overview of OpenGL
OpenGL extensions
Chapter 1 19
OpenGL extensions
The extensions listed in this section are extensions that Hewlett-Packard
has created; that is, in addition to those standard functions described in
the OpenGL Programming Guide, OpenGL Reference Manual, and
OpenGL Programming for the X Window System.
visibility test extensions
HP supports extensions for visibility testing and occlusion culling.
See the on-line Reference Manual for information on HP’s visibility test
extensions, glVisibilityBufferHP and glNextVisibilityTestHP.
occlusion extension
This occlusion culling extension defines a mechanism whereby an
application can determine the non-visibility of some set of geometry
based on whether an encompassing set of geometry is non-visible. In
general, this feature does not guarantee that the target geometry is
visible when the test fails, but is accurate with regard to non-visibility.
Typical usage of this feature would include testing the bounding boxes of
complex objects for visibility. If the bounding box is not visible, then it is
known that the object is not visible and need not be rendered.
occlusion culling code fragments
The following is a sample code segment that shows a simple usage of
occlusion culling.
/* Turn off writes to depth and color buffers */
glDepthMask(GL_FALSE);
glColorMask (GL_FALSE, GL_FALSE, GL_FALSE);
/* Enable Occlusion Culling test */
glEnable(GL_OCCLUSION_TEST_HP);
for (i=0; i < numParts; i++) {
/* Render your favorite bounding box */
renderBoundingBox(i);
/* If bounding box is visible, render part */
glGetBooleanv(GL_OCCLUSION_RESULT_HP, &result);
OGLImp.book Page 19 Tuesday, February 14, 2006 9:15 AM
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