This is most likely old hat to most of you here, but I figured for people new to GIMP it might be helpful to have a small walkthrough tutorial of what the curves tool does, and how it can be applied against images for toning/grading.

The example is over the top on purpose (to illustrate what is going on), but I have a follow up coming that looks more closely at a serious application, as well as some film emulation (especially what Petteri has done over over at his blog).

Very impressive and informative. Excellent job! I use Curves extensively but only for value in making chrome settings. I never knew you could do so much with Curves. You just opened up a whole new world for me. This sort of explanation should be incorporated into the Gimp Help file or something so people learning Gimp for the first time can understand this tool better. Thank you for making this tut.

Thanks, I'm glad it's useful!

Wow! These are pretty in depth tutorials. Well worth studying.

Nice site Pat, some helpful info there. The link to Petteri's has good info as well.

@patdavid:
thanks for your infos and the funny talk about orangeblue invasion.
I should risk myself in using curves on RGB channels more often, because when I asked myself, while reading your tut', how would I increased yellow ? I thought: Strengthen the blue and the green curves ! But... It doesn't work that way

Note: I've also bookmarked that "philosopher" font you used for your blog, she's lovely.

Glad it's been helpful! I'm always game for input/corrections/suggestions/criticisms as well, so please feel free to fire away!

And yes, I lucked into finding Philosopher, and am quite taken with it. Nice and clean, with a unique set of serifs and a good sense of weight.

Now that I use color curves more often, I'd like to know if I have a monitor calibration issue of if it is simply a drawback of editing color curves: I run into a "green problem" when editing, every time I manipulate the green curve it greatly increases the value, to counterbalance I use the value curve to diminish this value jump. And it's a bigger issue when I use the "color balance" tool (no possible value adjusting, even with the "preserve brightess" check-marked).

So, how would you adress this ?
Editing curves on a value mode layer ?

I am not 100% sure exactly what you refer to when you describe the "green problem"...

Is it a grayscale image you are editing, or a full color one? And if it's color - does the green curve significantly effect the overall value in that channel in a strange way (numerically, or to your eye)?

Green is the heaviest component (in RGB) for effecting our perceived relative luminance of a color (green will contribute twice as much as red and blue combined in the final luminance our eyes perceive).

Do you perhaps have some examples of the problem we could see to help with?

It seems this fact I was unaware of adresses my "green problem".

Trying to be more precise, what I experience is that the green curve, when I edit a full color image, heavily modifies the overall value of my image (here, speaking of what my eyes perceive, a perceived value boost), way more than the blue or red curve, forcing me to finetune the value curve immediately after adjusting the green curve in order to balance the overall value. So, working with green makes things more difficult, but, if I read you correctly, it is unavoidable.

As for examples, well, I don't know how to easily demonstrate this with an image... But maybe you already got my point ?