RE: [ECI-EN] ECIRGB versus LstarRGB 2
by Bestmann, Guenter RD-PN32
Hello!
The starting colour space of PrintOpen is something like the envelope of the so called Pointer Colours. Pointer colours are a list of natural colors. Colors outside this large colour space (artificial colors) are clipped to the bounderies of the gamut of the printing color space. Colors inside this space (natural colors) but outside the gamut of the printing color space are mapped into the gamut and colours inside the gamut are slightly compressed. The compression curves are know how derived from cmyk color scanners.
Other profiling softwares use different starting gamuts and different compressions. You can see this when transforming highly saturated images with profiles based on the same characterization data set (f.e. FOGRA27L.txt) from ECI (ISOcoated.icc, PrintOpen) or from Gretag (ISOcoated...icc, ProfilMaker) or from Adobe (EuropeanOffset...FOGRA27.icc).
Guenter Bestmann
-----Original Message-----
From: eci-en-admin(a)lists.transmedia.de [mailto:eci-en-admin@lists.transmedia.de] On Behalf Of Marttila Jouni
Sent: Wednesday, June 01, 2005 1:20 PM
To: 'eci-en(a)lists.transmedia.de'
Subject: RE: [ECI-EN] ECIRGB versus LstarRGB 2
Chris!
I have been following this conversation and I must admit that I have had the same misperception about gamut mapping. So I went to test this straight away and I must say that you are right. So this mean that ICC-profile software manufacturers assume some kind of wide colour space to start with. And this also means that if one is using small RGB space and perceptual rendering one never fully utilise the whole CMYK colour space.
Is there somewhere info what kind of starting colour spaces different software (e.g. Gretag ProfileMaker, PrintOpen) have? It must be quite large
- otherwise colours that would be outside this assumed colour space would be clipped?
BR Jouni
-----Original Message-----
From: eci-en-admin(a)lists.transmedia.de
[mailto:eci-en-admin@lists.transmedia.de] On Behalf Of Chris Murphy
Sent: 1. kesäkuuta 2005 12:46
To: eci-en(a)lists.transmedia.de
Subject: Re: [ECI-EN] ECIRGB versus LstarRGB 2
On Jun 1, 2005, at 3:05 AM, Karsten Krüger wrote:
>
> When using perceptual rendering, the whole source gamut has to fit
> into the destination colorspace. Rendering does not look for how much
> of the source gamut your image is using. If it would do so you would
> receive different results when moving from image to image. So when
> your source gamut is huge, it gets compressed a lot when rendering to
> a small CMYK output gamut.
This is not an uncommon misperception. The reality is that gamut compression is not dynamic. The CMS does not do "more" gamut compression for a large source space because it defers to the gamut compression that's "pre-baked" into the output device profile. Gamut mapping in the output profile is done at the time the profile is built, when a source space is not even selected yet.
If you take an sRGB image and convert it to ProPhoto RGB, then convert both to CMYK you will get effectively identical conversions. You do not get more gamut compression by virtue of having your image in a giant color space. Color management isn't that smart.
>
> This does not happen with colorimetric rendering (either absolute or
> relative). In this case everything outside the smaller output
> colorspace is just clipped to the edge of it. If your image is small
> enough, you don't see any problem. If it is bigger, you end up loosing
> details.
There is still some gamut compression with relcol otherwise we'd have images with even a little bit of out of gamut colors that would get totally clobbered by clipping. But there should theoretically/ideally not be much gamut compression for in-gamut colors. Only out of gamut colors would get compressed.
> Take a picture of neon advertisement at night or flowers with vivid
> colors in bright sunlight with an EOS 20D. Render them to ISOcoated.
> Using photographic rendering intend you get decent results. Using
> colorimetric rendering intend you end up loosing a lot of details and
> the emotions from your pictures are gone.
I've experienced this also. But practically speaking, the vast majority of images in the world do not have highly saturated fine detail that's really important - we can accept some detail loss or the detail isn't highly saturated. So relcol works very well for most images, including those in ProPhoto RGB.
> I like to use an other setup. Why should I keep and manage data I am
> not able to reproduce.
You can't reproduce it today perhaps. That doesn't mean you won't be able to reproduce it tomorrow. A majority of images destined for print to a press aren't likely to get reused ever again, so it's not a big deal to limit them from the get go. I have no problem with this. I'm just saying that there's a good reason to use a wide gamut RGB space that is not normally talked about, and that's about shadow detail. I'm not looking to tell people what they are using successfully today is a bad choice and that they should change. It depends on what you do and what's important to you.
Chris Murphy
Color Remedies (TM)
www.colorremedies.com/realworldcolor
---------------------------------------------------------
Co-author "Real World Color Management, 2nd Ed"
Published by PeachPit Press (ISBN 0-321-26722-2) _______________________________________________
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