preemptive use of the bandwidth-limit in video encoders

[Antoine Martin]

The bandwidth-limit is used to raise the batch delay (variable refresh rate) so that we stay within budget (and that in turns has an effect on quality and speed), but we should pass the information down to the video encoders so that they can be tuned adequately earlier.

This bitrate constraint overrules the speed and quality settings so this should only be applied when the bandwidth-limit is relatively low.

Somewhat related to #1951, #1840, #1952

[Antoine Martin]

Done in r20399 for x264 and vpx.
We only use the bandwidth-limit value when speed and quality are on "auto" and when min-quality is lower than 50%.

• vpx will cap its bitrate attribute using the bandwidth-limit
• x264 switches from CRF mode (aka "contant quality" - more info here: ​CRF Guide) to VBV mode (aka "constrained encoding ", see ​Understanding Rate Control Modes (x264, x265, vpx)) - but only when the bandwidth-limit is lower than 5Mbps

The change also improves the amount of x264 encoder configuration details shown in xpra info, so we can see which "rc-method" is in use, ie trimmed:

client.window.2.encoder=x264
client.window.2.encoder.b-frames=1
client.window.2.encoder.bandwidth-limit=2000000
client.window.2.encoder.content-type=video
client.window.2.encoder.fps=159
client.window.2.encoder.ms_per_frame=3
client.window.2.encoder.params.rc.bitrate=1953
client.window.2.encoder.params.rc.rc-method=ABR
client.window.2.encoder.params.rc.vbv_buffer_init=1.0
client.window.2.encoder.params.rc.vbv_buffer_size=1953
client.window.2.encoder.params.rc.vbv_max_bitrate=3906
client.window.2.encoder.preset=fast
client.window.2.encoder.profile=high10
client.window.2.encoder.tune=film

[Antoine Martin]

Regression introduced by r20399 is fixed: ticket:2087#comment:4

[Antoine Martin]

Summary: this new code should allow the video encoders to be better tuned for the amount of bandwidth available, lowering the picture quality (sometimes drastically - see #2087 for what a 2Mbps limit does) but allowing for a higher refresh rate and a better quality for the non-video areas around it.

[Antoine Martin]

Not heard back.

[migration script]

this ticket has been moved to: ​https://github.com/Xpra-org/xpra/issues/1957