[FFmpeg-user] Ticket#5936: Purple haze on ffmpeg mpeg4 decoding

[Toerless Eckert]

Just opened https://trac.ffmpeg.org/ticket/5936

Text appended. Sorry, no Jimi Hendrix included.

Thanks!

Summary:

I have ca. 3 TB/10 years of free-TV recordings created with (older versions) of Mencoder (aka: ffmpeg). These play back fine with older versions of XBMC/VLC (decoded by ffmpeg/libavcodec / mpeg4), but since XBMC 13.1 and VLC 2.2, ffmpeg produces purple haze that is creeping in from the side of the screen when the video is panning.

How to reproduce:

ffplay purple-haze-bug.avi
(on upload.ffmpeg.org or ticket)

    Duration: 00:00:16.63, start: 0.000000, bitrate: 1454 kb/s

        Stream #0:0: Video: mpeg4 (Simple Profile) (FMP4 / 0x34504D46), yuv420p, 720x576 [SAR 64:45 DAR 16:9], 1383 kb/s, 25 fps, 25 tbr, 25 tbn, 25 tbc

...

Latest working versions:

    <=ffmpeg 2.2.x (tested 2.2.16, 2.1, 2.0)
    ffmpeg

        tag 03bb99ae1a99fa315621308b885a8fc70702c9bc Jun 1, 2014 master branch

    ffplay version N-63655-g03bb99a Copyright (c) 2003-2014 the FFmpeg developers

        built on Nov 9 2016 14:43:54 with gcc 4.9.3 (Gentoo 4.9.3 p1.5, pie-0.6.4)
        configuration:
        libavutil 52. 88.100 / 52. 88.100
        libavcodec 55. 66.100 / 55. 66.100
        libavformat 55. 42.100 / 55. 42.100
        libavdevice 55. 13.101 / 55. 13.101
        libavfilter 4. 5.100 / 4. 5.100
        libswscale 2. 6.100 / 2. 6.100
        libswresample 0. 19.100 / 0. 19.100

First purple haze version:

    =ffmpeg 2.3.0 (tested 2.3 and later).

    ffmpeg

        tag 9236f7b5a23b4907f7b2bf6346ecd88e6d76f1e0 Jun 14, 2014 master branch

    ffplay version N-63957-g9236f7b Copyright (c) 2003-2014 the FFmpeg developers

            built on Nov 9 2016 14:50:33 with gcc 4.9.3 (Gentoo 4.9.3 p1.5, pie-0.6.4)
            configuration:
            libavutil 52. 89.100 / 52. 89.100
            libavcodec 55. 66.101 / 55. 66.101
            libavformat 55. 43.100 / 55. 43.100
            libavdevice 55. 13.101 / 55. 13.101
            libavfilter 4. 8.100 / 4. 8.100
            libswscale 2. 6.100 / 2. 6.100
            libswresample 0. 19.100 / 0. 19.100

I am only _guessing_ this is an avcodec issue because it looks very
much related to mpeg4 decoding of blocks panning in from "offscreen".

git diff 03bb99ae1a99fa315621308b885a8fc70702c9bc 9236f7b5a23b4907f7b2bf6346ecd88e6d76f1e0
produce 700k diff and i can not figure out what a likely cause is ;-(

I am sure there is some problem in mencoder, but i hope that ffmpeg will continue to try to decode as good as possible even potentially buggy videos as it did in the past. Especially videos encoded with (older versions of) ffmpeg.

The example video is encoded with MEncoder 1.0rc1-4.1.2, but the problem exists with at least up to Mencoder-1.1(which uses 0.10.2.git).