I noticed that a small video near the top of the page on cnn.com causes us to continue compositing after the video has been scrolled out of view. This is bad for battery on mobile.

STR:
(1) Enable the WR profiler overlay (compact is fine).
(2) Load http://fiddle.jshell.net/ovdjseLm/show/light/ , click play.
(3) Scroll down, and watch the FPS counter.

Presumably, WebRenderImageHost::UseTextureHost could check whether the texture is in view before scheduling composition.

[1] https://searchfox.org/mozilla-central/rev/00f3836a87b844b5e4bc82f698c559b9966e4be2/gfx/layers/wr/WebRenderImageHost.cpp#69

Sotaro, would this be straightforward to fix?

It seems possible to pause/resume video's ScheduleComposition() by using result of build frame.

video's ScheduleComposition() is triggered in WebRenderImageHost::UseTextureHost() and WebRenderBridgeParent::MaybeGenerateFrame().

https://searchfox.org/mozilla-central/rev/78b2129696eaad2162fd26256b5a19a2b0d9840a/gfx/layers/wr/WebRenderImageHost.cpp#75
https://searchfox.org/mozilla-central/rev/78b2129696eaad2162fd26256b5a19a2b0d9840a/gfx/layers/wr/WebRenderBridgeParent.cpp#1203

Gecko has 2 mechanisms to suppress video rendering
[1] Skip Composite at LayerManagerComposite::UpdateAndRender(), since invalid. region is empty.
[2] Suspend video playback by element visibility at MediaDecoder::UpdateVideoDecodeMode()

At STR in comment 0 with Compositor, [1] was suppressed, but [2] still happened.

WebRender is going to handle [1] in comment 3. It makes WebRender to similar situation to compositor.

https://treeherder.mozilla.org/#/jobs?repo=try&revision=4746e76e98c1b7590f0340a00e29404bb71cab38

Link to a discussion on IRC.
https://mozilla.logbot.info/gfx/20190607#c16375960

Implementation seems to become simpler if we use a list of used external image ids than Attachment 9068648 [details] as we discussed in Comment 10.

WebRenderTextureHostWrapper updates its data outside of WebRender. If WebRenderTextureHostWrapper's update is the only resource update, Webrender does not need to re-build frame for rendering. In this case, WebRender just does rendering. If an Image of the WebRenderTextureHostWrapper's update is not visible, WebRender does not need rendering.

During frame build, Document detects if it has an visible Image that might be update outside of WebRender(async image). invalidate_async_image() invalidates WebRender's rendered frame if there is visible async image.

Attachment 9070899 [details] does not have necessary change around WebRenderBridgeParent::RecvEmptyTransaction()

Attachment 9070899 [details] needs to be updated by Bug 1558106 and Bug 1559284

Hmm, Attachment 9070899 [details] caused timeout failure on talos g1.
https://treeherder.mozilla.org/#/jobs?repo=try&selectedJob=252763516&revision=c7d72ab0d17b31913be4cefb46bb83b6643dbd90

https://treeherder.mozilla.org/#/jobs?repo=try&revision=43e8967dba3f817275b82b8a4614a001f4315a42

Attachment 9070899 [details] is simplified a bit more.

https://treeherder.mozilla.org/#/jobs?repo=try&revision=1f8d457a5940447f0f891c37b388b75170ac7bf2

https://treeherder.mozilla.org/perf.html#/compare?originalProject=try&originalRevision=0c03f6d9a7139c57b1bcdf57f15f5f33d438256f&newProject=try&newRevision=1f8d457a5940447f0f891c37b388b75170ac7bf2

https://treeherder.mozilla.org/#/jobs?repo=try&revision=8f0fbe8017b4411ea5c0c84c6739e02d14f6700a
https://treeherder.mozilla.org/perf.html#/compare?originalProject=try&originalRevision=85619ee572f1b797fdbb2b069b44980d6556f6b1&newProject=try&newRevision=8f0fbe8017b4411ea5c0c84c6739e02d14f6700a

https://treeherder.mozilla.org/#/jobs?repo=try&revision=b5f771258a84152d5d7f1e2d6ebb163feec6b10c

https://treeherder.mozilla.org/perf.html#/compare?originalProject=try&originalRevision=cc7043bf750bbae894c0a821c73a2b01b7d1aeb8&newProject=try&newRevision=b5f771258a84152d5d7f1e2d6ebb163feec6b10c

https://hg.mozilla.org/mozilla-central/rev/8f1f4702a5c2