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Rendering acceleration techniques

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As I know rendering acceleration techniques include view-frustum and occlusion culling, and image-based rendering such as impostors. But I''m not clear with in which conditions occlusion culling should be used or in which conditions impostors should be better used. Is it possible that occlusion culling and impostors are used together at the same time? Anybody help me out?

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Impostors and occlusion culling are two very different systems. Occlusion culling removes geometry, that is hidden by other geometry. Example: you are standing in front of a wall, with several houses behind it. The wall, as well as all houses, are in the view frustum, so normally they would all be rendered. But in the end, all the houses would simply be hidden by the wall, so their render time was wasted. Occlusion culling will avoid that, by culling away the houses.

Impostors are different. They are used on visible , but far away geometry. If you have a complex object, such as a tree, then it's a good idea to render it fully 3D when the camera is near. But the further away it is, the more depth parallax you'll lose, ie. the perspective distortion of the object becomes so small, that the human brain doesn't actually recognize it as 3D object anymore. At this point, it is useless to render the full 3D mesh. Instead, you can simply use a pre-rendered 2D texture (an impostor) with the image of the object from the side it is being viewed. It's also possible to use multiple perpendicular textures. That's a lot quicker to render than the full 3D object. But you have to be careful to switch to impostors at the right distance, otherwise it will look weird. Some people use impostors all the way through. The (unfortunately) still used 'crossed quad' trees are an exmaple. They look awful when near the camera, but their quality is pretty good from a distance.

Occlusion culling and (dynamic) impostors can and should be used at the same time.

/ Yann

[edited by - Yann L on February 1, 2003 2:26:48 PM]

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Guest Anonymous Poster
I read in other threads that you use quite a lot NURBS for the geometry in your 3D engine Yann, why is that? Shouldn''t (compiled) vertex arrays be faster? I haven''t searched for information about NURBS in 3D rendering - one think at the time - so I go for the lazy way: the forum.

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