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

[Ned Batchelder]

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")

--Ned.
http://nedbatchelder.com

Karsten Hiddemann wrote:
> Mike Meisner schrieb:
>> I'd like to use PIL to prep an image file to improve OCR quality.
>>  
>> Specifically, I need to filter out all but black pixels from the 
>> image (i.e., convert all non-black pixels to white while retaining 
>> the black pixels).
>
> You could do something like the following:
>
> from PIL import Image
>
> img = Image.open("sample.png")
> (xdim, ydim) = img.size
> # this assumes that no alpha-channel is set
> black = (0, 0, 0)
> white = (255, 255, 255)
>
> if Image.VERSION >= "1.1.6":
>     data = img.load()
>     for y in range(ydim-1, 0, -1):
>         for x in range(xdim):
>             if data[x,y] != black:
>                 data[x,y] = white
> else:
>     data = img.getdata()
>     for y in range(ydim-1, 0, -1):
>         for x in range(xdim):
>             if data[x+y*xdim] != black:
>                 data[x+y*xdim] = white
>
> img.save("sample-filtered.png")
> _______________________________________________
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>
>
>

-- 
Ned Batchelder, http://nedbatchelder.com