Btw, not all fillrate-limited scenes can benefit from this optimization. It has to be fillrate-limited *and* having a lot of overdraw. It doesn''t help on, for example, strategy games (view from above without any good occluder).

quote:Original post by Yann L
You could use a lowpoly approximated mesh instead of rendering the scene geometry twice. Prefilling the zbuffer with an occlusion skin (as in occlusion culling) would probably work pretty well - early z rejection is a kind of degenerated (perpixel) occlusion culling method after all.

Idea is nice, but practically you need to ensure that low res mesh has a "smaller" projection that the high res one. Else you might reject some pixels at the edges of objects that should have been rendered.

quote:That''s what he meant by "occlusion skin".

Yes, I had understood. However, practically, I don''t see how this would help. calculating an exact outline seems too expensive to be profitable (this outline has to be computed each time the point of view is changing on the original mesh).

Maybe you can fake and render the low res version for a point of view that is a bit further that the actual one. This could be what Yann meant.

Regards,

Vincent

I think he rather meant have a low-res version of your geometry, that''s enclosed by the hi-res version. like occluders used for occlusion culling...

for instance:


see his last post here: http://www.gamedev.net/community/forums/topic.asp?topic_id=226960

Actually fill rate limited is a strong word that covers multiple cases.

Doing a Z only pass doesn''t necessary help your fill rate either. Think that not only you pay the cost in transform (which can be limited with the occlusion skinning) and you also pay the price of the extra Z pass in fill rate (which can be accelerated on some modern hardware).

What you gain is :
- if you''ve got a hardware that is able to early reject pixels based on their depth (which they could or could not depending of what you''ve drawn before or how you did draw it) then any heavy pixel shaders (think sufficiently heavy so that those 10% extra pixels rendered does cost you 5/10% extra time between two frames.) will have to be cut off early.

Generally you arrange your rendering by opaque/transparent-materials-rendertargets such that little liberty on the spatial ordering is left. The early pass does allow you to not think about spatial ordering at the cost of extra vertex transform and a little extra fillrate (Z-Only, sometimes ambiant color). All those costs needs to be compared against the cost of having textured/rendered heavy pixel shaders that won''t be present on the screen.

For eg: FarCry wouldn''t necessarily benefit in their outdoor scene. Most of the pixels on the screen in some jungle scene are alpha/tested/blended plants billboards that do not benefit on some hardware of the early rejection. If a structure is entirely occluded by alpha blended objects, then the cost of drawing/transforming/texturing/shading this structure is lost). Alpha testing instead of alpha blending doesn''t necessarily helps because alpha tested polygons are bad occluders (don''t include them in your Z only pass).

Stencil rendering adds yet another dimension to the problem.

Everything has to be pondered and of course tested cause all hardwares do not react the same.