Re: In search of a D50 Editing colorspace
Re: In search of a D50 Editing colorspace
- Subject: Re: In search of a D50 Editing colorspace
- From: Peter Miles <email@hidden>
- Date: Thu, 04 Sep 2014 06:54:09 -0700
Hi list members.
I realize I might not have been explaining my self very clearly. And the thread of logic behind what I see as happening at our print facility is spread through a string of posts. So please pardon me for repeating myself while I put it all together in a single post. Also in the interests of clarity, rather than continually prefacing all my assertions with "… in my limited understanding..." I will do it once, here. Feel free to point out my mistakes. If what I have been trying to say is clear to you already, please ignore this post.
Goal
My intention (in part) is to measure the color of a real world object for a given observer viewing illuminant. Calculate the absolute RGB coordinates for that color in a given RGB color space. And then absolutely render that color on a paper surface for viewing under D50 viewing conditions. Printing using Adobe color management.
1: measure the color of a real world object for a given viewing illuminant.
this I'm doing in Spectrashop4 and an i1pro
2: Then calculate the RGB coordinates for that color in a given color space.
this I'm doing in Spectrashop4
3: Absolutely render that color on a paper surface for viewing under D50 viewing conditions. Printing using Adobe color management.
Because of the way Adobe (mis) handles absolute colorimetric rendering for RGB color spaces, this step is not as easy as it could be.
Converting colors from one color space to another is actually a two step process.
(except in the case of device link profiles)
Step1: Scale the colors encoded within the source RGB space into the Profile Connection Space (PCS).
An Absolute colorimetric transform will preserve the color meaning of pixels as they are first moved from AdobeRGB to the PCS.
However what Adobe actually delivers is a relative colorimetric scaling instead. in the case of AdobeRGB with relative colorimetric scaling, its D65 white point gets mapped to the D50 white point of the PCS.
In this mapping process the color meaning of pixels embedded within the AdobeRGB space get scaled too, and so the original color meaning of them gets lost.
But…
in the case of ProPhotoRGB the relative colorimetric scaling Adobe delivers results in the white point being scaled from D50 of the RGB space to D50 of the PCS. This is a null transform. So with source RGB spaces that have a D50 white point, the color meaning of the RGB pixels embedded within them is not altered. We effectively have an absolute colorimetric rendering.
Step2: Scale the colors encoded in the Profile Connection Space (PCS) into the destination (printer) colorspace.
Requesting an Absolute colorimetric transform will preserve the color meaning of pixels as they are moved out from the PCS into the destination printer colorspace.
If the printer colorspace is CMYK then this is exactly what Adobe does.
If the printer colorspace is RGB then an Absolute rendering request to Adobe will deliver a relative colorimetric rendering instead. The color meaning of pixels changes as the D50 of the PCS get scaled to the white point of the media. Again the original color meaning of our pixels get lost.
Conclusions
Printing absolute colorimetric from an RGB color space will work, using Adobe color management…
1: Only if the source RGB space has a D50 white point.
2: if the destination printer colorspace is CMYK. (or non-RGB)
Hence my interest in D50 editing colorspaces and moving away from D65 based ones.
Hope that s a bit clearer.
Best regards
Peter Miles
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