why OpenGL?

Obviously cross-platform. Not everyone is using Windows.

"Cgpnz" wrote:
Other than non-windows platforms

Ignoring OSX would be stupid on the part of Oculus.

"Cgpnz" wrote:
yet really the VR 90 hertz presence premise necessitates newest PC+GPUs

Why? What is it about Linux or OSX that makes them unable to drive a 90 Hz display? The performance is basically down to the rendering engine and the hardware, and on comparable hardware, you should be able to render the same scene, regardless of whether you're running Linux or OSX or Windows.

In fact, in many ways the Linux OS makes it easier to deal with VR. Unlike the custom display driver required on Windows to enable 'Direct HMD' mode, a Linux box can simply assign the HMD to a distinct X screen, basically providing all the benefits of Direct HMD without a custom driver.

"Cgpnz" wrote:
why are developers bothering with openGL. It is well known that it has certain state-change related issues that do not make it competitive with native DirectX.

I'm sorry, where do you get that it's not competitive with Direct3D? First off, Direct3D also has internal state and has costs associated with state changes, and guides on performance tuning D3D will still tell you to minimize state changes. Modern OpenGL and modern Direct3D are pretty competitive both in terms of feature set and performance.

"Cgpnz" wrote:
Are your API's too entwined with it?

You can browse the SDK source code and see this isn't the case. The bulk of the SDK render-api neutral. Even in the CAPI implementation, it's entirely possible to avoid any code paths that use OpenGL or Direct3D (on the SDK side) by doing client side rendering.

"Cgpnz" wrote:
why are developers bothering with openGL. It is well known that it has certain state-change related issues that do not make it competitive with native DirectX.

Valve 2012:

After this work, Left 4 Dead 2 is running at 315 FPS on Linux. That the Linux version runs faster than the Windows version (270.6) seems a little counter-intuitive, given the greater amount of time we have spent on the Windows version. However, it does speak to the underlying efficiency of the kernel and OpenGL.

http://blogs.valvesoftware.com/linux/faster-zombies/

After this work, Left 4 Dead 2 is running at 315 FPS on Linux. That the Linux version runs faster than the Windows version (270.6) seems a little counter-intuitive, given the greater amount of time we have spent on the Windows version. However, it does speak to the underlying efficiency of the kernel and OpenGL.

the original L4D2 runs on DX9.0c (sm3), which is somewhat equivalent to OpenGL 2.0-2.1
now if valve uses any optimization techniques enabled by newer opengl versions, the fps increase is not that surprising.

DX11 (and opengl 4), when used right, can provide a lots of performance improvements compared to DX9 (opengl 2).

You could find an example here: http://federico-mammano.github.io/Oculus-SDK-0.4.4-beta-OpenGL-Demo
I hope you find it useful.