We’ve worked closely with Unity to enable Asymmetric Field of View (FOV) on Rift starting from Unity 2018.1.b5. Asymmetric Field of View rendering is a more efficient way to render VR eye textures, about 10% improvement in our tests. Over the next few months we’ll be moving to make Asymmetric Field of View rendering the default in Unity, and this post is intended to answer common questions about the technique.
1. What is Asymmetric FOV?
3D scenes are usually rendered using a virtual camera with a symmetric field of view. The camera represents a projection from 3D space to 2D space (to be displayed on a screen), which is defined by a horizontal FOV and a vertical FOV. Unity exposes these values as a vertical FOV value and an aspect ratio, from which the horizontal FOV of the projection can be computed. Importantly, this method assumes that the center of the screen is also the center of the projection, and that the distance from the center to each side is equal along each axis:
In an asymmetric FOV system, each of the four sides of the projection are placed independently from each other. This allows for a lopsided projection that can also be offset from the center of the render target.
Though a symmetric FOV can be expressed in just two values, an asymmetric projection requires four parameters to control Field of View in your projection matrix (for example, LeftFov/RightFov/TopFov/DownFov).
2. Why use Asymmetric FOV on Oculus Rift?
VR headsets can benefit from an asymmetric field of view because the center point of the lenses is not always the center point of the screen. By controlling the center of the projection (and matching it to the lens), we can produce an image that maximizes the visible field of view and minimizes artifacts like aliasing and aberration. In particular, there is more pixel data available to the lower and outer parts of the visible field.
What actually happens when we render with a symmetric field of view is that the GPU must fill many more pixels than can actually be seen. Because the lens itself is not symmetric, a large render target must be used to ensure that the whole visible FOV is covered. This yields considerable waste—much of the render target is unused. Up until now we've cut the cost of these unused pixels using an occlusion mesh, but with an asymmetric FOV we can avoid rendering extra pixels altogether. On Oculus Rift, symmetric FOV rendering requires a 1535x1776 target texture per eye. Using an asymmetric FOV we can cut that size to 1344x1600 per eye, a 22% reduction in filled pixels with no quality degradation. This method also yields improvements in areas that an occlusion mesh cannot, such as post-process, blitting, and resolve costs.
Though most of this savings is GPU-side, asymmetric FOV also saves us some CPU in the form of a tighter frustum for culling, which can result in fewer draw calls.
3. Ok, I’m switching from Symmetric FOV to Asymmetric FOV, are there any visual detriments to Asymmetric FOV?
No. Asymmetric FOV does not change visible pixels on the display, nor the pixel density of the projection. It allows us to cut pixels that were already out of the viewer's field of view, so there should be no visible change compared to the symmetric approach.
4. Is this a guaranteed performance improvement?
No. Asymmetric FOV produces better culling, requires less bandwidth, and results in rendering fewer pixels. However, an app that is not bottlenecked on its fragment shaders or culling cost will not see any benefit. If your GPU has free cycles using the symmetric FOV approach, further reducing its load is not going to yield a performance improvement.
That said, in our tests we've seen an 8% - 10% performance gain on average for many applications. Post-process effects and complex pixel shaders are common sources for GPU overhead in VR, so this is an optimization that will benefit many titles.
5. My image effects stopped working!
Switching to Asymmetric FOV mostly works “out of the box.” But there are a few side-effects, especially for image effects. This is usually caused differences in the way screen coordinates are projected by an Asymmetric FOV.
Most issues come down to two specific differences:
First, the size of the allocated eye buffer size and the size of the viewport have changed. This is necessary to achieve the same visible pixel density (1 texel per display pixel on the Rift):
This can affect effects that expect to map specific UV values to pixels.
Second, the projection center may differ: Whether you are using Asymmetric or Symmetric FOV, the Normalized Device Coordinate (NDC) space is always [-1, -1, 0] ->[1, 1, 1] (per DirectX conventions) and the screen center is always at [0, 0, depth]. However, under Asymmetric FOV the projection center is no longer the same as the screen center, and an offset is used. This makes the asymmetric projection matrix an off-centered projection matrix.
To reconcile this issue you need to decide where to align the center of your image effect. If you want it at the viewport center, then using NDC at [0, 0, depth] is fine. If you want the center to be at the projection center, this NDC space center is no longer sufficient.
Fortunately, computing the projection center of an asymmetric matrix is easy: ProjectionMatrix * (0, 0, zNear, 1). Unity follows OpenGL conventions, so use (0, 0, -zNear, 1).
Deciding where to put the center of your image effect is a concern specific to each effect. The vignette effect below provides an example. Aligning the vignette with the projection center makes sense (as the projection center is what you see when you look straight ahead), but a different effect might need to align with the screen center instead.
We can use this example to show what an image effect that needs to be fixed for Asymmetric FOV looks like. Before Asymmetric FOV, the projected center is at the screen center. If you look straight, you see the brightest pixel. After Asymmetric FOV is enabled, the projection center is no longer the brightest pixel, and without adjustment the shader effect will look wrong.