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 
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 
coordinates.
If the depth buffer test fails,
the incoming fragment is discarded.
The stencil value at the fragment's 
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 
location is set to the fragment's 
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.