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clarify the viewport in relation to space warp #335

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toji merged 1 commit into from
Jun 12, 2026
Merged

clarify the viewport in relation to space warp #335

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12 changes: 8 additions & 4 deletions webxrlayers-1.bs
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Original file line number Diff line number Diff line change
Expand Up @@ -753,7 +753,7 @@ Rendering {#rendering}

XRSubImage {#xrsubimagetype}
----------
The {{XRSubImage}} object represents what viewport of the GPU texture to use.
The {{XRSubImage}} object represents what viewport of the color GPU texture to use.

<pre class="idl">
[Exposed=Window] interface XRSubImage {
Expand All @@ -763,7 +763,7 @@ The {{XRSubImage}} object represents what viewport of the GPU texture to use.

NOTE: this class is designed to accomodate future extensions

The <dfn attribute for="XRSubImage">viewport</dfn> attribute returns the {{XRViewport}} to use when rendering the sub image.
The <dfn attribute for="XRSubImage">viewport</dfn> attribute returns the {{XRViewport}} to use when rendering the sub image's color attachment.

XRWebGLSubImage {#xrwebglsubimagetype}
---------------
Expand All @@ -784,6 +784,7 @@ The {{XRWebGLSubImage}} object is used during rendering of the layer.
</pre>

The <dfn attribute for="XRWebGLSubImage">colorTexture</dfn> attribute returns the color [=opaque texture=] for the {{XRCompositionLayer}}.
The {{XRSubImage/viewport}} describes the region of the {{XRWebGLSubImage/colorTexture}} that corresponds to the requested view. It does not describe the region of the {{XRWebGLSubImage/depthStencilTexture}} or {{XRWebGLSubImage/motionVectorTexture}}.

The <dfn attribute for="XRWebGLSubImage">depthStencilTexture</dfn> attribute returns the depth/stencil [=opaque texture=] for the {{XRCompositionLayer}}.
If the layer was created without depth/stencil, this attribute returns null.
Expand Down Expand Up @@ -2094,12 +2095,15 @@ If "[=space-warp=]" is enabled, the [=XR Compositor=] MUST make use of depth val
When the [=feature descriptor/space-warp=] [=feature descriptor=] is enabled, the experience should render representative depth values into the {{XRWebGLSubImage/depthStencilTexture}} for each {{XRProjectionLayer}} it submits.
The {{XRWebGLSubImage/depthStencilTexture}} and {{XRWebGLSubImage/motionVectorTexture}} will have the same dimensions, which may be different from the dimensions of the {{XRWebGLSubImage/colorTexture}}.
Authors can use {{XRWebGLSubImage/depthStencilTextureWidth}} and {{XRWebGLSubImage/depthStencilTextureHeight}} to determine the dimensions of both the depth/stencil and motion vector attachments.
Authors should render depth/stencil and motion vector information to the full extent of those attachments, rather than to the {{XRSubImage/viewport}}.

NOTE: Since the depth/stencil attachment can have different dimensions than the color attachment, it is not intended to be attached to the framebuffer used to render into the {{XRWebGLSubImage/colorTexture}}.
Authors should render depth/stencil information separately at the depth/stencil attachment dimensions.
NOTE: Since the depth/stencil and motion vector attachments can have different dimensions than the color attachment, they are not intended to be attached to the framebuffer used to render into the {{XRWebGLSubImage/colorTexture}}.
Authors should render depth/stencil and motion vector information separately at their attachment dimensions.

The {{XRWebGLSubImage/motionVectorTexture}} MUST be in {{RGBA16F}} format. The author SHOULD fill in the RG components of this texture with the 2D screen-space motion vector for that area, expressed as a delta in <dfn>Normalized Device Coordinates</dfn> (<dfn>NDC</dfn>) between the current frame and the previous frame. The motion vector is computed as `(currentClipPos / currentW) - (prevClipPos / prevW)`, where `currentClipPos` and `prevClipPos` are the <dfn>clip space</dfn> positions of the fragment in the current and previous frames, respectively. The red channel corresponds to the delta in [=NDC=] X (horizontal), and the green channel corresponds to the delta in [=NDC=] Y (vertical). The blue channel MAY contain the delta in [=NDC=] Z (depth), and the alpha channel is unused.

If the {{XRWebGLSubImage/motionVectorTexture}} or {{XRWebGLSubImage/depthStencilTexture}} were not submitted during the processing of the {{XRFrame}}, the [=XR Compositor=] MUST process the {{XRFrame}} as if [=space warp=] was not enabled.

WebXR Device API Integration {#webxrintegration}
============================

Expand Down
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