basic color management question


(Peter Bardsley) #1

I would appreciate advice or assistance with the following issue. I have generated an RGB black and white profiling target in Colorport, which I can print (Epson ABW Advance Black and White on an Epson P800) and measure with an i1pro. I would like to linearise output with a curve in RT. This seems straight forward.
The problem is that when I read in the tiff target file into RT the RGB numbers in the navigator do not reflect the correct values (not even close) that I know are in the file. Colorport generates a tiff in a gamma 2.2 space, and I can verify these numbers exactly in photoshop, applying Adobe RGB1988 or any other gamma 2.2 profile. I have read the RT manual, and tried all sorts of permutations to the color management settings. I have imported the file with an embedded AdobeRGB profile, and also tried importing it untagged. I would like see the correct values shown as a baseline for the target before I attempt any printer linearisation.
Any advice or assistance would be welcome.


(Karlheinz Lehmann) #2

RGB values are shown as percentage value. If you click in the histogram on one of the rgb values it switches the representation to [0-255]. Maybe you will then recognize the values …


(Morgan Hardwood) #3

Let me start off by saying that no color management question is basic. Now I will go forth to eat breakfast.

In the meanwhile you can upload all relevant files (ICC/DCP, target, etc) using https://filebin.net/

Ping @Claes


#4

Morning, @Peter_Bardsley,

I fully agree with @Morgan_Hardwood: do not ever call anything concerning colour management basic, simple, easy, &c, &c, &c :disappointed_relieved:

About a year ago, I experimented with all-black-profiles for my colour printer, so I believe that I understand what you would like to achieve (which is a good start).

Are you on Windows, a Mac or on Linux? Which?
To begin with, please upload the files that Morgan mentioned, plus some indications of expected values.

Have fun!
Claes in Lund, Sweden


(Peter Bardsley) #5

Dear all,

Thank you very much for the interest and prompt response.

I have some progress to report; user error (myself) seems to be the problem. There is nothing like writing up a question to prompt the brain to sort something out.

Investigating further, I generated some stepwedges in photoshop and imported them into RT; this time values were correct (using the embedded profile). Going back, I think that the problem arose because the profile was not properly embedded although I thought that it was. It seems that even if one tells PS to use a particular profile when opening an untagged file (and it shows up in the PS information bar) it forgets it when saving the file unless one formally applies the profile.

Anyway, I do not want to waste anybody’s time at this stage as things seem to be working so far, so I won’t upload any files.

I do appreciate the interest and I will report back with further progress (or problems).

Regards

peter


#6

Morning, Peter,

In case you need clever black and white test images, this link might be of interest: http://www.northlight-images.co.uk/digital-black-and-white-photography/

Have fun!
Claes in Lund, Sweden


(Peter Bardsley) #7

Dear all, thank you for your responses. I have looked further into the problem and this is what I can report.

The good news: I can get accurate results if I use the output profile to render the main histogram and Navigator, provided that I tag the input file (that is embed a profile), choose to use the embedded profile, and set the same profile as an output profile. I can also set a different output profile: then the input file is converted to this profile (Lab values are preserved, RGB values adjusted). If the input file is untagged, then I am OK if I use as custom pofile the profile of the space in which it was created and also set this (and only this) as the output profile. Changing the working profile has no effect. This behaviour is more or less as expected. The bad news: no other settings seem to work. In particular, if I choose the option to use the working profile to render the main histogram and Navigator, then results are completely unpredictable.

What I would expect: if I choose to use the working profile to render the main histogram and Navigator, then I would expect this request to be honoured (if my input file is correctly tagged gamma2.2, and the working profile is gamma2.2, then dispayed RGB and Lab values should be correct). If the input file has an embedded prifile then this should be respected (either converted to the working profile or with the working profile applied, in photoshop terminology).

It would be useful if somebody could clarify this for me. Is this behaviour as expected? Could somebody explain how profiles are actually managed, and the rationale? Obviously I have found by trial and error some settings that work. But I would be more confident if I understood how the system worked as a whole. As requested, I have uploaded relevant files to Filebin 3r3wrbeyukj6z3b4

Regards
Peter


#8

In case you have not seen this:
http://rawpedia.rawtherapee.com/Color_Management

/Claes in Lund, Sweden