AW: [ECI-EN] RE: Why ECI-RGB
by Bestmann, Guenter RD-PN32
Hello,
taking two RGB step wedges and assigning eciRGB and AdobeRGB to these images and then making a profile conversion to CMYK with rendering intent Absolute Colorimetric gives two different images: one is a little bit yellowish (eciRGB), one is very strong bluish (AdobeRGB). This is caused by a wrong MediaWhitePoint in AdobeRGB. The ICC specification says that the white point should be D50 or something measured with a D50 light source. In AdobeRGB the white point is D65. So you should not use AdobeRGB together with rendering intent Absolute.
A lot of RGB profiles delivered with Photoshop have wrong MediaWhitePoints. As long as you use these profiles only with rendering intent Perceptual or Relative Colorimetric all works fine, but not with rendering intent Absolute.
Regards,
Guenter Bestmann
-----Ursprüngliche Nachricht-----
Von: eci-en-admin(a)lists.transmedia.de
[mailto:eci-en-admin@lists.transmedia.de]Im Auftrag von Johannes
Hoffstadt
Gesendet: Dienstag, 6. Juli 2004 19:21
An: eci-en(a)lists.transmedia.de
Betreff: Re: [ECI-EN] RE: Why ECI-RGB
><ECI RGB is based on D50 instead of = D65 it works better as a editing
>space for prepress work.>
>
>I this why, for example, when an Image is converted from Adobe RGB to
>Whatever-CMYK using absolute colorimetric, a major color shift occurs,
>while this is not the case with ECI RGB (not that I'm often inclined to
>do such a conversion), or is there something else invlolved that I am
>unaware of?
>
>Michael Eddington
>
>
>
Mike,
that's about half the truth - because absolute colorimetric transforms use
the white points of your source RGB as well as your destination CMYK
- and now comes the important part - to keep the colors the same! (More
specifically, the Lab values.)
So why is there an apparent color shift? There shouldn't be any.
It's because Photoshop will normally display colors relative to the white
point. So your RGB picture is going to look similar (with respect to a
color cast) with both Adobe RGB and ECI RGB, but the absolute Lab
values are quite different, on the bluish side for Adobe RGB - but all
that blue is taken out when Photoshop displays the image.
I cannot remember if there is a way to force Photoshop to show absolute
colors for RGB matrix profiles like Adobe RGB and ECI RGB.
When you then convert to CMYK, you suddenly see that blue.
But only if your CMYK paper white is next to neutral. Then the
abscol conversion will need to put some cyan on to keep the bluish
colors in the case of Adobe RGB. It doesn't matter too much that
Photoshop displays relative to the paper white because its almost
neutral.
But should your CMYK paper white be rather blue (lots of fluorescent
whitening agents, for example), the Adobe RGB won't need that cyan,
the blue cast is already there. The ECI RGB, on the other hand, with its
generally warmer colors, will lead to lots of yellow in the CMYK.
The absolute color should be neutral (bluish white + yellow), but
Photoshop displays yellow because it discounts the bluish paper white.
Confusing. But you can make Photoshop show absolute colors for
CMYK profiles: use "proof setup", create a custom setup with the
same profile that is assigned to your data, and check the box
"simulate paper white." In the case of the bluish paper, the wrong
yellow cast will disappear. Photoshop no longer treats the paper as
being neutral.
Hanno Hoffstadt
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