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From: zap@lysator.liu.se (Zap Andersson)
Newsgroups: comp.graphics
Subject: Re: RGB to gray
Message-ID: <406@lysator.liu.se>
Date: 20 Nov 90 06:20:35 GMT
References: <9011200035.AA21751@lilac.berkeley.edu>
Sender: news@isy.liu.se (Lord of the News)
Organization: Lysator Computer Club, Linkoping University, Sweden
Lines: 60

twdorr01@ulkyvx.BITNET (ThomasD) writes:

>In some article andrew@ee.su.oz.au (Andrew Ho) writes:
>>  I am trying to convert some full color images (RGB) into
>>gray level images (because I hope to get some non-colorful
>>laser printouts after doing the convertion).
>>
>>  Are there any algorithms/lookup tables to do the "RGB to
>>gray level" matching ?

If I understood your question correctly:

  Gray = 0.39 * Red + 0.50 * Green + 0.11 * Blue...

...says the NTSC spec. I _*NEVER*_ got this to look goot, coz to my mind the
blue turns out WAY to dark. Perhaps my eyes aren't NTSC standard, right? I use
a beefed 0.30 Red + 0.50 Green + 0.20 Blue coz i like it better...

>Or, (you're trying to print the RGB image to a laser printer, right?),
>what I did in my laser routine was to use the common value as an index (in
>conjunction with the X and Y coordinate of the pixel) into a look-up table
>of bit map pixels (on and off) that matched the greyscale value as closely
>as possible.  The results were very nice, but I suspect there's a better way
>to do it.  Anybody know?

>ThomasD

Converting RGB to Gray is easy, as in the above... converting GRAY to halftone
(i.e. black OR white dots like from a laser) ain't so easy...

Dithering of sorts can be used, whoa, this is a whole field of science here..
You could use patterned dithering, either rastered like in a newspaper,
where small dots grow larger as they grow darker, or you could use a
dithermatrix where a gray pattern is

  50%          75%       25%
01010101    01010101  00000000
10101010    11111111  10101010
01010101    01010101  00000000
10101010    11111111  10101010


But a pattern like this often introduces incredibly ugly stripes and moire
effects.... Well, like I said, it's a science. My best shot at it is to
convert the durn thing to postscript, and thump it in to the printer.
That'll leave the problem to the printer (or at least the printer
manufacturer ;-)


/Z
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