Bug 1582294 - Set up HTMLMediaElement to convey video track alpha channel when playing a MediaStream. r=bryce

Differential Revision: https://phabricator.services.mozilla.com/D92898

dom/html/HTMLMediaElement.cpp

@@ -2660,6 +2660,9 @@ void HTMLMediaElement::NotifyMediaTrackEnabled(dom::MediaTrack* aTrack) {
       if (mSecondaryMediaStreamRenderer) {
         mSecondaryMediaStreamRenderer->AddTrack(mSelectedVideoStreamTrack);
       }
+      if (mMediaInfo.HasVideo()) {
+        mMediaInfo.mVideo.SetAlpha(mSelectedVideoStreamTrack->HasAlpha());
+      }
       nsContentUtils::CombineResourcePrincipals(
           &mSrcStreamVideoPrincipal, mSelectedVideoStreamTrack->GetPrincipal());
     }
@@ -5825,6 +5828,9 @@ void HTMLMediaElement::UpdateReadyStateInternal() {
     }
     if (hasVideoTracks) {
       mediaInfo.EnableVideo();
+      if (mSelectedVideoStreamTrack) {
+        mediaInfo.mVideo.SetAlpha(mSelectedVideoStreamTrack->HasAlpha());
+      }
     }
     MetadataLoaded(&mediaInfo, nullptr);
   }

dom/media/MediaStreamTrack.h

@@ -141,6 +141,12 @@ class MediaStreamTrackSource : public nsISupports {
    */
   void GetLabel(nsAString& aLabel) { aLabel.Assign(mLabel); }
 
+  /**
+   * Whether this TrackSource provides video frames with an alpha channel. Only
+   * applies to video sources. Used by HTMLVideoElement.
+   */
+  virtual bool HasAlpha() const { return false; }
+
   /**
    * Forwards a photo request to backends that support it. Other backends return
    * NS_ERROR_NOT_IMPLEMENTED to indicate that a MediaTrackGraph-based fallback

dom/media/VideoStreamTrack.h

@@ -35,6 +35,11 @@ class VideoStreamTrack : public MediaStreamTrack {
   void RemoveVideoOutput(VideoFrameContainer* aSink);
   void RemoveVideoOutput(VideoOutput* aOutput);
 
+  /**
+   * Whether this VideoStreamTrack's video frames will have an alpha channel.
+   */
+  bool HasAlpha() const { return GetSource().HasAlpha(); }
+
   // WebIDL
   void GetKind(nsAString& aKind) override { aKind.AssignLiteral("video"); }