[Ffmpeg-devel] H.264 decoding bug in either lavc/JM (x264 too?)

[Tuukka Toivonen]

I encoded a video clip using x264 and decoded with both
Mplayer (lavc) and JM 9.6. The two decoded videos differ
slightly on some frames. I think this is bug, as
H.264 should be bit exact, shouldn't it?

Here's frame-by-frame comparison:
Comparing size 640x480

              ssd_y       ssd_u      ssd_v     ssd       psnr_y   psnr_u   psnr_v   psnr
Frame    0          0          0          0          0       inf      inf      inf      inf
Frame    1          0          0          0          0       inf      inf      inf      inf
Frame    2          0          0          0          0       inf      inf      inf      inf
Frame    3          0          0          0          0       inf      inf      inf      inf
Frame    4       9342          1          8       9351   63.3006  96.9844  87.9535  65.0573
Frame    5          0          0          0          0       inf      inf      inf      inf
Frame    6          0          0          0          0       inf      inf      inf      inf
Frame    7          0          0          0          0       inf      inf      inf      inf
Frame    8          0          0          0          0       inf      inf      inf      inf
Frame    9          0          0          0          0       inf      inf      inf      inf
Frame   10      66739        150         46      66935   54.7612  75.2235  80.3568  56.5094
Frame   11          0          0          0          0       inf      inf      inf      inf
Frame   12          0          0          0          0       inf      inf      inf      inf
Frame   13          0          0          0          0       inf      inf      inf      inf
Frame   14          0          0          0          0       inf      inf      inf      inf
Frame   15          0          0          0          0       inf      inf      inf      inf
Frame   16        668          0          4        672   74.7573      inf  90.9638  76.4922
Frame   17          0          0          0          0       inf      inf      inf      inf
Frame   18          0          0          0          0       inf      inf      inf      inf
Frame   19      69884         55         25      69964   54.5612  79.5808  83.0050  56.3172
Frame   20          0          0          0          0       inf      inf      inf      inf
Frame   21          0          0          0          0       inf      inf      inf      inf
Frame   22         11          0          0         11   92.5911      inf      inf  94.3520

I used latest CVS Mplayer and latest SVN x264 (but same results were same 
with slightly older versions).

x264 command line:
--bframes 2 --8x8dct --b-pyramid --qp 26 --direct spatial
The problem occurs only if using all three options --8x8dct, --b-pyramid, 
and --direct spatial. Most other options seem not to matter.

The funny thing is that the image quality is actually a bit better with 
Mplayer decoded video than with JM decoded video, which would indicate
that the bug would be in JM. However, as JM should be quite mature, I 
would rather guess there's a bug in both x264 and in Mplayer.
(Tell me if it's otherwise, so I can go to bug JM developers :)

Test files:
http://www.ee.oulu.fi/~tuukkat/tmp/bug/

(Frame #11 below is frame 10 above, as my yuvcmp
counts from 0, Mplayer apparently from 1)

bug.tgz          - includes all

diff-fr11.png    - Frame #11 difference picture, contrast enhanced
diff.yuv         - Difference YUV file, contrast enhanced
differs.txt      - Frame-by-frame PSNR between two decoded videos
jm-fr11.png      - Frame #11 decoded with JM 9.6
mpl-fr11.png     - Frame #11 decoded with Mplayer
test-640x480.yuv - Original test video
test.264         - Bitstream produced with x264
test_dec-mpl.yuv - Decoded video with Mplayer
test_dec.yuv     - Decoded video with JM 9.6
testme.sh        - Command lines used