This article uses an old version of Gimp so the instructions are no longer valid due to some menus changes. An updated article was written here for the new version of Gimp (2.6.x): Using Auto-Level to fix dull gray-ish photos.
Last post I showed you a photo that was awesome except it had a slightly gray or "flat" look to it and hopefully you saw how dramatically you can improve a photo by removing it. I also told you that the real reason for that gray look is the lack of a "true" or 100% black and/or white area in the photo. So, now we'll talk about how to go about fixing this issue with photos you may have that suffer from the same problem...
Did you download Gimp? If so, awesome, you can follow along with me here. If not, I'm guessing you have Photoshop or some other software that you use and you've decided to stick with what you have. That's cool, your program will probably have the same tools, you'll just have to find where they are in your menus on your own. ...ready?
Step 1. Find a photo with a gray cast that you'd like to remove, then open it in Gimp.
(if you can't find one, download this one to use as practice by right-clicking on the photo and choosing "Save Picture As")
Once you have the photo you want to use, right-click on it and choose "Open With" > "The Gimp".
**NOTE:** If "Gimp" isn't on the list of programs, click on "Choose Program" and find "Gimp". Or you can also just find "Gimp" in your Start Menu under "Programs" and open it from there, then go to "File" > "Open" and browse to the photo you want to open.
You'll notice that Gimp doesn't quite work like Word or other programs you are used to. It has 2 different parts that open in separate windows -one is a toolbox that has a bunch of icons in it, and the other window has your photo in it. Don't worry about the window with all the icons in it, we won't be using it this time around.
Step 2. Using "Auto Levels" to remove the gray cast.
In the window that has the photo in it, click on the "Layer" menu. When it opens, click on "Colors" > "Levels". The "Levels" window will then be displayed. -We'll go into Levels more in-depth later, but for now, just click on the button that says "Auto".
If you're using the photo I linked to in Step 1, you'll notice that doing just that improved the image greatly. If you are using your own photo, there's a good chance it fixed it, but sometimes "Auto Levels" can make it worse. No biggie. If you like it, click "OK", if not, click "Cancel".
That's it! You're done!
Assuming you like the result, just go to the "File" menu and choose "Save As" in order to save the changes you've made to the photo. If you didn't like what "Auto Levels" did to this particular photo, stay tuned, we'll talk about other ways to fix such problems soon.
For simple, minor gray-cast or "flatness" issues, the "Auto" button in "Levels" works great! It's definitely something you should try whenever you have an image that seems to suffer from this problem.
If you don't understand something or have trouble with any of my tips, feel free to contact me via a comment on this (or any) article, or see my website for my email address and I'll be happy to help you out!