640328 - <video> does not start using prescaling until it exhausts its initial decoded buffer

Status: RESOLVED INACTIVE

(Core :: Audio/Video: Playback, defect)

Description

[Timothy B. Terriberry (:derf)]

+++ This bug was initially created as a clone of Bug #639415 +++

I'm breaking this out into its own bug.

We don't use the prescaling path when rendering the first few frames of video, because BasicImageContainer::SetScaleHint has not yet been called when they are decoded and BasicPlanarYCbCrImages are created around them.

roc says:
> We could reorganize nsVideoFrame::BuildLayer so that SetScaleHint gets computed
> and called after
>   if (videoSize.width <= 0 || videoSize.height <= 0 || area.IsEmpty())
>     return nsnull;
> and before we check for an actual image. That should make scale hints available
> to the decoder much earlier.

Comment 1

[Matthew Gregan [:kinetik]]

We're bailing on the videoSize check because it's not set yet.  The decoder doesn't set it until the first nsMediaDecoder::Invalidate fires (via nsBuiltinDecoder::MetadataLoaded dispatched to the main thread).

It's further complicated by the fact that a bunch of frames are decoded very early during the call to nsBuilinDecoderReader::FindStartTime, which is called as part of the metadata loading process.

Comment 2

[Matthew Gregan [:kinetik]]

So one fix would be to change the metadata loading logic so that it fires a synchronous event to update the media element's size as soon as it's known, then carry on with the rest of the metadata loading logic (including FindStartTime).

Comment 3

[Matthew Gregan [:kinetik]]

And that turns out to be tricky because we need the logic in nsMediaDecoder::SetVideoData/nsMediaDecoder::Invalidate to compute the element's size... and for that we need an Image.

Comment 4

[Robert O'Callahan (:roc) (email my personal email if necessary)]

Seems like we could factor out that code so the size can also be computed from th nsVideoInfo. Then nsBuiltinDecoderStateMachine::LoadMetadata could compute the size and do a synchronous event dispatch to the main thread so set the media size early. This would probably be good for getting the video frame size set correctly earlier for intrinsically-sized video elements as well.

Comment 5

[Robert O'Callahan (:roc) (email my personal email if necessary)]

It might make sense to dispatch that new event asynchronously instead of synchronously.

Comment 6

[Nils Ohlmeier [:drno]]

Mass closing do to inactivity.
Feel free to re-open if still needed.