AW: [ECI-EN] RE: Why ECI-RGB

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 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 _______________________________________________ ECI-EN mailing list ECI-EN(a)lists.transmedia.de http://lists.transmedia.de/mailman/listinfo/eci-en