HP c8000 manual Enumerated Types for Occlusion

overview of OpenGL

OpenGL extensions

	if (result) {
	glColorMask(GL_TRUE, GL_TRUE, GL_TRUE);
	glDepthMask(GL_TRUE);
	renderPart(i);
	glDepthMask(GL_FALSE);
	glColorMask (GL_FALSE, GL_FALSE, GL_FALSE);
	}
	}
	/* Disable Occlusion Culling test */
	glDisable(GL_OCCLUSION_TEST_HP);
	/* Turn on writes to depth and color buffers */
	glColorMask(GL_TRUE, GL_TRUE, GL_TRUE);
	glDepthMask(GL_TRUE);
	The key idea behind occlusion culling is that the bounding box is much
	simpler (i.e., fewer vertices) than the part itself. Occlusion culling
	provides a quick means to test non-visibility of a part by testing its
	bounding box.
		It should also be noted that this occlusion culling functionality is very
	useful for viewing frustum culling. If a part’s bounding box is not visible
	for any reason (not just because it’s occluded in the Z-buffer) this test will
	give correct results.
	To maximize the probability that an object is occluded by other objects in
	a scene, the database should be sorted and rendered from front to back.
	Also, the database may be sorted hierarchically such that the outer
	objects are rendered first and the inner are rendered last. An example
	would be rendering the body of an automobile first and the engine and
	transmission last. In this way, the engine would not be rendered due to
	the bounding box test indicating that the engine is not visible.
Table 1-9	Enumerated Types for Occlusion
Extended Area		Enumerated Type	Description
Enable.Disable/IsEnabled		GL_OCCLUSION_TEST_HP	pname variable.
		Default: Disabled
Get*		GL_OCCLUSION_TEST_RESULT_HP	pname variable
		Default: Zero (0)
	For related information, see the functions: glGet, glEnable, glDisable,
	and glIsEnabled.

20 Chapter 1