMode.Max: When using this parameter, SourceFactor and DestinationFactor are ignored.

FinalColor.Red = max( SourceColor.Red, DestinationColor.Red );
FinalColor.Green = max( SourceColor.Green, DestinationColor.Green );
FinalColor.Blue = max( SourceColor.Blue, DestinationColor.Blue );
FinalColor.Alpha = max( SourceColor.Alpha, DestinationColor.Alpha );

BlendEquationMode.FuncAdd: This is the default.

FinalColor.Red = SourceColor.Red*SourceFactor.Red + DestinationColor.Red*DestinationFactor.Red;
FinalColor.Green = SourceColor.Green*SourceFactor.Green + DestinationColor.Green*DestinationFactor.Green;
FinalColor.Blue = SourceColor.Blue*SourceFactor.Blue + DestinationColor.Blue*DestinationFactor.Blue;
FinalColor.Alpha = SourceColor.Alpha*SourceFactor.Alpha + DestinationColor.Alpha*DestinationFactor.Alpha;


FinalColor.Red = SourceColor.Red*SourceFactor.Red - DestinationColor.Red*DestinationFactor.Red;
FinalColor.Green = SourceColor.Green*SourceFactor.Green - DestinationColor.Green*DestinationFactor.Green;
FinalColor.Blue = SourceColor.Blue*SourceFactor.Blue - DestinationColor.Blue*DestinationFactor.Blue;
FinalColor.Alpha = SourceColor.Alpha*SourceFactor.Alpha - DestinationColor.Alpha*DestinationFactor.Alpha;


FinalColor.Red = DestinationColor.Red*DestinationFactor.Red - SourceColor.Red*SourceFactor.Red;
FinalColor.Green = DestinationColor.Green*DestinationFactor.Green - SourceColor.Green*SourceFactor.Green;
FinalColor.Blue = DestinationColor.Blue*DestinationFactor.Blue - SourceColor.Blue*SourceFactor.Blue;
FinalColor.Alpha = DestinationColor.Alpha*DestinationFactor.Alpha - SourceColor.Alpha*SourceFactor.Alpha;

If the color buffer is using fixed-point precision, the result in FinalColor is clamped to [0.0 .. 1.0] before it is passed to the next pipeline stage, no clamping occurs for floating-point color buffers.

OpenGL 2.0 and later supports separate equations for the RGB components and the Alpha component respectively. The command GL.BlendEquationSeparate( modeRGB, modeAlpha ) accepts the same parameters as GL.BlendEquation( mode ).

The command GL.BlendColor( R, G, B, A ) is used to specify a constant color that can be used by GL.BlendFunc(). For the scope of this page it is used to define the variable Color4 ConstantColor;.

The command GL.BlendFunc( src, dest ) is used to select the SourceFactor (src) and DestinationFactor (dest) in the above equation. By default, SourceFactor is set to BlendingFactorSrc.One and DestinationFactor is BlendingFactorDest.Zero, which gives the same result as if blending were disabled.

  • .Zero: Color4 (0.0, 0.0, 0.0, 0.0)
  • .One: Color4 (1.0, 1.0, 1.0, 1.0)
  • .DstColor: Color4 (DestinationColor.Red, DestinationColor.Green, DestinationColor.Blue, DestinationColor.Alpha)
  • .SrcColor: Color4 (SourceColor.Red, SourceColor.Green, SourceColor.Blue, SourceColor.Alpha)
  • .OneMinusDstColor: Color4 (1.0-DestinationColor.Red, 1.0-DestinationColor.Green, 1.0-DestinationColor.Blue, 1.0-DestinationColor.Alpha)
  • .OneMinusSrcColor: Color4 (1.0-SourceColor.Red, 1.0-SourceColor.Green, 1.0-SourceColor.Blue, 1.0-SourceColor.Alpha)
  • .SrcAlpha: Color4 (SourceColor.Alpha, SourceColor.Alpha, SourceColor.Alpha, SourceColor.Alpha)
  • .OneMinusSrcAlpha: Color4 (1.0-SourceColor.Alpha, 1.0-SourceColor.Alpha, 1.0-SourceColor.Alpha, 1.0-SourceColor.Alpha)
  • .DstAlpha: Color4 (DestinationColor.Alpha, DestinationColor.Alpha, DestinationColor.Alpha, DestinationColor.Alpha)
  • .OneMinusDstAlpha: Color4 (1.0-DestinationColor.Alpha, 1.0-DestinationColor.Alpha, 1.0-DestinationColor.Alpha, 1.0-DestinationColor.Alpha)
  • .SrcAlphaSaturate: f = min( SourceColor.Alpha, 1.0-DestinationColor.Alpha );
    Color4 ( f, f, f, 1.0 )
  • .ConstantColor: Color4 ( ConstantColor.Red, ConstantColor.Green, ConstantColor.Blue, ConstantColor.Alpha )
  • .OneMinusConstantColor: Color4 ( 1.0-ConstantColor.Red, 1.0-ConstantColor.Green, 1.0-ConstantColor.Blue, 1.0-ConstantColor.Alpha )
  • .ConstantAlpha: Color4 (ConstantColor.Alpha, ConstantColor.Alpha, ConstantColor.Alpha, ConstantColor.Alpha)
  • .OneMinusConstantAlpha: Color4 (1.0-ConstantColor.Alpha, 1.0-ConstantColor.Alpha, 1.0-ConstantColor.Alpha, 1.0-ConstantColor.Alpha)

