[Image-SIG] New fast quantizer for PIL

[Oliver Tonnhofer]

Hi everyone,

I tried to improve the performance of the existing PIL image quantizer, but the best improvements I got were only 10%. While investigating how the median cut algorithm works, I came across the octree color quantization algorithm. I've implemented a variation of this algorithm and the results are very impressive.

It shows up-to-10x improvements. The algorithm shows good image quality for rasterized vector images like maps; gradients do not look as good as with the median cut algorithm.
For our use case[0], serving maps, we get an overall performance boots of ~x3.5.

Here are some times in ms, best of 5 runs.

                   rgb adaptive  octree octree+rle    jpeg
baboon.jpg      122.42   403.77   34.76      20.71   16.35 
gradient.png      1.45     6.60    1.01       1.21    0.95 
lena.jpg        167.83   325.08   35.53      19.26   13.11 
map.png         194.42   305.59   89.83      37.78   34.59 
rainbow.png      11.84   229.73    3.83       3.74    3.13 
wiki-en.png       7.27    12.45    2.78       1.65    1.81                   

All times include a convert/quantize and save call.
 - rgb is a plain save
 - adaptive is `convert('P', palette=ADAPTIVE)`
 - octree the new algorithm
 - octree+rle the new algorithm with RLE encoding enabled with my compress_type patch[1] 

The images are online [2], and there is also a .tar.gz with all images to download.

The new quantizer is available at bitbucket[3]. You can use the new algorithm with `img.convert(256, 2)`. 

I'd love to see that in the next PIL release. I will add some more comments and will clean up the code a bit more, then I'm up for a code review. Comments are welcomed already, though.

[0] http://osm.omniscale.de/ http://mapproxy.org
[1] http://bitbucket.org/olt/pil-117/changeset/8d4661695edd
[2] http://bogosoft.com/misc/pil-octree-tests/
[3] http://bitbucket.org/olt/pil-117-octree


Regards,
Oliver