[Scribus] OT simulating CMYK printing in RGB space

Rainer Heilke rheilke
Sun Apr 2 09:36:49 CEST 2006

Hal V. Engel wrote:

>On Saturday 01 April 2006 10:41 am, you wrote:
>>What I tried out was to invert each of the layers, then use the
>>'Addition' layer mode on each layer, and invert the result back again.
>>Theoretically, this should be what I'm looking for, since each CMY-color
>>is the invert of one of the RGB-colors (isn't it?). In practice, it
>>looks like the right result...
>You can use LCMS to create a proofing transform something along the lines of 
>(image CMYK profile) --> (output device CMYK profile) --> (display RGB 
>profile).  The end result is a transform that converts the image from it's 
>original CMYK color space into the display color space with an intermeadate 
>step that will gives you the limited gamut of your intended output device.  
Just to help clarify a little, you _will_ need the profiles Hal 
mentions. This is because, to answer your question above, no, the CMY 
colours are not really the invert of the RGB colours. If you study 
colour theory as it relates to digital devices, the palette of colours 
you get with RGB does not cover the same palette as CMYK. They overlap, 
but each has ranges outside of the other (picture two circles that 
overlap, but don't have the same center point). RGB is an additive 
process, painting by combining light of various frequencies. CMYK is a 
subtractive process, painting with ink--which actually absorbs light 
frequencies. Pantone is yet another colour palette that overlaps, but 
doesn't equal either of the other two. And none of these cover the 
entire palette visible to the human eye. To make matters worse, each 
device--be it monitor or printer--is limited in how much of the RGB 
(monitor) or CMYK (printer) range they can produce, due to manufacturing 
quality and technical trade-off decisions.

The profiles help convert the colours in one palette to those in the 
other. Good profiles may do all sorts of tricks to get an accurate 
match. On screen, the image changes (sometimes significantly) when you 
do the profile conversion. Once you've converted the image to the 
printer profile, the on-screen image may look totally wrong, but it 
prints out correctly.

I hope that explanation helps a little.

Rainer Heilke

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