Gimp 101: Curves II

September 15, 2009

Ok, I wondered if I went from moving the white point to getting the S-Curve in the previous post too fast.

According to Karyn, I did. So, let’s back track, shall we?

Remember we already moved our black point so our curves looked like this:

Move the black point

And we decided that moving the white point blew out the highlights too much.

Remember in Levels we could move our midpoint in order to make the midtones of our photo either lighter or darker, but we only had that one mid-point to move.

With curves, we can grab the line at any point and move it. Remember our histogram? It’s basically a bar graph of how many points in the photo are pure black, pure white and all the tones in between (the ones on the left being pure black and the ones on the right being pure white).

Also, remember I mentioned that moving your Curves line *above* the diagonal will make that point lighter, and moving it below the curves line makes it darker.

So, when I made my curve look like this:

S Curve

I was saying “hey, make the stuff that’s between pure black and about 2/3rds of the way to pure white and make it darker. There’s not much of my S-curve that’s above the line, but what is above the line is the not-quite-pure white part – because I didn’t want really bright highlights.

Remember how each point along the x-axis of your histogram represents a tone in your photo? And how multiple points can have the same tone (thus the “bar graph” of the histogram?) Well, how far above or below the diagonal any point in your curves line is determines how much darker or lighter to make that tone. So, there’s a point where my Curves line is crossing over the diagonal – the tone represented by that point (somewhere between pure black and pure white, but closer to white) will remain unchanged. The tones immediately to the left of that point are going to get just a little bit darker, and then ones to the right just a little bit lighter. And so on.

Let’s take a look at another photo for a different example. Here’s the unedited photo:

Unedited Fire Hydrant

Here’s what the histogram looks like for this photo (from the Curves dialog):

No Adjustments Made

The first thing I’m going to do is move over the black and white points, just like I would have done in Levels. But in this photo, I want the snow to be light, bright, and white, so I don’t have to worry about bright highlights like I did with the sky in yesterday’s photo, so this time I’m going to make more of the S curve above the line and less below:

A Different S-Curve

This results in a final image like so:

Edited Fire Hydrant

I’m not sure if that cleared things up or not? Either way, I think the best way to start understanding Curves is to really start playing with them in your own photos.

(btw, this is completely unrelated to this post, but I do sometimes reply to the comments you guys leave – and I reply within the comments – I’m not sure if you get notified of this if you leave your email address? Anyway, if you’ve ever asked me a question in the comments and I haven’t actually answered it in an entry, chances are I replied to your comment on that page.)

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Filed under: Photo Editing
  • Okay, I think that cleared it up. Thanks for explaining! I'm definitely gonna have to play with Curves, because the blown highlights thing makes me nuts and I never knew how to get around it without, like, selecting different parts of the picture and doing different levels in different spots, which makes some weird blending issues sometimes.

    In fact... I might go find one of the school photos I remember having a particularly rough time with and see what I can make happen.

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