[Gimp-developer] Requests for curves for photo retouching

[Stephane Chauveau]

Andrea Olivotto wrote:
> Hello to everyone!
>
> I'm Andrea from Italy, this is my first post.
>
> I'm an amateur photographer, and I'm using GIMP to retouch my photos. I 
> wrote some articles in italian on the argument, and I'm here to suggest 
> some hints for this beautyful program.
>
> Histogram:
>
> - It's an invaluable tool for photo retouching, for valuating contrast, 
> brightness, clipping (even channel by channel), posterization.
>   
I just want to warn you that using the Curve tool for photo retouching 
is not always a good idea because the so-called 'Value' curve  is 
nothing else than the same curve applied to the Red, Green and Blue 
channels. In practice, that means the 'Value' curve does not preserve 
the Hue and the Saturation at all.
The curve tool should be fine for small adjustments but for serious 
works, you should prefer the brightness/contrast and 
hue/lightness/saturation tools.

If you are not convinced then try the following:
  - Fill your image with a color composed of  100% Red, 50% green and 0% 
Blue. That should give you a nice orange.
  - Open the 'Value' curve and add a point in the middle.
  - By moving the 3 points of the curves (left,  middle and right) up 
and down, you now control the Blue, Green and Red components of the 
image color. You should be able to produce all colors and when I say all 
I mean ALL: blue, pink, yellow, white, purple, black...

You will also notice that the 'value' reported by the curve when 
clicking in the image has little to do with the colors that are really 
affected by the curve: In my example, the initial color has a 'value' of 
100% but it is affected by moving the left (value 0%) and middle (value 
50%) points.