How to Use Raster Images in Photoshop CS6
The primary goal of Adobe Photoshop Creative Suite 6 is to give the user awesome raster images (also known as bitmap images). The best way to achieve this is to arm yourself with the necessary knowledge to deal with raster images.
Raster images are usually the result of the digitizing of continuous-tone images, such as photographs or original painted or drawn artwork. Raster images are made up of a grid of squares called pixels. (Pixel is short for picture element; it’s the smallest component of a digital image.)
If you’ve ever looked at a bathroom wall made up of those small square tiles reminiscent of the ’40s, you’re familiar with what a grid of pixels looks like up close: Each pixel lives in a specific location on that grid, and each contains a single color. When you edit a raster image, you’re editing one or more pixels, rather than an object-oriented shape.
But how do you fit a round peg into a square hole? By faking it. Unlike the true mathematical curve that’s possible when you’re drawing vector shapes, raster images must try to approximate a curve by mimicking the overall shape with square pixels.
So, the elliptical shapes of a beanie have to fit within this system of squares. Fortunately, the pixels’ mimicry is unnoticeable in high-resolution images viewed at a reasonable distance. But when you zoom in, you can see that a curve in an image is indeed made up of square pixels.
Raster graphics work great for photorealistic or painterly images in which subtle gradations of color are necessary. On the downside, because they contain a fixed number of pixels, raster graphics can suffer a degradation of quality when they’re enlarged or otherwise transformed. They’re also large in file size.
Bitmap (another name for raster) images are resolution-dependent. Because they contain a fixed number of pixels, the resolution of the device they’re being printed to is only one of two factors that influence the quality of the image. The quality of the output also depends heavily on the resolution of the image.
For example, an image with 72 dots per inch (dpi) doesn’t look any better printed on a 1200-dpi printer than it does on a 300-dpi printer. Likewise, a 300-dpi image doesn’t look as good printed on an old 72-dpi dot-matrix printer as it does on a 1200-dpi printer.