definition and separation of colors

definition and separation of colors

[Henning Hraban Ramm]

I stumpled upon a ambiguous color definition in my automatically  
generated documents:

\definecolor[Custom][c=0,m=1,y=0,k=0,r=0,g=0,b=0,]

I.e. the color is defined in CMYK mode as 100% magenta and in RGB as  
0/0/0 black.
In the PDF the color is shown as black, according to Acrobat's  
preflight defined as RGB 0/0/0; CMYK separation depends on the  
profile, e.g.:
Generic CMYK:        76/71/65/81
ISO Coated FOGRA27:  86/85/79/100
Euroscale Coated:    95/83/82/90
(I didn't expect the Euroscale Coated profiles to differ that much!)

If I say \setupcolors[rgb=no], I get CMYK 100/100/100/0.
This is not a good separation algorithm - GCR or UCR would be nice,  
but at least RGB 0/0/0 should render as CMYK 0/0/0/100.
(Perhaps you could include something like littleCMS in LuaTeX and even  
deal with images?)

It doesn't change if I define in different order, like  
\definecolor[Custom][r=0,g=0,b=0,c=0,m=1,y=0,k=0,]
I get magenta only if I leave out the RGB values.

It seems that in ConTeXt the RGB definition always takes precedence?  
Is this intended?


Greetlings from Lake Constance!
Hraban
---
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https://www.cacert.org (I'm an assurer)

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Re: definition and separation of colors

[Hans Hagen]

On 16-11-2010 9:37, Henning Hraban Ramm wrote:
> I stumpled upon a ambiguous color definition in my automatically
> generated documents:
>
> \definecolor[Custom][c=0,m=1,y=0,k=0,r=0,g=0,b=0,]

you should not mix definitions (so, either cmyk or rgb or g of h of hsv 
keys)

> It seems that in ConTeXt the RGB definition always takes precedence? Is
> this intended?

just a side effect of definitions

when a rgb color is defined, internally conversions to cmyk and gray 
take place and depending on the color space one of them is used

when an cmyk color is defined, ....

Hans


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