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Texturelookups in the vertex unit and software vertexprocessing

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To use texture lookups in the vertex unit, you need a "fully" SM3 capable hardware or a hotfix, like ATi's "Render to Vertexbuffer" - R2VB. However, the transformation stage of the programmable pipeline can be executed on the cpu, by enabling software vertexprocessing. One of the reasons that I need vertex-based texturelookups is that I've planned to write some vertex morph-data into a texture instead of bloating my vertexformat and use multiple streams. What kind of perfomance loss can I expect using software vertexprocessing instead of hardware vertexprocessing for vertex-based texture lookups?

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I wouldn't want to hazard a guess, but this is the sort of thing that is far easier to profile by experimantation than to esitmate. Make the necessary changes to your code and see what happens to your framerate.

Be sure to let us know how it turns out, of course.

Regards
Admiral

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SWVP requires the vertex texture to be in D3DPOOL_SCRATCH so you shouldn't incur any horrifically expensive CPU-samples-VRAM situations. Then again, it does mean you'll have problems getting the data into the samplers - either having to keep two copies of static data or streaming back render targets to SysMem backing stores.

Note that the filtering/formats for SWVP's Vertex Texturing seem much lighter than what I've read about for Nvidia's SM3 hardware (supports only one format and point filtering iirc)...

But as TheAdmiral says - it'll be difficult to say for definite and you'll be better off testing it and seeing.

hth
Jack

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