Gimp 101: Curves I

September 14, 2009

Remember when I talked about Levels? and how you can use it to increase the contrast in your photos?

Well. Get ready to do even more. With Curves!

Remember with Levels, we could adjust the black point, white point and mid-point? With Curves we have the ability to adjust up to 15 points, and because we’re moving these points in a 2-dimensional way instead a 1-dimensional way, we have more control over how we can adjust the photo, too.

First, let’s open up the Curves dialog by going to Colors –> Curves:

Curves Dialog

You can see the same histogram that you did in the Levels dialog, but now we have a diagonal line we can move instead of just three points to move side-to-side. The idea is still the same though – dark tones on the left and white tones on the right. In short – moving anything above the line makes it lighter and moving anything below the line makes it darker. For instance, if we were to take the left-most point and move it all the way up to the upper left hand corner like this…

How to Make the Photo All White (but why?)

…we get an image that’s completely white.

So how would we really use this Curves thing? Let’s take a look at this photo:

Frozen Pond

Let’s adjust the contrast using Curves. First we’ll move the left-most dot to the right – in the exact same manor that we would have moved it in Levels – until we get to the point in the Histogram that it starts to rise up:

Move the black point

Here’s what the image looks like now (original image on the left, new image on the right – it’s a very subtle difference):

Moving Just the Black Point

At this point, if were still using Levels our next option would be to move the white slider to the left, the equivalent in Curves is to move the top point to the left (along the top of the histogram):

Making the line steeper

… which we can do, but since there’s already some blown highlights in this photo, that would blow them out even more (once again, original on the left, edited on the right):

Blowing out some highlights

By the way, notice how my line is steeper now in the Curves dialog – the steeper the line, the higher contrast there is in the photo. As an aside, if you made the line less steep, like so…

Less Contrast

…the photo would actually lose contrast:

Less Contrast

…which I don’t like, so let’s go back to our image. I didn’t like moving my white point, so let’s look at the real power behind Curves over the Levels dialog. In Levels, I could move the midpoint, but there was only ONE midpoint. In Curves, we can create points anywhere on the line to move around, so let’s make an S-shaped curve instead:

S Curve

Check out the difference it makes:

With S Curve

I like this, and so I’ll save this as my final image:

Final Image

There’s a lot more to Curves than just this, but I’ll leave you with that much for now, hopefully it’ll make a difference on some of your photos!

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Filed under: Photo Editing
  • travelour

    Okay, I have a pic with a very washed out snow scene in the foreground and a dark forest in the background. I know there is texture in the snow that I want to bring out. Just can't figure out how to get the curves to do that. Can you do a curves part 3?

  • This was extremely helpful. Thank you! Levels made sense to me. This is definitely helping illuminate curves some more.

  • Just wanted to give you kudos for your site. I can't remember how I found it, but I appreciate the way you explain things in clear language. I'm not a novice, but in some respects I am. For instance, I'm just learning the GIMP. So thanks again & keep it up!!

  • I <3 curves. :)

  • Wait, so, what do the new points in the middle making the S-curve *do*? What contrast is it changing? In words a Kindergartener could understand ;). Because, that made a HUGE difference and looks AWESOME and is way more than levels could do, obviously, but I haven't ever touched curves so I have no idea where to make a middle-dot or how to angle it or anything. Splain more?

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