Allow depth direction preference option.

[AdaRoseCannon]

In WebGPU it is possible to format your depth buffer in reverse Z for an improvement of depth fidelity.

Many WebGL experiences today use forward-z. For Safari on VisionOS we need to convert forward-z buffers to reverse-z for os compatibility which is expensive.

It would be good for the Session to be able to indicate to Websites that the session prefers reverse Z and for the Website to indicate that it has indeed provided reverse-Z. This would allow an additional performance improvement for WebGPU experiences.

[toji]

As far as the site indicating that it's using reverse depth goes, that should already be supported via setting depthNear to a greater number than depthFar. See the RenderState section in the spec:

depthNear and depthFar are used in the computation of the projectionMatrix of XRViews. ... They also determine how the values of an XRWebGLLayer depth buffer are interpreted. depthNear MAY be greater than depthFar.

That allowance for near and far to be reversed was added specifically to enable reverse-Z rendering.

It should be noted that the reason that reverse-Z rendering is preferred in many cases doesn't have anything to do with performance, but instead is about more evenly distributing depth precision across the full range of the depth buffer, which can help quite a bit with z fighting in distant geometry. I've always liked how this post visualizes the benefit.

That page also demonstrates why the technique is not used very often with WebGL. The [-1, 1] depth range used by OpenGL/WebGL's clip volume counteracts many of the benefits of a reversed Z buffer, bunching up most of the precision in the middle of the range. This is probably why almost nobody has bothered to use it for WebXR content to date. For the same reason I'm a bit hesitant to think that a property advertised from by the API as to which method is preferred with be very impactful. Most WebGL-based content will probably continue to be forward-Z, and I suspect a great deal of WebGPU content will be reverse-Z, especially if we can advocate for it as a best practice. Implementations will have to deal with either regardless, since we've been advertising this as a capability of the API since day 1.

[AdaRoseCannon]

I think what would be really useful when we start making samples for WebXR+WebGPU we include ones for both depth types.

[klausw]

I think there may have been a misunderstanding here. Brandon is right that reverse Z by itself does not provide a performance benefit, but Ada said that Vision OS requires reverse Z, so if an app uses non-reversed Z the WebXR runtime will need to do an expensive conversion of the depth data every frame. Providing a hint that reverse Z is more efficient could be useful for that scenario if developers could be made aware of that property. (Though I see the argument that it may be easier to just convince everyone to use reverse Z for WebGPU in general to also gain the benefit of the improved depth precision.)

Is there any implementation where reverse Z has a performance cost due to requiring extra operations, basically the inverse of the Vision OS siruation? That seems unlikely, but if it does exist it would be an argument for exposing such a runtime preference.

On Thu, Aug 22, 2024, 00:55 Ada Rose Cannon @.***> wrote:

I think what would be really useful when we start making samples for WebXR+WebGPU we include ones for both depth types.

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[AdaRoseCannon]

Is there any implementation where reverse Z has a performance cost due to requiring extra operations, basically the inverse of the Vision OS siruation? That seems unlikely, but if it does exist it would be an argument for exposing such a runtime preference.

I would be really interested in this. As much as I would love for there to be a developer hint I feel developers would probably pick their depth direction early on in the project for whatever is appropriate for their content and then probably be unwilling to change it on the fly since it will likely expose unexpected depth issues. Or if it's something that is handled at the library level then a display which differs from the status-quo with regards to this hint will see experiences not work simply because the developers didn't test their depth both ways.

If it does seem unlikely that there will be a performance impact for other platforms it would be great if reverse-z was the default for WebXR+WebGPU.