[x264-devel] Degrading video quality over time

[Jan Willamowius]

Jason Garrett-Glaser wrote:
> >  // Rate control set to CBR mode
> >  context.rc.i_rc_method        = X264_RC_ABR;
> >  context.rc.f_rate_tolerance   = 1;
> >  context.rc.i_vbv_max_bitrate  = 0;
> >  context.rc.i_vbv_buffer_size  = 0;
> >  context.rc.i_lookahead        = 0;
> >  context.rc.i_bitrate = 768;
> 
> That's not CBR mode, that's ABR mode!  To set CBR, you need to set
> bufsize and maxrate.  *THAT* is why you're not getting the results you
> expect.

The comment is actually the error, ABR is ok.
The problem seems to be that x264 in ABR mode tries to reduce the
bitrate to be way under the set limit when dealing with my easily
compressible still images.

As an experiment, I set x264 to CRF mode instead
  _context.rc.i_rc_method       	= X264_RC_CRF;
  _context.rc.f_rf_constant       	= 16.0;

This gives great video quality, as expected. When I transmit still
images, the bitrate in CRF mode is even a bit below the bitrate of AMR
(both well under the set limit in stats from the receiver). When feed
motion pictures, the bitrate goes through the roof with this CRF
setting.

Could it be that the estimate how much bitrate x264 is currently
producing is wrong (buggy ?) and it just starts to reduce the bitrate
too early in my case ?

Thanks,
Jan

-- 
Jan Willamowius, jan at willamowius.de, http://www.willamowius.de/