In this article I'm going to demonstrate how the GD library can be used to apply some basic graphic filters (yes, like Photoshop, but much simpler) by utilizing the versatility of a brand new function called "imagefilter()". As you'll see shortly, with this function it is possible to reverse, blur and highlight the colors of a determined image, as well as convert it to its grayscale version, among other things.

They use the imagefilter function in a few different examples - altering the colors in an image, working with contrast and brightness and various other filters (like smoothing, embossing, blurring and colorizing).