Contrast

What do you think of the following photograph (Le Fort Carré in Antibes near Cannes on the French south coast)? It is an old photograph, and rather faded - the contrast is too low and needs increasing. This is a straightforward adjustment to make, but first you will need to copy the photo into your space and then open it in the GIMP, as explained on the Basics page.

To adjust the contrast, choose Colors>Brightness-Contrast (you can find the same set of 8 color tools in Tools>Color tools). Make sure that the Preview option is turned on, then adjust the contrast using the slider until the image is as good as possible (check what happens if you use much too much contrast). Do NOT save the modified image for we will see a more advanced method of adjusting contrast in a moment.

By the way, do NOT be tempted to use this panel to adjust the brightness for there is a much better, more sophisticated way which we will discover in the next lesson.

The contrast control offers a good and easy way of increasing the contrast in a picture, but I advise you not to use it to treat pictures that are suffering from too much contrast. This is because the use of the contrast control in that situation would leave you with a picture that contains no pure black or pure white, and most pictures should contain a little of each.

It's a bit difficult to know when you have got just the right amount of contrast so that the above condition (a little bit of black and a little bit of white) is satisfied, so let's get some help from the computer. Do Ctrl-Z to return to the original image - do it several times if necessary or you could even choose File>Revert to make sure you have returned to the version of the image that is on the disk. Now choose Colors>Levels. You should see a complicated panel appear - focus on the 'Input Levels' graph which should look like this:

What this graph shows you is the pattern of brightness in the picture - the height of the 'landscape' at the extreme left indicates how much black there is in this picture (none) and the height of the landscape at the extreme right indicates how much white (none). Our picture contains no pure black or pure white but the 'mountains' towards the right of the graph indicate that there are large areas of the picture which are 'quite bright' (the clouds, boats, water and fort). You can fix the problem of the 'missing black and white' by sliding the two triangles at either end of the graph until they are just within the landscape, like this:

The part of the landscape above or to the left of the left triangle will be pure black, so we can be sure that our picture now contains a little pure black and similarly on the right we have made sure that our picture contains a little pure white. Your picture should now have almost perfect contrast.

You must be wondering what happens when you move the middle triangle - try it and see and you will realize that this is a way of adjusting the brightness of the picture - it's actually better than using the brightness control (!) but usually not as good as using the technique that you will learn in the next lesson. Now put the middle triangle back where it was before you moved it, then save (export!) your image.

Both the methods mentioned above (contrast control and levels control) are good ways to fix pictures which have too little contrast. For pictures that have too much contrast neither method works well - both methods will result in a picture with no pure black or pure white. See the next lesson for a better approach.

Finally, you might be wondering whether the GIMP has an automated contrast 'enhancement' feature -Colors>Auto>Equalize - if you care to try it on the original image you will find that it does an amazingly poor job, making the image much too contrasty. The GIMP also has Colors>Auto>Stretch Contrast but this seems to adjust colors as well as contrast, perhaps in ways you don't want.