This option renders each eye using a single viewport, which gives it the highest image quality for peripheral regions of the frame. It also makes it the most expensive in terms of GPU fill-rate. This option should only be used with an extremely high-end GPU.
In order to achieve the high resolutions needed for VR, we take advantage of part of the human eye’s physiology, the fovea. This part of the eye is responsible for the sharp central vision we see and so we can focus our pixel density in a fixed central area of the screen, allowing the rest of the screen to be rendered at a lower resolution. The fixed foveated option of multi-resolution rendering does this by dividing the screen into two parts: a central circular high resolution region, and an outer lower resolution region. By doing this, we can achieve 90fps at foveated ‘virtual resolutions’ above the native resolution of the headset’s LCD screen as described in Pixel Density section above.
This option enables NVIDIA’s Multi-Res Shading, part of the NVIDIA Gameworks platform. Similar to fixed-foveated rendering, it divides the frame into areas of low-resolution and high-resolution. The frame is divided into a 3x3 grid, as shown in the image below. The central cell is rendered at full resolution, whilst the remaining cells are rendered at 30% of the full resolution.
Unlike fixed-foveated rendering, this method only needs to draw geometry once per eye, with the hardware relaying it to each of the Multi-Res viewports. It may offer better performance than fixed-foveated rendering in cases where the title is draw-call bound.
The percentage value relates to the native resolution of the LCD screen where 100% on Vive and Oculus headsets means a resolution of 2160x1200. Due to the lenses on the headset, a percentage resolution of 140% is required to give a 1:1 pixel resolution at the center of the screen. 140% is a resolution of 3024x1680 ((2160x1200)*1.4). We allow the user to double this 140% resolution to 280% which is a resolution of 6048x3360.
This allows the user to change the size of the central circular region that is rendered at the resolution defined by the ‘Pixel Density’ option. We have three settings: Small, Medium and Large, which relate to the central circular high resolution region’s diameter as a ratio of the screen width.
Outer Pixel Quality
This allows the user to change resolution of the outer regions resolution as a percentage of the value in Pixel Density. This ranges from 30% to 90% (100% is achieved by using the ‘Full Resolution’ option of Multi-Resolution Rendering). If the Pixel Density is set to 100% and the Outer Pixel Quality is set to 90%, then the inner central high resolution area will be rendered at 2160x1200 and the outer resolution will be rendered at 90% of that resolution at 1944x1080 ((2160x1200)*0.9).