[FFmpeg-devel] PATCH: RTP/MJPEG low contrast image on low quality setting

Michael Niedermayer michael at niedermayer.cc
Thu Mar 24 15:38:42 CET 2016

On Thu, Mar 24, 2016 at 02:31:38PM +0100, Ico Doornekamp wrote:
> Original mail and my own followup on ffmpeg-user earlier today:
> I have a device sending out a MJPEG/RTP stream on a low quality setting.
> Decoding and displaying the video with libavformat results in a washed
> out, low contrast, greyish image. Playing the same stream with VLC results
> in proper color representation.
> Screenshots for comparison:
>   http://zevv.nl/div/libav/shot-ffplay.jpg
>   http://zevv.nl/div/libav/shot-vlc.jpg
> A pcap capture of a few seconds of video and SDP file for playing the
> stream are available at
>   http://zevv.nl/div/libav/mjpeg.pcap
>   http://zevv.nl/div/libav/mjpeg.sdp
> I believe the problem might be in the calculation of the quantization
> tables in the function create_default_qtables(), the attached patch
> solves the issue for me.
> The problem is that the argument 'q' is of the type uint8_t. According to the
> JPEG standard, if 1 <= q <= 50, the scale factor 'S' should be 5000 / Q.
> Because the create_default_qtables() reuses the variable 'q' to store the
> result of this calculation, for small values of q < 19, q wil subsequently
> overflow and give wrong results in the calculated quantization tables. The
> patch below uses a new variable 'S' (same name as in RFC2435) with the proper
> range to store the result of the division.

patch applied


Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Avoid a single point of failure, be that a person or equipment.
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