Merge pull request #1217 from Squareys/predicted-display-time

cabanier · web-flow · commit cc6b625d63d9 · 2021-10-05T12:31:19.000-07:00
Add XRFrame.predictedDisplayTime
diff --git a/index.bs b/index.bs
@@ -1052,6 +1052,7 @@ 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|:
@@ -1095,7 +1096,7 @@ window.requestAnimationFrame(onWindowAnimationFrame);
 
 function onXRAnimationFrame(time, xrFrame) {
   xrSession.requestAnimationFrame(onXRAnimationFrame);
-  renderFrame(time, xrFrame);
+  renderFrame(xrFrame.predictedDisplayTime, xrFrame);
 }
 
 function renderFrame(time, xrFrame) {
@@ -1137,6 +1138,7 @@ 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);
@@ -1147,6 +1149,14 @@ 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.
+
+<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">
