[Galene] About VP9 SVC in Galene

[Juliusz Chroboczek <jch@irif.fr>]

Hi,

I've spent the last two nights implementing spatial VP9 SVC in Galene
(temporal was already working fine).  I think it's correct, although not
quite optimal.

In order to test, set up a group with VP9, enable simulcast, and then
toggle between « Receive: all » and « Receive: all (low) ».  You should
see the quality change without any visible artefacts and with no
renegotiation.  (It is normal to see a temporary degradation in quality,
due to the second point below.)

Two issues.  First, once in a while Chromium's rate estimator gets stuck
in a state where it's sending a very low data rate; the only fix appears
to be to restart the stream.  I've spent hours trying to debug the issue,
and I don't see anything wrong with my code.

Second, we're not parsing the dependency descriptor, which means that we
cannot switch seamlessly at arbitrary points.  The result is that we
require a keyframe when a switch is required, which causes extra traffic
and a visible degradation in stream quality.  I think this is good enough
for 0.4, especially as we currently tend to prefer temporal degradation
when we're congested.

-- Juliusz