clarify spacewarp and mark as no longer unstable

Author: cabanier

webxrlayers-1.bs

@@ -502,18 +502,7 @@ The <dfn attribute for="XRProjectionLayer">ignoreDepthValues</dfn> attribute, if
 is <code>false</code> it indicates that the content of the depth buffer attachment will be used by the
 [=XR Compositor=] and is expected to be representative of the scene rendered into the layer.
 
-<div class="unstable">
-
- The <dfn attribute for="XRProjectionLayer">deltaPose</dfn> attribute controls the amount of foveation used by the
-[=XR Compositor=]. If the user agent or device does not support this attribute or the
-"[=feature descriptor/space-warp=]" [=feature descriptor=] was not requested, they
-should return <code>null</code> on getting, and setting should be a <code>no-op</code>. This attribute describes the
-incremental application-applied transform, if any, since the previous frame that affects the view. When artificial
-locomotion happens, the experience transforms the whole world from one application pose to another pose between frames
-and {{XRProjectionLayer/deltaPose}} MUST be populated with the difference in the scene's world position.
-The {{XRProjectionLayer/deltaPose}} should be identity when there is no transformation between the previous and the current {{XRFrame}}.
-
-</div>
+The <dfn attribute for="XRProjectionLayer">deltaPose</dfn> attribute describes the incremental application-applied transform, if any, since the previous frame that affects the view. If the user agent or device does not support this attribute or the "[=feature descriptor/space-warp=]" [=feature descriptor=] was not requested, they should return <code>null</code> on getting, and setting should be a <code>no-op</code>. When artificial locomotion happens, the experience transforms the whole world from one application pose to another pose between frames and {{XRProjectionLayer/deltaPose}} MUST be populated with the difference in the scene's world position. The {{XRProjectionLayer/deltaPose}} should be identity when there is no transformation between the previous and the current {{XRFrame}}.
 
 XRQuadLayer {#xrquadlayertype}
 -----------
@@ -796,30 +785,22 @@ The <dfn attribute for="XRWebGLSubImage">colorTexture</dfn> attribute returns th
 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.
 
-<div class="unstable">
-
 The <dfn attribute for="XRWebGLSubImage">motionVectorTexture</dfn> attribute returns the motion [=opaque texture=] for the {{XRProjectionLayer}}.
 If the {{XRSession}} was created without the [=feature descriptor/space-warp=] [=feature descriptor=] or the layer is not a {{XRProjectionLayer}}, this attribute MUST return null.
 
-</div>
-
 The <dfn attribute for="XRWebGLSubImage">imageIndex</dfn> attribute returns the offset into the texture array. Valid only for layers
 that were requested with {{texture-array}}.
 
 The <dfn attribute for="XRWebGLSubImage">colorTextureWidth</dfn> and <dfn attribute for="XRWebGLSubImage">depthStencilTextureHeight</dfn> attributes
 return the width and height in pixels of the GL depth attachments
 
-<div class="unstable">
-
 The <dfn attribute for="XRWebGLSubImage">depthStencilTextureWidth</dfn> and <dfn attribute for="XRWebGLSubImage">colorTextureHeight</dfn> attributes
 return the width and height in pixels of the GL color attachments. If the layer was created without depth/stencil, this attribute returns null.
 
 The <dfn attribute for="XRWebGLSubImage">motionVectorTextureWidth</dfn> and <dfn attribute for="XRWebGLSubImage">motionVectorTextureHeight</dfn> attributes
 return the width and height in pixels of the GL motion vector attachments.
 If the {{XRSession}} was created without the [=feature descriptor/space-warp=] [=feature descriptor=] or the layer is not a {{XRProjectionLayer}}, these attributes MUST return null.
 
-</div>
-
 XRTextureType {#xrtexturetype}
 -------------
 The {{XRTextureType}} enum defines what type of texture is allocated.
@@ -2097,18 +2078,17 @@ Other layers types MUST continue to be sorted as before.
 
 Space warp {#spacewarp}
 ==========
-<div class="unstable">
 
 <dfn>Space warp</dfn> is a technology that improves the [=XR Compositor=]'s reprojection.
 By submitting a {{XRWebGLSubImage/motionVectorTexture}} along with a {{XRWebGLSubImage/depthStencilTexture}}
-the [=XR Compositor=] can do high quality frame extrapolation and reprojection which allows the user agent to run at a reduced framerate but still provide a smooth experience to users.
+the [=XR Compositor=] can do high quality frame extrapolation and reprojection which allows the user agent to run at a reduced framerate but still provide a smooth experience to users. The rate at which {{XRSession/requestAnimationFrame()}} callbacks are delivered may be lower than the display's native refresh rate. The [=XR Compositor=] will synthesize the missing frames using the motion vectors and depth information provided by the experience.
 
 To enable [=space warp=] the {{XRSession}} MUST be created with the "<dfn for="feature descriptor">space-warp</dfn>" [=feature descriptor=].
 If "[=space-warp=]" is enabled, the [=XR Compositor=] MUST make use of depth values, {{XRProjectionLayer/ignoreDepthValues}} MUST be set to `false` and {{XRWebGLBinding/usesDepthValues}} MUST be set to `true`.
 
-The {{XRWebGLSubImage/motionVectorTexture}} MUST be in {{RGBA16F}} format. The author SHOULD fill in the RGB components of this texture with the speed in meters per second of that area with the red pixel corresponding with the x axis, green the y axis and blue the z axis.
+When the [=feature descriptor/space-warp=] [=feature descriptor=] is enabled, the {{XRWebGLSubImage/depthStencilTexture}} for an {{XRProjectionLayer}} MUST be populated by the experience. The {{XRWebGLSubImage/depthStencilTexture}} and {{XRWebGLSubImage/motionVectorTexture}} will have the same dimensions, which may be different from the dimensions of the {{XRWebGLSubImage/colorTexture}}.
 
-</div>
+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.
 
 WebXR Device API Integration {#webxrintegration}
 ============================