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
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.
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.
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