The GIMP is a pixel-based image manipulation tool, but from the perspective of creative composition with digital images, pixels are neither the most convenient nor the most important component to work with. This honor is reserved for layers. Pixels are the basic stuff of layers, and layers are stacked to make images. This constitutes a hierarchy of scale. Pixels are small, which makes them too hard to work with. It would be difficult to build an image if it had to be done pixel-by-pixel. On the other hand, whole images are too unwieldy to work with comfortably. There are conceptual parts or particular components of images we want to work with without disturbing the rest. By constructing an image out of layers, it is possible to work with each component independently of the rest of the image. This makes layers the happy medium between pixels and images. They are ``right-sized'' for what we want to accomplish in the GIMP.
The images we want to make are usually constructed of conceptual pieces from other images: a part from here and there that we stitch together into a single whole. Layers allow us to combine all the pieces yet keep them separate. This collage view of working with digital images is extremely powerful. We can work on individual image pieces without affecting the others. Having the parts on separate layers allows for their separate processing. Layers can be positioned, repositioned, color-adjusted, and filtered independently. It is difficult to over-emphasize the utility of layers. Compositing, animation, selections, effects, and enhancements are all made easier because of layers.
The objective of this chapter is to develop a firm foundation for layer mechanics. You will learn how to use them effectively while avoiding common pitfalls. The material covered here will be referred to often in later chapters.