This is a first draft. There may be errors, typos, broken links, etc.


It is my opinion that XviD beat libavcodec MPEG-4 in the 2005 Doom9 codec comparison on the basis of having more adaptable ratecontrol. lavc simply doesn't increase the bitrate in high-motion scenes nearly as much as XviD does, and such scenes in an lavc encode look comparatively horrible. Lavc has more bitrate available for low-motions scenes, but the difference is not readily apparent, nor is it a worthwhile trade-off.

When I was preparing a set of parameters for Doom9 to use, I had a feeling that might be the case. At the time, though, I didn't know how to fix it other than by increasing vqcomp from the default value of 0.5 to 0.6 or 0.7. This seemed to help, though not very much. After hearing mixed opinions with regard to how effective vqcomp is, I ended up being conservative and choosing 0.6.

Since the competition, Michael Niedermayer has committed several improvements to lavc. vb_strategy=2, in particular, reduces the usage of B-frames in high-motion scenes, which gives a substantial improvement (the older vb_strategy=1 does this too, but with a net quality loss). My own small contribution of sc_factor (specifically, sc_factor=6) makes some high-motion scenes look a bit better by being more likely to use an I-frame. Still, none of these improvements make lavc look nearly as good as XviD in high-motion scenes. Rate control is still the main underlying factor.


I've written a small program for analysis of MPEG-4 video. It parses the output of 'mplayer -lavdopts debug=1' and makes graphs and statistics. I will submit it for inclusion in mplayer's "TOOLS" archive soon, after I clean it up a bit and make it more general.

Take a look at these two pages. The first is for lavc, using the best mencoder command I have at this point (and the default ratecontrol equation).

for i in 1:turbo:vb_strategy=2 2 ; do
  mencoder matrix.vob -aid 128 -oac copy -vf \
  crop=718:356:0:60,scale=640:272 -sws 9 -ovc lavc -lavcopts \
last_pred=2:vqcomp=0.6:qpel:sc_factor=6:vmax_b_frames=2 \
  -ofps 24000/1001 -o lavc-default.avi
lavc: my parameters, default ratecontrol
The second is for my translation of the Doom9 XviD encode.
for i in 1:turbo 2 ; do
  mencoder ~/dumpstream/matrix.vob -aid 128 -oac copy -vf \
  crop=718:356:0:60,scale=640:272 -sws 9 -ovc xvid -xvidencopts \
  bitrate=581:psnr:qpel:nopacked:pass=$i -ofps 24000/1001 -o xvid.avi
XviD; Doom9 parameters
It helps if you open the above links in separate tabs so you can switch back and forth rapidly to compare.

There are several notable differences between the lavc information and the XviD information.

Failed Attempts

I tried a few things...

Increasing vqblur didn't have the desired effect; there wasn't much of a change, but some of the QP spikes on the graph got higher and others got lower. Visually, I could find a few places that had lower quality and none really looked better. Also, PSNR dropped from 43.03 to 43.02.

Increasing vqcomp even more looks like it has the desired effect, but only according to the statistics. The bitrate spikes more, the quantizer spikes less, and the PSNR even increased from 43.03 to 43.13. Unfortunately I couldn't see a noticeable improvement in most high-motion scenes, and, at higher values of vqcomp, low-motion scenes did indeed look worse.

I started a vqmax test before I went to bed, and when I got up I found some surprisingly bad results. Even though less than 0.5% of the frames in the original encode had a qp higher than 9, removing those qps dropped the PSNR to 42.87 and made the visual quality much worse.


I finally ended up trying to modify the ratecontrol equation itself. The relevant variable is mcVar, and my earlier attempts at making use of it had failed because, apparently, mcVar is small and doesn't change as rapidly as does tex. In order to use mcVar, I had to multiply it by a very large number. The ratecontrol equation listed below is from experimentation only -- I don't fully understand what I'm playing with, and I'm sure it can be improved upon.
The statistics page for the preceding equation is here:
lavc; experimental ratecontrol

Note that if you try to use that equation you may have to use quotes or escapes so the shell doesn't try to interpret the parenthesis. Also, that big number (100000000, or 10^8) isn't highly tuned, but I'm confident I like it better than the two surrounding orders of magnitude; smaller variations don't make much of a difference.

Although this new ratecontrol results in a qp/bitrate graph that still doesn't look much like the XviD one, the two encodes are actually quite similar looking. At this point, I cannot honestly say which encode is better. There are very slight variations, but they both look quite good in both high-motion scenes and low-motion scenes.

Also, this equation raises PSNR from 43.03 to 43.08.


As I said, I don't really know what I'm doing. I'd appreciate any suggestions. I'm going to keep trying miscellaneous alterations, and hopefully I can come up with something that ends up looking even better. I'll update this page and post notification on ffmpeg-devel.

Revision History:
2006-02-13 Wrote initial version.
Fixed mis-copied libavcodec command.

Corey Hickey, bugfood-ml [at] fatooh [dot] org
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