[Libva] gen7 h264 encode bitrate behaviour

[Chris Healy]

I've been encoding h264 content using HD 4000 HW and am not able to make
heads or tails of the way the encoder is behaving from the standpoint of
the data size coming out of the encoder.

I have a 24 fps 720p video that is the same image for ~8 seconds, then a
1.5 second fade to the next image followed by another ~8 seconds on that
image.  This goes on and on indefinitely.  I would have expected that the
bitrate would have been pretty low, then spike for 1.5 seconds then go back
to a similarly low value.

When I look at the data coming out of the encoder with a 4Mb/s bitrate set
and CBR, I'm seeing almost the inverse where most of the time, the bitrate
is pretty close to 4Mb/s then it spikes above 4Mb/s (presumably for the
fade), then it drops down to ~2Mbps for a second or so before going back up
to ~4Mb/s.

The strangest part is that for the first ~30 seconds of encode, across the
board, the bitrate is ~2x the bitrate from second 31 -> end of encode.
(So, I'm hitting a typical rate of 7Mbps and peaking out at 13Mbps.)

Is this behaviour expected with gen7 HW?  Is there something I can do in
the initial setup that will cap the MAX bitrate regardless of the impact on
encode quality?

Regards,

Chris
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