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kackurot
10-07-2004, 09:58 PM
Does the function glGetClipPlane give back sides of the frustum for culling? I haven't seen any html pages or anybody using this function. DO anyone here know that this function can be used for cliping instead of calculating frustum culling your selves?

dorbie
10-08-2004, 11:19 AM
In *addition* to the frustum clipping planes (which clip coordinates against their w etc.). The user can specify up to six user defined clipping planes using the glClipPlane calls and glEnable(GL_CLIP_PLANE0) etc. These clip planes are used in addition to post projection 'frustum' clipping.

Frustum clipping is effectively invisible and often does not happen in implementations as you might assume. It is a theoretical thing that is often worked around with optimizations like guard band clipping and homogeneous rasterization. In addition some types of clipping are performed at the fragment level in some implementations. Including the user defined clipping.

The effects of user defined clip planes can be visible and obvious. It can slow down geometry transformation significantly (or waste texture units) depending on implementation details.