OpenGL 4.1 (Compatibility Profile) - July 25, 2010

4.1. PER-FRAGMENT OPERATIONS 358
functions are both ALWAYS, and the front and back stencil mask are both set to the
value 2 s − 1, where s is greater than or equal to the number of bits in the deepest
stencil buffer supported by the GL implementation. Initially, all three front and
back stencil operations are KEEP.
If there is no stencil buffer, no stencil modification can occur, and it is as if the
stencil tests always pass, regardless of any calls to StencilFunc.
4.1.6 Depth Buffer Test
The depth buffer test discards the incoming fragment if a depth comparison fails.
The comparison is enabled or disabled with the generic Enable and Disable commands
using the symbolic constant DEPTH_TEST. When disabled, the depth comparison
and subsequent possible updates to the depth buffer value are bypassed and
the fragment is passed to the next operation. The stencil value, however, is modified
as indicated below as if the depth buffer test passed. If enabled, the comparison
takes place and the depth buffer and stencil value may subsequently be modified.
The comparison is specified with
void DepthFunc( enum func );
This command takes a single symbolic constant: one of NEVER, ALWAYS, LESS,
LEQUAL, EQUAL, GREATER, GEQUAL, NOTEQUAL. Accordingly, the depth buffer
test passes never, always, if the incoming fragment’s z w value is less than, less
than or equal to, equal to, greater than, greater than or equal to, or not equal to
the depth value stored at the location given by the incoming fragment’s (x w , y w )
coordinates.
If depth clamping (see section 2.23) is enabled, before the incoming fragment’s
z w is compared z w is clamped to the range [min(n, f), max(n, f)], where n and f
are the current near and far depth range values (see section 2.17.1)
If the depth buffer test fails, the incoming fragment is discarded. The stencil
value at the fragment’s (x w , y w ) coordinates is updated according to the function
currently in effect for depth buffer test failure. Otherwise, the fragment continues
to the next operation and the value of the depth buffer at the fragment’s (x w , y w )
location is set to the fragment’s z w value. In this case the stencil value is updated
according to the function currently in effect for depth buffer test success.
The necessary state is an eight-valued integer and a single bit indicating
whether depth buffering is enabled or disabled. In the initial state the function
is LESS and the test is disabled.
If there is no depth buffer, it is as if the depth buffer test always passes.
OpenGL 4.1 (Compatibility Profile) - July 25, 2010