OpenTK uses the enums BlendingFactorSrc and BlendingFactorDest to narrow down your options what is a valid parameter for src and dest. Not all parameters are valid factors for both, SourceFactor and DestinationFactor. Please refer to the inline documentation for details.

OpenGL 2.0 and later supports separate factors for RGB and Alpha, for source and destination respectively. The command GL.BlendFuncSeparate( srcRGB, dstRGB, srcAlpha, dstAlpha ) accepts the same factors as GL.BlendFunc( src, dest ).

State Queries

To determine whether blending for all draw buffers is enabled or disabled, use Result = GL.IsEnabled( EnableCap.Blend );
To query blending state of a specific draw buffer: Result = GL.IsEnabled(IndexedEnableCap.Blend, index);

The selected blend factors can be queried separately for source and destination by using GL.GetInteger() with

  • GetPName.BlendSrc - set by GL.BlendFunc()
  • GetPName.BlendDst - set by GL.BlendFunc()
  • GetPName.BlendSrcRgb - set by GL.BlendFuncSeparate()
  • GetPName.BlendSrcAlpha - set by GL.BlendFuncSeparate()
  • GetPName.BlendDstRgb - set by GL.BlendFuncSeparate()
  • GetPName.BlendDstAlpha - set by GL.BlendFuncSeparate()

The selected blend equation can be queried by using GL.GetInteger() with

  • GetPName.BlendEquation - set by GL.BlendEquation()
  • GetPName.BlendEquationRgb - set by GL.BlendEquationSeparate()
  • GetPName.BlendEquationAlpha - set by GL.BlendEquationSeparate()

08. sRGB Conversion

This stage of the pipeline is only applied if EnableCap.FramebufferSrgb is enabled and if the color encoding for the framebuffer attachment is sRGB (as in: not linear).

  • If those conditions are true, the Red, Green and Blue values after blending are converted into the non-linear sRGB color space.
  • If any of those conditions is false, no conversion is applied.

The resulting values for R, G, and B, and the unmodified Alpha form a new RGBA color value. If the color buffer is fixed-point, each component is clamped to the range [0.0 .. 1.0] and then converted to a fixed-point value. The resulting four values are sent to the subsequent dithering operation.

09. Dithering

Dithering is similar to halftoning in newspapers. Only a single color (black) is used in contrast to the paper (white), but due to using patterns the appearance of many shades of gray can be represented. In a similar way, OpenGL can dither the fragment from a high precision color to a lower precision color. I.e. dithering is used to find one or more representable colors to ensure the image shown on the screen is a best-match between the capability of the monitor and the computed image.

This is always needed when working with 8 Bit color-index mode, where only 256 unique colors can be represented, but the image to be drawn is actually calculated with higher precision. Dithering also applies when a RGBA32f color is converted to display on the screen, which is usually RGBA8. In RGBA mode, dithering is performed separately for Red, Green, Blue and Alpha.

Dithering is the only state that is enabled by default, the programmer has no control over how the image is manipulated (the graphics hardware decides which algorithm is used) besides enabling or disabling dithering with EnableCap.Dither.

GL.Enable( EnableCap.Dither ); // default
GL.Disable( EnableCap.Dither );

State Query

The state of dithering can be queried through Result = GL.IsEnabled( EnableCap.Dither );

10. Logical Operations

Before a fragment is written to the framebuffer, a logical operation is applied which uses the incoming fragment values as source (s) and/or those currently stored in the color buffer as destination (d). After the selected operation is completed, destination is overwritten. Logical operations are performed independently for each Red, Green, Blue and Alpha value and if the framebuffer has multiple color attachments, the logical operation is computed and applied separately for each color buffer.

