CMYK vs RGB RE: PDF query

[Davis, David]

Are colour conversions really that "complex"?
They're just converting colour coordinates in one colour-space to those in another, using some straightforward formulae that are hidden under the hood.
(The choice of formula, and what to do if a colour is 'out of gamut', depends on the 'rendering intent', of which we usually have a choice of four).
The conversions will give 'round-tripping' errors if they're out of gamut.
This is why better Adobe software (Illustrator, Photoshop, InDesign) maps RGB and CMYK via a device-independent ("LaB") representation, rather than straight from one to the other.
The problem with FrameMaker is that the conversion algorithms it uses are crude rubbish. This is why they frequently give such awful results, not because it's "complex".

>From previous threads on the Adobe Forums, it seems that the conversion formula Frame uses is:

RGB to CMYK:
C = max(R, G, B) - R
M = max(R, G, B) - G
Y = max(R, G, B) - B
K = 100 - max(R, G, B)

CMYK to RGB:
R = min(100 - K - C, 0)
G = min(100 - K - M, 0)
B = min(100 - K - Y, 0)

...which gives rubbish results.

David

____

From: Jacob Sch?ffer (Grafikhuset) <js at grafikhuset.dk>
To: <info at heiko-haida.de>,      "'Craig Ede'" <craigede at hotmail.com>
Cc: 'framers' <framers at lists.frameusers.com>
Subject: RE: CMYK vs RGB RE: PDF query
Message-ID: <00a601cfec93$39b38e10$ad1aaa30$@grafikhuset.dk>
Content-Type: text/plain; charset="utf-8"

It?s definitely NOT easy to explain this. Color conversions are COMPLEX. Anyway, RGB -> CMYK conversion is lossy, and CMYK -> RGB is also lossy.



What is the point of the RGB switch afterall? Why not just pass through what?s input and remove this switch?

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