• Ico Doornekamp's avatar
    avformat/rtpdec_jpeg: fix low contrast image on low quality setting · e3e6a2cf
    Ico Doornekamp authored
    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.
    Signed-off-by: 's avatarMichael Niedermayer <michael@niedermayer.cc>
    e3e6a2cf
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