Whether you acquire an image from a scanner, a digital camera, a royalty-free CD with 1,000,000 images, or a stock photography vendor, it's made out of pixels. Pixel is shorthand for picture element, the smallest unit of information in a digitized image. Even though pictures on your monitor look like smooth transitions of color, zoom in sufficiently and you'll see all the little square pixels that actually make up the image (Figure 4.1).
Figure 4.1 Images are made of pixels. Think of them as little-bitty mosaic tiles.
Ancient Times: B.P. (Before Pixels)
In the olden days of graphic arts, enormous cameras were used to capture artwork such as drawings, reflective photographic prints, transparencies, or painted illustrations. Highly skilled specialists commandeered these monstrosities, some of which occupied entire rooms. The use of colored filters, masking, and exposure methods to produce color separations (a separate piece of film for each printing ink) was rather arcane and required years of apprenticeship and study to perfect. And since every step required the use of specialized film, there were a lot of trips to a darkroom to develop the results in chemical baths. It all seemed very high-tech at the time (well, compared to cave paintings), but the process was quite time-consuming.