For this tutorial I'm using GIMP 2, the free open-source image editing program, but I will also be referring to Adobe Photoshop Elements 8 and Corel Paint Shop Pro Photo X2. The techniques used in this tutorial will also work in all recent versions of Adobe Photoshop, as well as other advanced image editing programs.
Last week I explained how to use the Curves function to boost contrast and alter the tonal balance of a digital image. This week I'll cover some more extreme effects including tone reversal, posterisation and solarisation. I strongly suggest reading at least the first page of last week's tutorial so that you know where to find the Curves function.
As I explained in the first part of this tutorial, the Curves function is a graph representing the distribution of tonal values within an image, with a line representing the relation between input tones (the original image) and output tones (the image after the effect is applied). In GIMP, Photoshop and Paint Shop Pro this line can be manipulates directly by dragging points to new positions to alter the balance of the tones. These alterations can be used to make subtle corrections to the tone and contrast, or to produce striking colour effects.
As with most extreme image alterations these effects don't work well on all images, and will seldom make a dull picture into a work of art, but with the right sort of image they can produce fascinating effects resembling the pop-art of the 1960s. The best type of image to use is one with good contrast, bold colours and strong shapes, so that the picture will remain recognisable after the alteration has been carried out.