glDepthFunc -	specify	the value used for depth buffer
comparisons

glDepthFunc
	glDepthFunc(func) -> None

void glDepthFunc( GLenum func	)

func	Specifies the depth comparison function.  Symbolic
      constants GL_NEVER, GL_LESS, GL_EQUAL, GL_LEQUAL,
      GL_GREATER, GL_NOTEQUAL, GL_GEQUAL, and	GL_ALWAYS are
      accepted.  The initial value is	GL_LESS.

glDepthFunc specifies	the function used to compare each
incoming pixel depth value with the depth value present in
the depth buffer.  The comparison is performed only if depth
testing is enabled.  (See glEnable and glDisable of
GL_DEPTH_TEST.)

func specifies the conditions	under which the	pixel will be
drawn.  The comparison functions are as follows:

GL_NEVER	  Never	passes.

GL_LESS	  Passes if the	incoming depth value is	less
                than the stored depth	value.

GL_EQUAL	  Passes if the	incoming depth value is	equal
                to the stored	depth value.

GL_LEQUAL	  Passes if the	incoming depth value is	less
                than or equal	to the stored depth value.

GL_GREATER	  Passes if the	incoming depth value is
                greater than the stored depth	value.

GL_NOTEQUAL	  Passes if the	incoming depth value is	not
                equal	to the stored depth value.

GL_GEQUAL	  Passes if the	incoming depth value is
                greater than or equal	to the stored depth
                value.

GL_ALWAYS	  Always passes.

The initial value of func is GL_LESS.	 Initially, depth
testing is disabled.	Even if	the depth buffer exists	and
the depth mask is non-zero, the depth	buffer is not updated
if the depth test is disabled.

GL_INVALID_ENUM is generated if func is not an accepted
value.

GL_INVALID_OPERATION is generated if glDepthFunc is executed
between the execution	of glBegin and the corresponding
execution of glEnd.

glGet	with argument GL_DEPTH_FUNC
glIsEnabled with argument GL_DEPTH_TEST

glDepthRange,	glEnable, glPolygonOffset