The Curves Tool

The curves tool is a very basic tool that can be used to improve photos with a few clicks of the mouse. It is used to change the brightness and contrast of an image. It can also modify the separate Red, Green and Blue channels of an image too. The Curves Tool has a histogram to represent the shadow, midtone and highlight detail in the image. In the GIMP, you access it by right clicking on an image and go to Colors->Curves.

This is the second article in my GIMP for Photographers series, but as usual, all of this applies to Photoshop, or any other image application with a Curves Tool. The first tutorial was on The Levels Tool, and worth a read if you missed it!

Here’s an image I shot at the Lord Mayor’s Picnic in Fitzgerald’s Park a few months ago, and the Curve Tool below it. Notice the histogram? The photo is fairly well exposed, but some highlights are “clipped”, as the histogram hits right hand side without sloping off.


Brightening Curve

It’s easy to brighten an image. Just drag points on the line up.


Darkening Curve

Now, let’s darken the image by dragging points down.


Contrast Curve

A classic use of the Curves Tool is to increase contrast in an image. You do this by darkening the shadows, and brightening the highlights. The curve looks sort of like an “S” when you do this. Don’t go overboard on this though, because it’s easy to lose detail in either direction.


If for some reason your image has too much contrast, a quick inverted S curve will solve that problem,


Wacky Colours

You can select any of the Red, Green and Blue channels and do strange things to your photos. Here’s what happens when you play with the Red Channel.


And here’s what happens when you change multiple colour channels in different ways.

Colour Picker

After you have opened the Curves Tool, click anywhere in the image. Notice how a vertical line goes up and down the histogram/line? That vertical line is the colour of the pixel where you clicked. That can be useful if you’re trying to modify a particular part of a photo. This is what you get when you click on the black coat on the left of the image above.

Aperture ƒ/6.3
Camera Canon EOS 20D
Focal length 18mm
ISO 200
Shutter speed 1/250s

GIMP For Photographers: Levels

GIMP Colors Many people find using the GIMP or Photoshop a daunting prospect but in fact those packages are quite easy to use once you’ve practiced a few times. This will be the first in an occasional series to help photographers use the GIMP to post process their photos.

The Levels tool (right click on your image, select Colors, then Levels) is used to adjust the levels of the colours in your image by manipulating a histogram representing the image. In simple terms, you can make broad changes to the Red, Green, Blue and overall “Value” parts of your image.

The single most useful function of the Levels tool is the “Auto” button. Click that button and the histogram will be stretched out. Your image should look better. If the photo lacked contrast, it can suddenly become a lot “punchier”!

Here’s an example which will make things clearer.

GIMP Levels before example

This is a nice photo I took in Galway in 2005 with a Sony 717. Unfortunately, there’s a nasty yellowish sheen to the image. I probably shot this with the white balance set to cloudy. That can give a pleasing golden look to images but it’s not always welcome. The image also lacks contrast and looks under exposed. How do I fix that?

GIMP Levels

Fire up the GIMP Levels tool. Right click on the image, go to Colors, then Levels. This is the histogram for the image above. See how it’s all bunched into the middle? Now, click on the “Auto” button.

GIMP Levels after example

Wow! One click did that? The image looks so much better now! The swans actually look white and it’s brighter and shiny!

GIMP Levels

I opened the Levels tool again, just to see what effect “Auto” had on the histogram. Sure enough. It’s stretched from side to side.

Levels before and after
Before and After Auto Levels

Wasn’t that easy?

Advanced Usage
You may have noticed the eyedropper buttons next to to the Auto button. Those are “Black”, “Grey” and “White” selectors. Click on one of those, your cursor will change to a eyedropper and then click on the corresponding colour in your image. They work pretty well, but can be confused. If it all goes wrong, just click the Reset button, or CTRL-z to undo if you’ve clicked OK.

You can also manipulate the histogram manually. Just drag the sliders left and right until your image looks ok. You can change the channel with the drop down at the top of the Levels dialog. Changing individual channels does interesting “cross processing” things to an image.

External links:

  1. The Levels tool on gimp.org docs
  2. Levels Tool – white, black and grey to the rescue! – a tutorial I wrote about the levels tool back in 2004!

Want to know more? Leave a comment. I’d love to hear what you’d like to know and learn about the GIMP.