[FFmpeg-cvslog] rtpdec_jpeg: fix low contrast image on low quality setting

[Ico Doornekamp]

ffmpeg | branch: master | Ico Doornekamp <libav at zevv.nl> | Thu Mar 24 14:31:38 2016 +0100| [edf54887e2935a30f9d9a46dd806802c3c867c0e] | committer: Martin Storsjö

rtpdec_jpeg: fix low contrast image on low quality setting

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.

Instead, use a new variable 'S' (same name as in RFC2435) with the
proper range to store the result of the division.

Signed-off-by: Martin Storsjö <martin at martin.st>

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=edf54887e2935a30f9d9a46dd806802c3c867c0e
---

 libavformat/rtpdec_jpeg.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/libavformat/rtpdec_jpeg.c b/libavformat/rtpdec_jpeg.c
index 15324bf..bc86b15 100644
--- a/libavformat/rtpdec_jpeg.c
+++ b/libavformat/rtpdec_jpeg.c
@@ -186,16 +186,17 @@ 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);