If Logic Op is enabled, OpenGL behaves as if Blending is disabled regardless whether it was previously enabled.

In order to apply the Logicial Operation, use EnableCap.ColorLogicOp

GL.Enable( EnableCap.ColorLogicOp );
GL.Disable( EnableCap.ColorLogicOp ); // default

Note: If you use EnableCap.LogicOp or EnableCap.IndexLogicOp, only indexed color buffers (8 Bit) are affected.

To select the logical operation to be performed, use GL.LogicOp( op ); where op is by default LogicOp.Copy.

  • LogicOp.Clear: 0
  • LogicOp.And: s & d
  • LogicOp.AndReverse: s & !d
  • LogicOp.Copy: s
  • LogicOp.AndInverted: !s & d
  • LogicOp.Noop: d
  • LogicOp.Xor: s XOR d
  • LogicOp.Or: s | d
  • LogicOp.Nor: !(s | d)
  • LogicOp.Equiv: !(s XOR d)
  • LogicOp.Invert: !d
  • LogicOp.OrReverse: s | !d
  • LogicOp.CopyInverted: !s
  • LogicOp.OrInverted: !s | d
  • LogicOp.Nand: !(s & d)
  • LogicOp.Set: all 1's

State Queries
The state of LogicOp can be queried with Result = GL.IsEnabled( EnableCap.LogicOp );

Which operation has been set through GL.LogicOp() can be queried with GL.GetInteger( GetPName.LogicOpMode, ... )

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How to check if an OpenGL extension is supported

Before using an OpenGL feature, you need to check that it is supported by your GPU drivers. It is an error to call functions that belong to an unsupported feature and doing so will result in undefined behavior.

For a core feature, i.e. all functions exposed directly by the GL class, it is sufficient to check the version string of the driver against the minimum version required by the feature. The minimum version is listed in the documentation tooltip of each OpenGL function.

// Retrieve the OpenGL version string. Do this once on startup.
string version_string = GL.GetString(StringName.Version);
int major = int.Parse(version_string.Split(' ')[0]);
int minor = int.Parse(version_string.Split(' ')[1];
Version version = new Version(major, minor);
// Create Version objects for each OpenGL feature you wish to use.
// You can find the minimum version for each feature in the
// documentation tooltips or in the OpenGL specification.
// For example, GL.GenFramebuffer() says in its documentation:
// "[requires v3.0]"
static class RequiredFeatures
    public static readonly Version FramebufferObject = new Version(3, 0);
// Before using a feature, check that it supported by the OpenGL driver
if (version >= RequiredFeatures.FramebufferObject)
    int fbo = GL.GenFramebuffer();

For an extension feature, i.e. functions exposed under the nested classes GL.Arb, GL.Ext, GL.NV etc, you must check that the required extension string is advertised by the driver.

// For OpenGL 3.0 and higher
Dictionary<string, bool> extensions =
    new Dictionary<string, bool>();
int count = GL.GetInteger(GetPName.NumExtensions);
for (int i = 0; i < count; i++)
    string extension = GL.GetString(StringNameIndexed.Extensions, i);
    extensions.Add(extension, true);
// For OpenGL 2.1 and lower
Dictionary<string, bool> extensions =
    new Dictionary<string, bool>();
string extension_string = GL.GetString(StringName.Extensions);
foreach (string extension in extension_string.Split(' '))
    extensions.Add(extension, true);
// Check that an extension is supported before using it.
// The required extension string is reported in the
// documentation tooltip for each extension function.
// For example, GL.Ext.GenFramebuffer says:
// [requires: EXT_framebuffer_object]
if (extensions.ContainsKey("GL_EXT_framebuffer_object"))
    int fbo = GL.Ext.GenFramebuffer();

More information can be found in the OpenGL wiki:

How to save an OpenGL rendering to disk

You can use the following code to read back an OpenGL rendering to a System.Drawing.Bitmap. You can then use the Save() method to save this to disk.


