13.01.2010 21:45, Gilbert Soucy пишет: > Hello, > > I would like to know whether the algorithm to determine the JPEG quality of > an image ( > http://trac.imagemagick.org/browser/ImageMagick/trunk/coders/jpeg.c , line > 683) was taken from some papers or other references or whether it was > developped internally by one of the imagemagick developers (ex: by trying to > reverse the quantization tables in libjpeg) ? >
It is not an algorithm to determine the JPEG quality. It is an algorithm to guess the parameters that were given to libjpeg, or to reject the JPEG image if it wasn't created by libjpeg. > > This algorithm works perfectly for all images created with libjpeg but it > does not work well with images saved from Photoshop ( an image saved with > low quality, say less then 60, will be found to have a quality factor of 90 > and above). I have an example of an image saved with 6/12 in photoshop and > imagemagick reports 91%. Did anybody experiement with other encoders then > libjpeg? > See my blog post on this subject: http://patrakov.blogspot.com/2008/12/jpeg-quality-is-meaningless-number.html Basically, the comparison is as meaningless as directly comparing temperatures expressed in Centigrade and Fahrenheit degrees. -- Alexander E. Patrakov _______________________________________________ Magick-developers mailing list Magick-developers@imagemagick.org http://studio.imagemagick.org/mailman/listinfo/magick-developers
