One tool I have been finding very useful in Gimp lately is the Color Curves adjustment dialog so I figured I would make a quick tutorial or guide on it. I have been finding that it allows you to make nice smooth appearing adjustments to contrast in the image or layer you are working on. Often times I want to increase contrast just a little bit and this tool works very well. Here's an example of a crumpled paper image with the contrast increased for fun:
To bring up the Color Curves tool, navigate to Colors - Curves... and it will be displayed. The Color Curves tool will display the current layer's histogram in the background with a linear histogram adjustment line. You can also switch this to logarithmic in the upper right corner if you want but I usually find linear to work well for me. From here, you can adjust the histogram curve while watching the preview image to the contrast levels you like.
The channel can be changed between the combined RGB, or the red, green, and blue channels individually. The alpha channel can also be selected and I have found this to be tremendously useful when adjusting layers with transparency and for making brushes for Photoshop or Gimp. Say we wanted to turn this crumpled paper into a brush for use later. After converting it to greyscale and converting white to your alpha channel, Color Curves can be used to adjust the histogram and resulting contrast produced by the alpha channel. This allows us to make the page 'crumbles' stand out more in our brush:
Well, that's all I've got to say about adjusting your images with the useful Color Curves dialog!