Re: Why did NVIDIA skip the revision of Direct X 10.01 on their 9800 GTX/GX2/GTX 260/
NVidia and ATI GPUs have their own featuresets. And one could argue that NV GPUs has more features based on the number of functionalities exposed through their OpenGL extensions. However, it seems that for some reason ATI successfully manage to lobby MS to include in DX 10.1 some features which they are better prepared to implement than NVidia. DirectX is Microsoft's proprietary, so nothing can stop them from giving one IHV a favor over another. And Microsoft may have some good reasons to favor ATI in this regard. Think about how much ATI did to help them making DirectX and Windows OS the only viable gaming platform on PC by impairing OpenGL in their drivers troughout history. So, no, DX 10.1 is not in anyway an "equal ground" to start with as it only benefits ATI. NVidia supporting DX 10.1, at least at this time, is as much as illogical as ATI supporting CUDA from a business perspective. The sales of their G92 GPUs and its derivatives currently in stock and production could be instantly killed if they do so.
If the devs are so willing to give you 10% extra performance due to multi sample read-back capability (as DX 10.1 offer) then it can also be done on NV hardware via custom extension. But why would one wanted to support three different APIs (DX 9, 10, 10.1) with all the coding and maintenance nightmare when they could choose to support just two of them and spend their effort more on the game's stability, efficiency and features for much more broader audience?
There could be more to it in regard of GT2XX though. Believe it or not GT2XX is an relatively older technology than G92. It designed around the same time with G80 or slightly later. (Hence they're using very similar boards.) Back then DX 10.1 hasn't been specified yet. There were rumors/plans that NV was going to release it at late 2007, but it was cancelled. Probably because the lack of competition from ATI (at that time) doesn't justify its production cost.