Custom Colour Space?!?!

[roy]

I was warming up a picture this evening.  I was using curves because I wanted to warm up the (darker) foreground more than the (lighter) sky and sun.  I was in RGB, with two control points on each of the Red and Blue channels.  I had to adjust the red curve, adjust the blue curve down, look at result, go back and adjust both curves again....... bit tedious and hard to get right.  So I thought.....

How about a custom colour space - user chooses a colour (in this case an orange) and gets a curve based on that very colour?  Actually it wouldn't need to be a full colour space - just a single curve would do the trick.

Or maybe there's already a way of doing this?  (I absolutely love the slider in Lab to rotate both a and b curves together by the way - that's partly what gave me this idea).

[-default]

The idea of a custom color space - or rather mapping one color onto a particular space is an interesting one that I will give some thought to for a later version of Curvemeister.  Two considerations would be the additional complexity of the interface (your single curve idea addresses this), and an exact definition of what this color space would be.  One idea there would be to rotate the a/b plane of the Lab color space, but this would allow hue and saturation adjustments, independent of Lightness.

There are a couple of techniques that I often use to adjust dark tones versus lighter ones.  Using the RGB curve directly, as you discussed, is one method that works very well. 

Another method would be to create an eyedropper sample over the color you want to change, set it to "Hue" only, double click the eyedropper pane to bring up the color picker, and set a warmer color.  A third method would be to use an inverted Lightness channel as a mask, and adjust in whichever color space you needed, fine tuning the mask with a curve.  This would allow you to use the Lab slider, and adjust the hue via the a and b curves.

Anyway - the custom color space suggestion is a good idea.