[Image-SIG] Image.convert() doesn't dither?

Bill Janssen janssen at parc.com
Tue Oct 25 19:44:40 CEST 2005

>     When converting to a bilevel image (mode "1"), the source image
>     is first converted to black and white. Resulting values larger than

I think it would be clearer to say "the source image is first
converted to grayscale."

>     Resulting values larger than
>     127 are then set to white, and the image is dithered. To use other
>     thresholds, use the point method.

This doesn't make sense to me.  If I take a four-color block, the four
colors having luminance values of 0, 33%, 66%, and 100%, the above
process would give me an image which is half gray (actually, black)
and half white, with some dithering on the gray half.  Instead, I get
four carefully delineated blocks, two of which are dithered gray, one
of which is white, and one of which is black.

I think it makes more sense to say, "the image is dithered to bitonal
black and white using the Floyd-Steinberg error diffusion filter and a
threshold value of 127."


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