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ATI Radeon 9800 Pro vs. NVidia GeForce 5900
HardcoreGamer writes "Today ATI shipped its Radeon 9800 Pro 256 MB DDR-2 card in time for E3 and nVidia announced the NV35-based GeForce 5900 which will be available in June. Early tests seem to say that while nVidia edges ahead of ATI in specific areas, overall ATI still has the better card. The caveat is that the next generation of DirectX 9-based games (like Doom 3 and Half-Life 2, demonstrated with ATI at E3) will truly determine which is the better card. Lots of coverage at PC Magazine, PC World, The Register (ATI) (nVidia), ExtremeTech, InternetNews, and Forbes/Reuters. Either way, at $450-$500, serious gamers are about to get another serious dent in their wallets."
It's really rather quite simple. A small subset of 3D games are OpenGL games. These hardware accelerators are incidentally designed to adhere to standards defined by DirectX. They simply expose this functionality as part of their OpenGL implementation, either as vendor-specific extensions or otherwise. Doom 3 will make use of features standardized between DirectX 8 to DirectX 9 3D hardware. No one is going to enumerate every possible OpenGL extension the engine can be run with, as there're numerous render paths. They're not incorrect for using DirectX as a benchmark for functionality the engine will make use of, even if it doesn't use the API. Most 3D engines, though, actually do use DirectX. Source and Unreal both do, for instance.
ATI's drivers were given to the X crew, they didn't commit them. Check out their archives for more info.
People call games "DX9 games" because the various DirectX revisions give a rough dilineation of the different generations of graphics hardware. Roughly, they are:
DirectX 6: Software Transform and lighting. Most games from this category use lightmaps for lighting, rather than goraud(per vertex) shading.
DirectX 7: Hardware T&L. All those new T&L enabled games you heard about belong here. The opengl equivalent is calling glTranslate, glRotate, etc do to transformations, and using glLight to do lighting
DirectX 8: Vertex and Pixel Shaders. Let's you program the vertex transform and lighting part, and to a lesser extent, the pixel processing part, of the graphics pipeline. Corresponds to the OpenGL extensions NV_VERTEX_PROGRAM, NV_TEXTURE_SHADER, and NV_REGISTER_COMBINERS(for nvidia, similar extensions for ATI)
DirectX 9: Highly programmable Vertex and Pixel Shaders. The old pixel shader model let you do something like 8 operations max, while the new model greatly extends this number. OpenGL extensions are ARB_VERTEX_PROGRAM and ARB_FRAGMENT_PROGRAM.
This is really only a brief overview, there are many, many more OpenGL extensions(which you can see here, some of which have no DirectX counterparts. It's easier to tell non-graphics programmers "It's a DX9 game" than "Oh, it uses OpenGL 1.4, ARB_VERTEX_PROGRAM, ARB_TEXTURE_PROGRAM, etc", especially since DirectX is a well-known name. People generally aren't as aware of the various revisions of OpenGL(which are mainly exposed through extensions).
Doom 3 uses OpenGL for its graphics. In fact, the basic tech required is really DirectX 8 level(bump mapping and stencil buffer), but it looks better on DirectX 9 hardware(due to the higher programmability). It likely uses other Direct X APIs for sound, networking, etc on Windows.