[Image-SIG] PIL histogram bin 255 empty

Fredrik Lundh fredrik at pythonware.com
Tue Sep 25 09:39:33 CEST 2007

Christopher Jones wrote:

> With mode I images, the histogram method sometimes returns bin 255 
> empty, counting the getextrema maximum value in bin 254 instead. Is this 
> a bug or is bin 255 treated in a special way, perhaps to count clipped 
> pixels?

what is the getextrema() when this happens?

my guess is that it's a rounding error in the histogram code; it 
basically does

     scale = 255.0F / (max - min);

     for each pixel:
        i = (int) ((pixel-min)*scale);
        if (i >= 0 && i < 256)

I don't have time to verify this right now, but I guess changing the 
index calculation to:

        i = (int) ((pixel-imin)*scale + 0.5);

would take care of this.

(the whole histogram-of-I/F approach feels a bit odd; not sure why it 
was done that way, really).


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