Editing Digital Photos with the GIMP
I've been using the GIMP for a while now but it was only recently that I started using it to edit digital photos. I started a couple weeks after starting my photoblog. I realized that many of my pictures were coming out too dark, usually because of bad weather, and they needed work. I read a photoshop tutorial on how to adjust photos and started applying the techniques to the GIMP. The results have been great.
I'm sharing this because I think it is useful for anyone who needs to edit their photos but can't/doesn't want to spend the money on an expensive application. GIMP is free (you can't argue with that). I'm going to walk through editing a photo in the GIMP 2.2 to show how easy it is. This is the photo:
As you can see, this photo did not come out good despite the fact that it was a sunny day. The sky looks gray, the grass is dark and the lighthouse is in shadow. The first thing I am going to change is the curves. The dialog for this is under Layer - Colors - Curves. I'm presented with this:
This shows the curves for different levels of brightness throughout the picture. Clicking anywhere in the graph will add a node to the line and tug it towards where I clicked. Two clicks pulls the curve up at two points and the picture is much brighter:
Now I could probably stop here as this picture looks as good as day but there are still issues with the colors and the grays. The next thing I am going to do is adjust the blues specifically. In the top left of the curves dialog, I change "channel" to blue. What I want to do is make the light blues brighter and remove the darker blues from the lighthouse. In the curves graph, the top right is light and the bottom left is dark. I adjust the curve as so, again only requiring two clicks:
Now the blue in the sky is very much to my liking, while the lighthouse has very little blue at all. Once again I could stop right here but I want to explore one more tool that will improve this image a little more. I close the curves dialog and go to Layer - Colors - Levels. I am presented with something similar to the curves dialog but on just one axis. There are sliders below the graph, one for black, one for white, and one for gray. Sliding them adjusts the position of the values.
Since I want to adjust the grays in the picture, I use the slider in the middle. I slide it left from 1.00 to 1.25 and most of the gray is gone.
This time I really am done. The picture looks very bright and much more attractive. While it may not be the real picture, scroll up and compare it to the first image and you will see how much better it is.
Obviously the adjustments that have to be made are different for each photo, and if your problem color is red instead of blue, you'll have to adjust that channel instead. A photo with a lot of colors might require you to adjust all three channels. You really have to just play with these tools and see the effects you get. Save backup copies of your photo and save copies often as you make changes. After a while of editing these values and seeing the results you will probably get the hang of it.
Then you can make any rainy day sunny 
Extra: this tutorial by Matt was easy to find, and covers the same tools, as well as other features. Read it!
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