I’ve just received an email from an user saying that he was’nt able to run the demos of the Vertex Displacement Mapping Tutorial on his brand new Radeon X1900XTX. VTF or Vertex Texture Fetching is a cool feature of high-end graphics chipsets and it’s part of Shader Model 3.0. The X1900 series is based on the R500 chipset (R580) that is a SM3.0 complient GPU. But in OpenGL side and especially in GLSL, VTF is not supported. The OpenGL query done with GL_MAX_VERTEX_TEXTURE_IMAGE_UNITS_ARB always returns 0. That means that no texture units are available in the vertex shader.

ATI confirms this fact in one of its whitepapers shipped with the ATI SDK (ATI OpenGL Programming and Optimization Guide.pdf). At the page 11, we can read this: [i]”All ATI graphics HW have a few items that deserve special consideration when using GLSL. The first major item of note is the absence of vertex texture units. This means that vertex texturing is never available, and all shaders attempting to use texture functions in the vertex shader will fail to link.”[/i]. I know, this is a rude reality. The R580 GPU is really powerful and it’s a pity that ATI does not support VTF in his chipsets. I don’t know how the R580 behaves in D3D side but I can suppose the GPU has the same limitations. VTF is currently supported by Geforce chipset from 6600 to 7900. Conclusion: if you wish to play with VTF, use a nVidia board.

Maybe, all these problems will be solved with the SM4.0. I hope! :winkhappy:

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