Width and height of XRQuadLayer are actually half width and half height

[mrxz]

According to the WebXR Layers API spec the width and height of an XRQuadLayer are the dimension of the layer in meters. In practice however it seems that they are "half" width and height. From the center position of the layer, the quad extends width to the left and right, and height up and down. Resulting in a 2 * width by 2 * height quad.

This can be observed with the Meta Quest Browser on Quest devices, as well as with the webxr-layers-polyfill. Not sure if there are other XR devices/browsers implementing it this way or if they implement it according to the spec.

Spec

The width and height attributes set and return the width and height of the layer in meters.

(Source: https://www.w3.org/TR/webxrlayers-1/#dom-xrquadlayer-width)

Usages

Various libraries and frameworks already account for this discrepancy when creating layers:

• Three.js: https://github.com/mrdoob/three.js/blob/dev/src/renderers/common/XRManager.js#L1523-L1524
• A-Frame: https://github.com/aframevr/aframe/blob/master/src/components/layer.js#L258-L262
• webxr-layers-polyfill: https://github.com/immersive-web/webxr-layers-polyfill/blob/main/src/gl/quad-renderer.ts#L32-L57
• WebXR Samples > Layer Samples > Layer VS Non-Layer
  • Quad layer is created with width = 1 and height = 1: https://github.com/immersive-web/webxr-samples/blob/main/layers-samples/quad-ab-test.html#L173-L174
  • Non-layer quad is created with size 2 (= 2x2): https://github.com/immersive-web/webxr-samples/blob/main/layers-samples/quad-ab-test.html#L88

Ideally the implementation(s) would be fixed so that the width and height properties match the actual dimensions. Though, given the above list, it might be easier to change the wording in the spec instead.

[svillar]

For what is worth, I'm implementing that for WebKit and I am using width and height, not double the sizes. Chromium upstream also uses those values so I suspect it's an issue in Meta's layers implementation, maybe @cabanier knows?

[cabanier]

I checked our browser code and we pass width and height through unchanged. I also checked the OpenXR spec and it says that width and height are the dimensions of the quad.
I think this means that there's a bug in Quest's OpenXR implementation :-\

At this point, there's too much content for us to work around this bug and we need to fix this by either changing the spec to match the current behavior, or introduce new attributes and implement them in the quest browser with the workaround.

[cabanier]

/agenda discuss incorrect width/height behavior

[svillar]

I checked our browser code and we pass width and height through unchanged. I also checked the OpenXR spec and it says that width and height are the dimensions of the quad.
I think this means that there's a bug in Quest's OpenXR implementation :-\

That's surprising because in Wolvic we use openxr quads for UI elements of the browser and we haven't seen any special bug related to that in Quest devices, we use the same code for all devices and the result is always the same.