Gimp and Inkscape are free and open-source programs for creating and manipulating two-dimensional images. Gimp is a painting program; that is, it is primarily concerned with setting the colors of individual pixels in an image. Inkscape is a drawing program; that is, it represents an image as a data structure that contains information about the objects in a 2D scene. The difference between painting and drawing is discussed in Section 1.1.
Even though the main focus of this book is on programming for three-dimensional graphics, it can be useful to get some experience with 2D image manipulation programs. Such programs illustrate some important concepts, such as color manipulation, transparency, shape creation and editing, Bezier curves, and (in drawing programs) grouping graphical objects into hierarchical structures. And the programs are often useful even in 3D graphics, for working with texture images. This appendix offers just a very brief introduction to Gimp and Inkscape, but maybe enough to let you start experimenting with the programs and to inspire you to learn more about them from other sources.
When I teach computer graphics, I often include a few labs on Gimp and Inkscape. The material in this appendix was adapted from those labs. While Gimp and Inkscape are not quite the equivalent of the commercial programs Photoshop and Illustrator, they are free, they can be used in serious graphics projects, and they already have far more features than can be covered in a couple of labs.