[FFmpeg-devel] Fwd: Re: [Ffmpeg-user] jpg frame export : gama problem

[Nicolas Jauffret]

Hello,
here is a discussion thread about a jpg export gama problem.
Thank you for your time and support.
Best regards,

Nick

------- Forwarded message -------
From: "Nicolas Jauffret" <nicolas at buf.fr>
To: "FFmpeg user questions and RTFMs" <ffmpeg-user at mplayerhq.hu>
Cc:
Subject: Re: [Ffmpeg-user] jpg frame export : gama problem
Date: Thu, 21 Jun 2007 18:58:17 +0200

On Thu, 21 Jun 2007 18:08:44 +0200, Michel Bardiaux
<mbardiaux at mediaxim.be> wrote:

> Nicolas Jauffret wrote:
>> Hello,
>>
>> When I try to extract a frame from a dv video with this command :
>>
>>    ffmpeg -i "input.mov" -vframes 1 -ss 0.0000 -t 0.0400 "out.%08d.jpg"
>
> You should always post the complete output messages too.
>
>>
>> I have a gama problem : the initial black RGB(0,0,0) becomes  
>> RGB(16,16,16).
>> ffmpeg doesn't seem to do a yuv level range conversion when exporting to
>> jpeg.
>>
>>
>> If I choose png file format for output, I have no problem.
>> I get a pure black RGB(0,0,0) in my exported frame.
>>
>>    ffmpeg -i "input.mov" -vframes 1 -ss 0.0000 -t 0.0400 "out.%08d.png"
>>
>
> If only a few pixels of that image are black, well, JPEG is lossy, so
> its no surprise it takes liberties with some pixels. Make an all-black
> image as a BMP or raw or other lossless format, and convert it to JPEG
> with ffmpeg. If still not black, then report the bug.

Of course, I have done that with a black images test sequence.

I confirm that there is a problem doing this:

$ ffmpeg -i input.bmp -f image2 output.%d.jpg
FFmpeg version SVN-r9377, Copyright (c) 2000-2007 Fabrice Bellard, et al.
    configuration: --enable-gpl --enable-static --enable-swscaler
--enable-pthreads --enable-libmp3lame --enable-libogg --enable-libxvid
--disable-ffserver --enable-memalign-hack --enable-libx264 --enable-liba52
--extra-ldflags=-l m
    libavutil version: 49.4.0
    libavcodec version: 51.40.4
    libavformat version: 51.12.1
    built on Jun 21 2007 16:36:33, gcc: 3.3.5 (Debian 1:3.3.5-13)
Input #0, image2, from 'input.bmp':
    Duration: 00:00:00.0, start: 0.000000, bitrate: N/A
    Stream #0.0: Video: bmp, bgr24, 420x300, 25.00 fps(r)
Output #0, image2, to 'output.%d.jpg':
    Stream #0.0: Video: mjpeg, yuvj420p, 420x300, q=2-31, 200 kb/s, 25.00
fps(c)
Stream mapping:
    Stream #0.0 -> #0.0
Press [q] to stop encoding
Compiler did not align stack variables. Libavcodec has been miscompiled
and may be very slow or crash. This is not a bug in libavcodec,
but in the compiler. Do not report crashes to FFmpeg developers.
frame=    1 fps=  0 q=3.5 Lsize=       0kB time=0.0 bitrate=   0.0kbits/s
video:3kB audio:0kB global headers:0kB muxing overhead -100.000000%

Although, the "input.bmp" is a full black RGB(0,0,0) image,
the "output.1.jpg" is not black : RGB(16,16,16).

I am going to report this issue to the devel mailing list.
Best regards,

Nick