[magick-users] JPG compression levels

[Wolfgang Hugemann]

> <http://www.xat.com/jpegopt/index.html> ("Selective optimization -
> compress some areas more than others") too.
> 
> If I am not mistaken, JPEG compresses images in blocks of 8x8 pixels?
> 
> The best idea I could come up with so far with is to create the
> different 8x8 blocks individually with different target compressions and
> then use the -drop option of jpegtran to merge them into a single image
> without loss/recompression. I will be testing the idea in the coming hours.

I think the problem is that JPEG compression is mostly based on dropping 
not so important frequencies after the discrete cosine transform. And 
what frequencies are "not that important" is jugded on the complete 
content of the picture and stored only for the complete picture.

There is a really nice visualisation of the single steps in JPEG Com 
pression (deplorably only in German, but you'll get through it, I guess) 
at http://www.spemaus.de/studium/visjpeg/applet.html

Greetings from Germany
Wolfgang Hugemann