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authorIco Doornekamp <libav@zevv.nl>2016-03-24 14:31:38 +0100
committerMichael Niedermayer <michael@niedermayer.cc>2016-04-28 03:47:13 +0200
commite112096885c556e45be5f973a3cbb4cfb8bf48fe (patch)
treeaf92e499bd97d12bae7625dc40bc3ceae73b2c0f /doc/t2h.pm
parentc6bb7659726728d9c523b168f08c8d04aec75433 (diff)
downloadffmpeg-e112096885c556e45be5f973a3cbb4cfb8bf48fe.tar.gz
avformat/rtpdec_jpeg: fix low contrast image on low quality setting
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: Michael Niedermayer <michael@niedermayer.cc> (cherry picked from commit e3e6a2cff4af9542455d416faec4584d5e823d5d) Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
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