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

[Nicolas Jauffret]

Hi,

On Wed, 04 Jul 2007 15:23:17 +0200, Nicolas Jauffret <nicolas at buf.fr>  
wrote:

> Hi
>
> On Fri, Jun 29, 2007 at 11:14:53AM +0200, Nicolas Jauffret wrote:
>> On Thu, 21 Jun 2007 19:08:54 +0200, Nicolas Jauffret <nicolas at  
>> buf.fr> wrote:
>>> 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
>> Ping ?
>> Have you well received my mail ?
>
> yes and patch welcome, its likely a problem with the colorspace  
> convertion
> see sws_setColorspaceDetails() and the code responsible for RGB->YUV

I won't be able to patch it, but found similar problems when compressing  
 from jpeg image sequences.

There are some strange gama differences when compressing from a jpg image  
sequece or a png image sequence.

I made a test example and wrote a detailed README file to go with.
You can find these in the mplayer FTP repository :

   upload.mplayerhq.hu:/MPlayer/incoming/gama_problem

Thank you for your time and support,
Best regards,

Nick