How to I use PIL Image.point(table) method to apply a threshold to a 256 gray image?

Question

I have 8-bit greyscale TIFF images that I want to convert to Monochrome using a 75% white (decimal 190) threshold. In the Image.convert(mode) method section, the PIL manual says:

"When translating a greyscale image into a bitlevel image (mode "1"), all non-zero values are set to 255 (white). To use other thresholds, use the point method."

The Image.point(table) method says that it maps each pixel through the given table.

im.point(table, mode) => image
im.point(function, mode) => image

"Map the image through table, and convert it on fly. In the current version of PIL , this can only be used to convert 'L' and 'P' images to '1' in one step, e.g. to threshold an image."

How do I create the table (or function) that corresponds to the 75% threshold I need?

pajton's answer is the one you want. You use 190, but I would use `p > 191` (I know, the difference is minor :). — tzot, Jun 26 '11 at 18:56
Thanks for the answers, but a funny thing happened on the way to implementation. My images actually start as TIFF,CCITT group4. I thought it would be easy to convert to 8-bit grey. However, PIL reports "decoder group4 not available". Is there another imaging library that supports group4? — tahoar, Jun 27 '11 at 05:40

score 18 · Answer 1 · edited May 23 '17 at 12:25

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I found the complete solution in this answer "Write TIFF file in python from String". The function must include "and 255"

threshold = 191  
im = im.point(lambda p: p > threshold and 255)

edited May 23 '17 at 12:25

Community

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answered Jun 27 '11 at 09:19

tahoar

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score 2 · Answer 2 · answered Jun 26 '11 at 16:48

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Try im.point(lambda p: p > 190) and post the results.

answered Jun 26 '11 at 16:48

pajton

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As tahoar says above, you do seem to need the and 255 – Ben May 10 '12 at 13:39

How to I use PIL Image.point(table) method to apply a threshold to a 256 gray image?

2 Answers2

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