Converting from 24-bit RGB to Black&White

Alexander Belchenko wrote:

> I don't understand why solid green color (#00FF00)
converted as mix of
> black and white dots (you can see this in my out1.bmp).
Is this result
> of dithering?

yes.

when converted to luminance (grayscale), green corresponds
to
a gray level, not white or black:

>>> import Image
>>> i = Image.new("RGB", (1,1),
"#00FF00")
>>> i.getpixel((0,0))
(0, 255, 0)
>>> i = i.convert("L")
>>> i.getpixel((0,0))
149

to represent149 in an image that only consists of 0 (black)
and
255 (white), PIL uses a dither pattern to make the image
look gray,
on average:

>>> i = Image.new("RGB", (100,100),
"#00FF00")
>>> i = i.convert("1")
>>> i.getcolors()
[(4140, 0), (5860, 255)]
>>> (4140*0+5860*255)/(100*100)
149

> And why paletted image works as described in 1.1.3
manual?

conversion to P uses dithering too, so it's probably only
your choice
of colors that makes things work as you want when you do
this.

if you want full control of the graylevel-to-bilevel
mapping, and you
don't want dithering, I'd recommend converting first to
"L", and then
using the point method with a suitable lookup table to
convert to
"1":

   im = im.convert("L") # get luminance
   im = im.point(table, "1")

</F>



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