Re: PS CMYK Conversions
Re: PS CMYK Conversions
- Subject: Re: PS CMYK Conversions
- From: Roger Breton <email@hidden>
- Date: Thu, 14 Aug 2008 12:01:46 -0400
Those are three interesting statements, Derek.
> No, as opposed to getting accurate colours in CMYK.
Accurate as in "absolute" accuracy?
Then we're talking about "absolute colorimetry", the only way I know to
convert from RGB to CMYK to get the closest from "absolute" accuracy.
Please correct me if I'm wrong.
> They always shift
> when going from RGB to CMYK,
I would tend to think that it depends on what the starting point of the
color is.
If the color is "in-gamut" and the RI selected is absolute then I don't see
why the colors could shift.
If that's what you have in mind.
But, keep in mind that the starting point is RGB. A lot do happen between
RGB and CMYK. If I want to trace RGB transforms between RGB and CMYK, I will
use a D50-based RGB space. That way, the colors map 1:1 to Lab. And from
there, I can follow what happens to CMYK. Now, a lot do happen also on the
CMYK side. There is the "absolute" white point of the output profile that
will influence the final "landing" point, in CMYK. If I want to trace the
RGB to CMYK conversion, taking every "negative" influence out of the
conversion as much as possible, I will make sure I use a media as neutral as
possible, something like a press paper that is as close to a* and b* = 0.
Otherwise, I won't know what to attribute the final outcome to. I'm refering
to optically brightned papers here.
So, why do colors shift? Many factors that are not necessarily under one's
control. But if all those factors are kept under control then the "absolute"
accuracy is conserved. And you have to convince yourself of this with some
physical samples you observe under strict lighting to the press sheet.
> and you have to spend quite a bit of
> time manually tweaking.
Assuming all of the above is true then this statement does not need to be.
But is that what you had in mind?
Roger Breton
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