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Add XRFrame.predictedDisplayTime #1217

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Oct 5, 2021
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Add XRFrame.predictedDisplayTime #1217

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12 changes: 11 additions & 1 deletion index.bs
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Original file line number Diff line number Diff line change
Expand Up @@ -990,6 +990,7 @@ When an {{XRSession}} |session| receives updated [=viewer=] state for timestamp
1. Let |now| be the [=current high resolution time=].
1. Let |frame| be |session|'s [=XRSession/animation frame=].
1. Set |frame|'s [=XRFrame/time=] to |frameTime|.
1. Set |frame|'s [predictedDisplayTime] to the timestamp when the [=XR Compositor=] is expected to display content drawn during this [=XR animation frame=].
1. For each |view| in [=XRSession/list of views=], set |view|'s [=view/viewport modifiable=] flag to true.
1. If the [=view/active=] flag of any [=view=] in the [=XRSession/list of views=] has changed since the last [=XR animation frame=], [=update the viewports=].
1. If the frame [=should be rendered=] for |session|:
Expand Down Expand Up @@ -1033,7 +1034,7 @@ window.requestAnimationFrame(onWindowAnimationFrame);

function onXRAnimationFrame(time, xrFrame) {
xrSession.requestAnimationFrame(onXRAnimationFrame);
renderFrame(time, xrFrame);
renderFrame(xrFrame.predictedDisplayTime, xrFrame);
}

function renderFrame(time, xrFrame) {
Expand Down Expand Up @@ -1075,6 +1076,7 @@ An {{XRFrame}} represents a snapshot of the state of all of the tracked objects
<pre class="idl">
[SecureContext, Exposed=Window] interface XRFrame {
[SameObject] readonly attribute XRSession session;
readonly attribute DOMHighResTimeStamp predictedDisplayTime;

XRViewerPose? getViewerPose(XRReferenceSpace referenceSpace);
XRPose? getPose(XRSpace space, XRSpace baseSpace);
Expand All @@ -1085,6 +1087,14 @@ Each {{XRFrame}} has an <dfn for="XRFrame">active</dfn> boolean which is initial

The <dfn attribute for="XRFrame">session</dfn> attribute returns the {{XRSession}} that produced the {{XRFrame}}.

The <dfn attribute for="XRFrame">predictedDisplayTime</dfn> attribute returns the {{DOMHighResTimeStamp}} that represents the point in time at which a rendered frame submitted for this {{XRFrame}} is expected to show up on the devices display.
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Maybe this is a bit clearer:
that represents the point in time at which the [=XR Compositor=] expects this {{XRFrame}} to be displayed on the [=XR Device=].

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+1


<div class=note>
The [=XRFrame/predictedDisplayTime=] is intended to allow rendering an animated XR scene in the state that it should be in when the frame is displayed rather than when the {{XRSession/requestAnimationFrame()}} callback was scheduled or when it was executed.

The [=XRFrame/predictedDisplayTime=] is not intended be used to infer how much time the application has for rendering, as the [=XR Compositor=] typically has to do extra processing after the frame is submitted. If the experience assumes that it can process up to [=XRFrame/predictedDisplayTime=], the [=XR Compositor=] will not be able to make use of the submitted frames, and the application would not make target framerate.
</div>

Each {{XRFrame}} represents the state of all tracked objects for a given <dfn for="XRFrame">time</dfn>, and either stores or is able to query concrete information about this state at the [=XRFrame/time=].

<div class="algorithm" data-algorithm="get-viewer-pose">
Expand Down