Make Your Photos Better Using GIMP Curves

How to Use GIMP Curves to Improve a Photo

If you enjoy taking photos with your digital camera, but sometimes don't achieve the results you hoped for, knowing how to use the Curves feature in GIMP can help you to produce better looking images.

The Curves feature in GIMP can look quite intimidating, but it is very intuitive to use. In fact you can get good results from fiddling with Curves without really understanding what you are doing. In the accompanying image, you can see the original photo on the left with poor contrast and how it has been improved markedly on the right by making a Curves adjustment in GIMP. You can see how this is achieved on the following pages.

Open the Curves Dialog in GIMP

Once you've opened a photo that you think has poor contrast, go to Colors > Curves to open the Curves dialog.

You'll see that there are a number of options available, but for this exercise, ignore the Presets, ensure that the Channel drop down is set to Value and the Curve type is Smooth. Also check that the Preview box is ticked or you won't see the effect of your adjustments.

You should also see that a histogram is displayed behind the Curves line, but it isn't important to understand this as we're just going to apply a simple 'S' curve.

Note: Before making adjustments to your photos, it may be advisable to make a copy of the original or even duplicate the background layer and edit this before saving a JPEG of the adjusted photo.

Adjust Curves in GIMP

An 'S' curve is a very simple way to make an adjustment with the Curves feature of GIMP and this is probably the most commonly made Curves adjustment in any image editor. It is a very quick way to boost the contrast of a photo and also tends to make colors appear more saturated.

In the Curves window, click on the diagonal line somewhere towards the right hand side and drag it upwards. This lightens the lighter pixels in your photo. Now click on the line towards the left and drag it downwards. You should see that the darker pixels in your photo have darkened. You should take some care not to make the effect look too unnatural, though this depends on taste. When you're happy with the effect, just click OK to apply the effect.

What is the Histogram?

As mentioned, the Curves dialog displays a histogram behind the Curves line. You can read more about what a histogram is in this definition of a histogram.

In the image, you can see that the histogram only covers an area in the middle of the window. That means that there are no pixels with very dark or very light values contained in the image – I reduced the contrast of the photo which has caused this effect. This means that the curve will only have any effect when it is within the area that is covered by the histogram. You can see that I have made some very extreme adjustments in the areas to the left and right of the curve, but the image behind seems largely unaffected because there are no pixels in the photo with matching values.