Hue Shift From Curves and Levels

Hi Cinepaint Developers,

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."

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 
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_)?

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

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.

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?

Thanks, in advance.

Regards,

Carl Cole


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