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Re: RGB to CMYK

• To: mathgroup at smc.vnet.net
• Subject: [mg25547] Re: RGB to CMYK
• From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
• Date: Sat, 7 Oct 2000 03:35:51 -0400 (EDT)
• Organization: Universitaet Leipzig
• References: <8rjik5$odn@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

Hi,

with version 4.x you can use

?ToColor

"ToColor[color, form] converts color to form if form is GrayLevel,
RGBColor \
or CMYKColor color is converted into it. Otherwise form[color] is
evaluated \
and the result is expected to be a valid color directive."

In[]:=ToColor[RGBColor[1, 0, 0], CMYKColor]

Out[]=CMYKColor[5.46908`*^-6, 0.9958244690799987`, 0.9958244690799987`,
5.46908`*^-6]

The correct conversion from RGB to CMY is

c=1-r
m=1-g
y=1-b

and from CMY to CMYK

k=min(c,m,y)

c'=(c - k)/(1-k)
m'=(m - k)/(1-k)
y'=(y - k)/(1-k)

Regards
  Jens

PS: Yes, the ToColor[] function is not documented in "The Book"

Bernd Brandt wrote:
> 
> Hi Group,
> 
> I am looking for a conversion function between RGB and CMYK.
> I thought this would not be a problem but i have not found one good function.
> 
> The Mathematica book (examples) includes:
> 
> CMYKfromRGB[RGBColor[r_, g_, b_]] :=
> CMYKColor[b, r  b, 1 - b, (r + g + b)/3]
> 
> But this is not correct.
> RGBColor[1,0,0] really red, is converted to CMYKColor[0, 0, 1, 0.5],
> which is clearly not red.
> 
> Does someone have a correct function?
> 
> Regards,
> Bernd