[Image-SIG] Filtering out all but black pixels for OCR

[Fredrik Lundh]

Ned Batchelder wrote:

> If your image is single-channel (mode "L"), then you can use the eval 
> function:
> 
>    img = Image.open("onechannel.png")
>    # at each pixel, if it isn't zero, make it 255..
>    better = Image.eval(img, lambda p: 255 * (int(p != 0)))
>    better.save("bilevel.png")

if you want to avoid the clever lambda, you can use "point" instead, 
with a precalculated lookup table:

     # build lookup table (you can do this once, at the module level)
     lut = [255] * 256 # make everything white
     lut[0] = 0 # except things that are already pure black

     ...

     better = img.point(lut) # for each image

</F>