  • Don't forget to call Dispose() on the returned Bitmap once you are done with it. Otherwise, you will run out of memory rapidly. If you wish to save a video, rather than a single screenshot, consider modifying this method to reuse the same Bitmap.
  • Call GrabScreenshot() from your main rendering thread, i.e. the thread which contains your GraphicsContext.
  • Make sure you have bound the correct framebuffer object before calling GrabScreenshot().
  • You can improve performance significantly by removing the bmp.RotateFlip() call and saving the resulting image as a BMP rather than a PNG file. This is especially important if you wish to record a video - it is the difference between a real-time recording and a slideshow.
  • This code can record 720p/30Hz video relatively easily, given suitable hardware and a little optimization (as outlined above). There are many programs that can encode a stream of consecutive BMP files into a high definition video.
using System;
using System.Drawing;
using OpenTK.Graphics;
using OpenTK.Graphics.OpenGL;
static class GraphicsHelpers
        // Returns a System.Drawing.Bitmap with the contents of the current framebuffer
        public static Bitmap GrabScreenshot()
            if (GraphicsContext.CurrentContext == null)
                throw new GraphicsContextMissingException();
            Bitmap bmp = new Bitmap(this.ClientSize.Width, this.ClientSize.Height);
            System.Drawing.Imaging.BitmapData data =
                bmp.LockBits(this.ClientRectangle, System.Drawing.Imaging.ImageLockMode.WriteOnly, System.Drawing.Imaging.PixelFormat.Format24bppRgb);
            GL.ReadPixels(0, 0, this.ClientSize.Width, this.ClientSize.Height, PixelFormat.Bgr, PixelType.UnsignedByte, data.Scan0);
            return bmp;

How to render text using OpenGL

The simplest way to render text with OpenGL is to use System.Drawing. This approach has three steps:

  1. Use Graphics.DrawString() to render text to a Bitmap.
  2. Upload the Bitmap to an OpenGL texture.
  3. Render the OpenGL texture as a fullscreen quad.

This approach is extremely efficient for text that changes infrequently, because only step 3 has to be performed every frame. Additionally, dynamic text can be reasonably efficient as long as care is taken to update only regions that are actually modified.

The downside of this approach is that (a) rendering is constrained by the capabilities of System.Drawing (i.e. poor support for complex scripts) and (b) it only supports 2d text. Moreover, care should be taken to recreate the Bitmap and OpenGL texture whenever the parent window changes size.

Sample code:

using System.Drawing;
using OpenTK.Graphics.OpenGL;
Bitmap text_bmp;
int text_texture;
window.OnLoad += (sender, e) =>
    // Create Bitmap and OpenGL texture
    text_bmp = new Bitmap(ClientSize.Width, ClientSize.Height); // match window size
    text_texture = GL.GenTexture();
    GL.TexParameter(TextureTarget.Texture2D, TextureParameterName.TextureMagFilter, (int)All.Linear);
    GL.TexParameter(TextureTarget.Texture2D, TextureParameterName.TextureMinFilter, (int)All.Linear);
    GL.TexImage2D(TextureTarget.Texture2D, 0, PixelInternalFormat.Rgba, text_bmp.Width, text_bmp.Height, 0,
        PixelFormat.Bgra, PixelType.UnsignedByte, IntPtr.Zero); // just allocate memory, so we can update efficiently using TexSubImage2D
window.Resize += (sender, e) =>
    // Ensure Bitmap and texture match window size
    text_bmp = new Bitmap(ClientSize.Width, ClientSize.Height);
    GL.TexSubImage2D(TextureTarget.Texture2D, 0, 0, 0, text_bmp.Width, text_bmp.Height,
        PixelFormat.Bgra, PixelType.UnsignedByte, IntPtr.Zero);
// Render text using System.Drawing.
// Do this only when text changes.
using (Graphics gfx = Graphics.FromImage(text_bmp))
    gfx.DrawString(...); // Draw as many strings as you need
// Upload the Bitmap to OpenGL.
// Do this only when text changes.
BitmapData data = text_bmp.LockBits(new Rectangle(0, 0, text_bmp.Width, text_bmp.Height), ImageLockMode.ReadOnly, System.Drawing.Imaging.PixelFormat.Format32bppArgb);
GL.TexImage2D(TextureTarget.Texture2D, 0, PixelInternalFormat.Rgba, Width, Height, 0,
    PixelFormat.Bgra, PixelType.UnsignedByte, data.Scan0); 
// Finally, render using a quad. 
// Do this every frame.
GL.Ortho(0, Width, Height, 0, -1, 1);
GL.BlendFunc(BlendingFactorSrc.One, BlendingFactorDst.OneMinusSourceAlpha);
GL.TexCoord(0f, 1f); GL.Vertex2(0f, 0f);
GL.TexCoord(1f, 1f); GL.Vertex2(1f, 0f);
GL.TexCoord(1f, 0f); GL.Vertex2(1f, 1f);
GL.TexCoord(0f, 0f); GL.Vertex2(0f, 1f);

This method can be easily generalized to use a more powerful text rendering library, like Pango#.