If you've ever looked through the iStockphoto illustration collections, you've probably noticed that most of the images we offer are vector images instead of raster or bitmap graphics. In fact, if you filter your search results by selecting "Illustrations" from the list of file types, you'll only see vector illustrations.
So what exactly are vector illustrations and why do we love them so much?
Unlike raster images, which are made up of pixels (tiny colored squares arranged in a rectangular grid to form an image), vector graphics use mathematical calculations to plot points and the paths that connect them to describe the image. While a raster image might have 2000 pixels from point A to point B, a vector graphic would simply plot the two points, calculate the distance between them and draw a line (this is why vector files are typically much smaller than bitmap images).
Vector illustrations also give you the freedom to use the image however you choose. The shapes and objects plotted in a vector can be easily manipulated, colored and re-sized without any loss of quality. This is why you'll only see one size of an illustration appear on the file close-up page – you can make these images any size you want.
Fortunately, you don't need to be a mathematician to create and edit vector artwork. Adobe Illustrator — the most widely-used vector graphics program out there — crunches the numbers for you.
You may hear the phrase "resolution independent" when talking about vectors. That's just a fancy way of saying that you can print a vector graphic on a business card or a billboard, and the image will remain sharp and crisp at any size.
The image on the right is the "outline" or "keyline" view of the vector ornament on the left. You can see the points and the curved paths connecting them — these are called Bézier curves. The shape contains only as many points necessary to plot the curves, and instead of pixels, it is made up of mathematically-described paths.
Now look at this example. On the left is a bitmap version of the same scroll, magnified 800%. At that size, you can start to see the individual pixels, and the image becomes jaggy. On the right is the vector at the same magnification — it remains smooth and sharp.
In addition to their resolution independence, vectors are great because they are easy to edit and adapt to your designs. Let's say you download a set of red icons, but you really need them to be orange. In this example, the colors of the buttons were changed, but the icon elements were left white, as they are separate objects.
It's as simple as selecting the object, then clicking on a different color in the Swatches panel. If you were to try to replace the color in a flattened bitmap image, it would be difficult to isolate the color you wanted to replace, without it looking a bit fuzzy.
Moreover, the vector file can be edited time and again without loss of quality. If you re-size and re-save a JPEG several times, you'll see a distinct degradation. But since vector illustrations are made up of objects, each object can be moved, rotated, rearranged, duplicated or deleted to suit your needs. You can easily delete shapes to isolate the object you want, or combine shapes to create a new composition.
Or hey, maybe you just need the perfect image for your project and you've found it right here. Go ahead and slap it into your layout just like you would a photo. Make it any size you like, and it will still look great.