[Ffmpeg-devel] Strange behaviour of jpeg & qscale

Philip Gladstone philip
Sat May 7 04:09:11 CEST 2005

I've been fiddling with ffserver a bit recently, and it is happier than 
it was. However, I uncovered a strange feature of encoding to jpegs.

The commands

ffmpeg -vd /dev/video0 -qscale 25 -s 320x240 -f singlejpeg -vframes 1 
-dct_algo 1 qs25.jpg

ffmpeg -vd /dev/video0 -qscale 25 -s 320x240 -f singlejpeg -vframes 1 
-dct_algo 3 qs25.jpg

behave very differently. The first produces a good jpg image. The second 
has some serious problems. This is with the latest CVS sources, built on 
Linux on Athlon.

I do not have enough DCT-fu (nor MMX fu) to figure out what is going 
wrong. Note also that when the dct_algo is not defined, it automatically 
chooses (I think) 3, and thus generates the busted image.


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