Originally posted by Hephasteus
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OpenGL 2.1 demands some features that are not supported by those GPUs, namely texture repeat/mirror modes for NPOT textures (implemented in R600), separate stencil (R500?) and separate blend equations (Geforce 7 series, IIRC).
The closed-source drivers for those cards advertize OpenGL 2.1 even if they don't fully support it. The reason they get away with this is that few games/apps use those unsupported features (mainly because they are not all that useful and because they are not that well-supported on those older GPUs).
DirectX 9 is slightly more in-tune with what the hardware actually supports, mainly because the spec was developed alongside the hardware. AFAIK, the only feature that's required but *not* supported is vertex texture fetch on R500-, which worked around the requirement on a technicality (the card claimed to support VTF but didn't expose any texture formats - which was was allowed by the spec in letter, if not in spirit).
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