Difference Between DX9 and OpenGl

[Bigpet]

Driver support on Windows can be really spotty, so some people have better performance with D3D and some with OpenGL. There also may still be some bugs that make things look different. Of course there's also various tools that hook into Direct3D or OpenGL.

Also, the postprocessing shaders aren't translated to hlsl, so you can't run them in D3D