1. ffmpeg
  2. Commit e112096885c556e45be5f973a3cbb4cfb8bf48fe
Browse code

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>

Ico Doornekamp authored on 2016/03/24 22:31:38
Showing 1 changed files
libavformat/rtpdec_jpeg.c
History View file @ e112096
... ... 
@@ -193,16 +193,17 @@ static void create_default_qtables(uint8_t *qtables, uint8_t q)
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 {
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     int factor = q;
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     int i;
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+    uint16_t S;
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     factor = av_clip(q, 1, 99);
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     if (q < 50)
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-        q = 5000 / factor;
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+        S = 5000 / factor;
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     else
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-        q = 200 - factor * 2;
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+        S = 200 - factor * 2;
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     for (i = 0; i < 128; i++) {
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-        int val = (default_quantizers[i] * q + 50) / 100;
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+        int val = (default_quantizers[i] * S + 50) / 100;
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         /* Limit the quantizers to 1 <= q <= 255. */
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         val = av_clip(val, 1, 255);