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. --- a/libavformat/rtpdec_jpeg.c +++ b/libavformat/rtpdec_jpeg.c @@ -207,16 +207,17 @@ static void create_default_qtables(uint8_t *qtables, uint8_t q) { int factor = q; int i; + uint16_t S; factor = av_clip(q, 1, 99); if (q < 50) - q = 5000 / factor; + S = 5000 / factor; else - q = 200 - factor * 2; + S = 200 - factor * 2; for (i = 0; i < 128; i++) { - int val = (default_quantizers[i] * q + 50) / 100; + int val = (default_quantizers[i] * S + 50) / 100; /* Limit the quantizers to 1 <= q <= 255. */ val = av_clip(val, 1, 255); -- :wq ^X^Cy^K^X^C^C^C^C _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
