seems it makes sense to write shaders in cg if it supports ATI cards.
so does it support non-nvidea cards?
Cg supports bacend for generation shader in GLSL language. And ATI supports GLSL in OpenGL (it is in OpenGL core since verion 2.0). So yes - Cg supports ATI card.
Check out these threads for more details:
The last one has Ilian showing how to use cross-compile GLSL to ARB_vertex_program/ARB_fragment_program using Cg.
thanx, guys )