As of [1], a single VP9 encoder instance can produce simulcast 4:2:1.
When it does, the EncodedImage has its simulcast index set (0, 1, 2).
The bug is that if you then go back to a single encoder instance,
either because you're doing singlecast or because you're doing
simulcast with scaling factors that are not power of two (not 4:2:1),
then the simulcast index which was previously set to 2 is not reset due
to the old code path never calling SetSimulcastIndex.
Example repro:
1. Send VP9 simulcast {180p, 360p, 720p}, i.e. 4:2.1.
2. Reconfigure to {180p, 360p, 540p}, i.e. no longer 4:2:1.
What should happen: all three layers are sent.
What actually happened: 180p is not sent and the 540p layer flips flops
between 180p and 540p because the EncodedImage says simulcast index is
2 for both encodings[0] and encodings[2].
The fix is a one-line change: `SetSimulcastIndex(std::nullopt)` in the
case that we don't have a `simulcast_to_svc_converter_` that sets it
(0, 1, 2) for us.
[1] https://webrtc-review.googlesource.com/c/src/+/360280
Bug: chromium:370299916
Change-Id: I52bd4428bd12528f0e98869ec61626c06f589b43
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/363941
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#43109}
a6fbb35ac1