Digital images fall into two camps:vector images, which are created based on mathematical formulas, andraster images, which are made up of pixels. Photoshop allows you to produce both types of images and even to combine both types within a single file.
Table 1 gives you the skinny on vector and raster images. For the details, keep reading.
Table 1: Characteristics of Vector and Raster Graphics
Graphic Type
How It Works
File Size
Image Degradation Possible?
Resolution Dependent?
Vector
Mathematical formulas precisely locate and connect geometric objects and segments
Usually smaller
No
No
Raster
Breaks pieces of an image into a grid made up of pixels
Usually larger
Yes
Yes
Vector images
One cool thing about vector images, also called object-oriented images, is that when you zoom in on them, they don't look blocky. That's because vector images are comprised ofsegments — curved or straight — andanchor points — elements that indicate the endpoints of the segments — that are defined by mathematical objects called vectors. Vectors use a unique mathematical formula to define the specific location of an object as well as its geometric shape.
Vector images are usually the product of drawing programs, such as Adobe Illustrator, but Photoshop is also capable of producing a vector or two. And not to be outdone, its cousin Illustrator can also rasterize (or convert into pixels) vector artwork, thereby providing you with raster images to work with.
Here is some additional information about vector graphics:
A curve is still a curve, even at 20,000 feet. Because they are mathematically defined, vector graphics can be sized and otherwise transformed without an inkling of quality loss. Take that little 2-inch spot illustration and size it up to mural size, and it appears identical. A perfect true curve remains a perfect true curve, whether it's 2 inches or 20 feet long.You can get pretty pictures in small packages. Vector images can be small in file size because the file size depends on the complexity of the vector objects, not the size of the illustration.Graphics that need clean lines, such as logos, typographic illustrations, and line art, work great in vector format. Vector images are independent — resolution independent, that is. Not only can they be transformed and printed without a degradation in quality, but they also have no built-in resolution — they take on the resolution of the output device. For example, print something to an imagesetter (a high-end printing device used for color separations) at 2400 dots per inch (dpi), and the image comes out at 2400 dpi. Print it to a 300 dpi laser printer and what do you get? A 300 dpi image.
Because your monitor can display images only on a grid, vector images display on-screen as pixels. This accounts for the jagged appearance you see when you zoom into a curved vector object. But don't worry; it will print just fine.
Raster images
Raster images are usually the result of the digitizing of continuous-toneimages, such as photographs or original painted or drawn artwork. Raster images are comprised of a grid of squares, which are called pixels. Pixel is short for PICture Element and is 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 1940s, you're familiar with what a grid of pixels looks like: Each pixel lives in a specific location on that grid and contains a single color. When you edit a bitmap image, you are editing one or more pixels rather than an object-oriented shape.
Although it doesn't seem like it when you're viewing an image that fits inside your computer screen, your entire image can be broken down into a grid of square pixels. But how do you fit a round peg in a square hole? By faking it. Unlike the true mathematical curve possible when drawing vector shapes, raster images must try to approximate a curve by mimicking the overall shape with square pixels.
Fortunately, the mimicry the pixels have to do is indecipherable with high-resolution images viewed at a reasonable distance. But when you zoom in, you can see that a curve in an image is indeed comprised of square pixels. Raster graphics work great for photorealistic or painterly images where 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 are also large in file size.
Bitmap images are resolution dependent. Because they contain a fixed number of pixels, the resolution of the device they are 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 600 dpi printer than it does on a 1200 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.
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