[Libav-user] MPEG2 decoding pixelation problem (but not in ffplay)

Jaka Bac jakabac at gmail.com
Tue Jun 6 23:42:01 EEST 2017

Hi Michael,

You are parsing the mpeg2 stream by yourself looking for the 0x00 0x00 0x01
0x00 startcode which means that you will only feed the mpeg2 encoded
pictures to the decoder.
This way your program will skip over the sequence headers which may include
a lot of important information for the mpeg2 decoder (for example custom
intra and non intra quantiser matrices, picture dimensions, aspect

The ffmpeg mpeg2video codec will try its best to decode the pictures, but
with the missing info it may not be fully successful.
Your PussInBoots file for example, includes custom quantiser matrices which
are probably slightly different from the default ones (I did not look in

My suggestion would be to use libavformat to read the source files. This
way you would get correctly parsed AVPacket(s).

If you for some reason can't use libavformat, you could at least
use av_parser_parse2 function to parse the stream for you. (See example
here: https://www.ffmpeg.org/doxygen/trunk/decode_video_8c-example.html)

Hope that helps,

On 4 June 2017 at 05:12, Michael Goffioul <michael.goffioul at gmail.com>

> Hi,
> I'm trying to use ffmpeg to decode a MPEG2 stream and I'm running into
> pixelation problem with some streams (not all of them). The fact is that
> the problematic streams play fine in ffplay (or vlc), so I figured the
> problem must be in the way I setup the decoder or feed data to it.
> I've written a very simple code to illustrate the problem I'm having, it's
> available here:
> https://drive.google.com/open?id=0BwljeX6541LuTjlLRmN2bG52M3M
> It doesn't do anything fancy: just initialize the mpeg2video codec, feed
> the data (segmented on the picture start code boundary), drain frames,
> convert to RGB and save as PNG. It accepts 2 arguments: the elementary
> MPEG2 stream filename and the number of frames to extract.
> I'm using this sample clip as test case, it's the elementary video stream
> extracted from a MPEG/TS clip (next link):
> https://drive.google.com/open?id=0BwljeX6541LuUFFHSElwaUFGYkE
> https://drive.google.com/open?id=0BwljeX6541LuamdieXZzamNkNFE
> The pixelation problem is illustrated in this extracted frame:
> https://drive.google.com/open?id=0BwljeX6541LubmhkS2VWekc5S00
> By comparison, playing the sample clip (elementary stream) with ffplay
> gives this:
> https://drive.google.com/open?id=0BwljeX6541LuQ0hHQmVZMlAxMmc
> Clearly I must be doing something wrong. Does anybody have a hint or
> suggestion?
> Thanks,
> Michael.
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> Libav-user mailing list
> Libav-user at ffmpeg.org
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