Working with Raster and Vector Art

In desktop publishing, there are two basic kinds of art: raster and vector. Raster art includes all scanned art or images created in image-editing applications, such as Adobe Photoshop. Vector art, on the other hand, are drawings made using paths in applications, such as Macromedia FreeHand and Adobe Illustrator.

One of the skills to master in desktop publishing is understanding the difference between raster and vector art and knowing when to use each one. In addition, once you have mastered both, you can move art back and forth from one form to another and combine the two to create special effects. To learn the difference between raster and vector art, you need to know the difference between scanned images and drawn ones, between pixels and paths, or more fundamentally, between dots and lines.

Raster, bitmap or pixel art all refers to art that is made up of a grid of dots. It consists of art created from scanned images or by painting on a digital canvas of pixels. Typically, raster art is realistic looking like a photograph. It is also very resource intensive. An 8 by 10 inch scanned image at 300 dots per inch contains 7,200,000 different dots, each of which can contain one of 16.7 million different colors. When you run a Photoshop filter on such an image, your computer has to compute the new color value of each and every one of those 7.2 million dots.

Vector art is not realistic-instead it looks like an illustration. It consists of paths and fills inside of those paths. Because they consist of paths and fills, vector drawings are less resource intensive than bitmaps. They also take up less hard drive space than bitmaps. Instead of storing the color and position of every pixel, computers save a description of all the paths and fills in the drawing. However, the popular vector drawing programs, Illustrator and FreeHand, all contain so many features, including bitmap editing tools, that they require a heavy-duty computer.

Vector art is basically geometry. If you converted a typical drawing program instruction into English, it would read something like this: "Draw a path with endpoints at these coordinates and make the width of the path a fixed amount." Because vector drawings only contain mathematical descriptions, they can be expanded in size without losing image quality. In other words, the path will look the same at 20 percent or 2000 percent. If you blow up a bitmap, however, you effectively reduce the resolution (the number of dots per given unit). If you reduce the resolution enough, you will start to notice that the dots and lines will appear jagged or stair-stepped.

Therefore, vector art is resolution independent and bitmap art is resolution dependent. The rough rule of thumb for bitmap art is that print-quality graphics should contain 300 dots per inch. The more precise rule is that a bitmap should contain a resolution equal to two times the intended printing line screen. In other words, if you are printing at 133 lines per inch, all of your bitmaps should contain at least 266 dots per inch. The exception to this rule is Internet or multimedia use, where images should contain roughly the same resolution as the screen they will appear on. Web bitmaps should have a resolution of roughly 72 dots per inch.