[x264-devel] x264 rate control

[eloise vidal]

I am working on CBR streaming applications and I compare x264 to Mainconcept
 in terme of quality (PSNR, SSIM), performance (frame encoded per second)
and rate control accuracy.

I read the "Video Codec Comparison" of CS MSU Graphics&Media Lab, Video
Group. This comparison shows that x264 is the best codec in terme of SSIM,
but I cannot find the same results because of the rate control.

I use sequences of 200 frames for my tests. Here is an exemple of the
command line I use for HD sequence :
x264 --input-res 1920x1080 --fps 25 --bitrate 8000 --vbv-maxrate 8000 --
vbv-bufsize 3000 --profile high -o Output.h264 Input.yuv

Mainconcept uses a buffer size of 3Mbit for HD sequence, and I choose the same
size for x264 in order to have the same latency.
With this buffer size, x264 does not succeed in achieving 8Mb/s but 6MB/s.
To achieve 8Mb/s, I have to choose a bigger buffer of 35Mbit, but firstly I
add more than 4sec of latency! And secondly, the rate control needs a lot of
 frames to be stable, a small number of bits are allocated to the frames at
the begining of the sequence and consequently, the average of PSNR/SSIM on
the 200 frames is always lower than Mainconcept.

Is there something that I don't understand about x264 ratecontrol which
allows to reach the target bitrate and does not need more than 1 GOP to be
stable?

Thanks for your help,

Eloïse
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