Re: Hue Shift From Curves and Levels

Hello Carl,

Am 18.11.07, 12:09 -0800 schrieb Carl Cole:

> I user cinepaint 0.22-1 on Debian, built from the tarball.
> 
> I recently read this article 
> _http://www.luminous-landscape.com/essays/curves.shtml_, "Do Curves 
> Throw You a Curve?" by Mark Segal, where Mark mentions the Adobe Camera 
> Raw hue lock. "Curves have a hue lock. They map the minimum and maximum 
> RGB values (in linear ProPhoto RGB) through the tone curve, and the 
> middle value is interpolated to exactly preserve hue."

Some strategy to let the brightness adjustments behave more perceptual is 
long missed. Reads like a reasonable aproach.
 
> I notice that my hue changes when using Cinepaint curves and levels. So 
> I mocked up a simple linear interpolation for curves and levels, and it 
> seems to work just slick. I am happy to contribute my code if you would 
> like it. However, I realize that I have several questions regarding the 

Yes, of course there is a quite some interesst for improving such things.

> technology of color:
> 
> 1) Linear Interpolation: Should the interpolation be linear? Should the 
> interpolation be differently nonlinear depending on the workspace 
> colorspace? When do we need to worry about gamma or about the relative 
> "weights" of the colors (Charles Poynton's Color FAQ Question 9, 
> _http://www.poynton.com/ColorFAQ.html_)?

Stronger saturated colours should be more dependent on gamma. The 
images editing colour space should be considered. 

> 2) Trade offs: When locking the hue in this manner, is something given 
> up elsewhere?

Basic channel manipulation should remain possible.
One approach, I can imagine how to present, is to add modifiers 
like toogle buttons to the Gui. Having your code switched on by default 
would be what most people expect.
Simply adding a new curve and name it to something reasonable would be an 
other way. 

> 3) How to Test: What are some good test images to show the potential 
> trade offs? I found this one, Printer Test File at 
> _http://www.digitaldog.net/tips/index.shtml_. In particular, the rainbow 
> bars at the top are shifted by curves, but appear to be fixed by my 
> re-interpolation.

Thats good.

> 4) Cinepaint Colorspaces: The Mark Segal article mentions the ProPhoto 
> colorspace. I don't notice an ICC profile by that name within Cinepaint 
> 0.22-1. Is ProPhoto available for Cinepaint?

CinePaint comes with no profiles. One have to install some from elsewhere. 
Provided that CinePaint uses Oyranos, the Oyranos profiles would be quite 
good. For a wider gamut profile use LStar-RGB. It should officially be now 
replaced by ECI RGB v2.

Provided that I could not find a download link to ProPhoto even not here
http://www.luminous-landscape.com/tutorials/prophoto-rgb.shtml
I'd expect it is not distributeable. 
Can we talk about profiles licensed for free distribution? The ECI ones 
are considered quite standard. Adobe at least provides a free download 
page and linux packages for theire profiles. They provide sufficient gamut 
to handle most colours.

> Thanks, in advance.

Could I see a prototype patch? If you would prefere then as well offline.
 
kind regards
Kai-Uwe Behrmann
--
developing for colour management 
www.behrmann.name + www.oyranos.org


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