We see computer images all the time, but we rarely stop to think about the composition of those images. How are they put together? How are they manipulated? The vast majority of computer graphics fall into two categories: vector and bitmap. One is more complicated and time-consuming to create than the other, but both are in wide use.

The most elementary kind of image in any discussion of vector and bitmap is the bitmap image, which sounds like something from the early days of computing, a map of bits. A bitmap image is quite literally a collection of squares that, taken together, make up the image. The squares might be of different colors, but they are all the same size. Reduce any bitmap image to its parts and you'll see this in detail.

Because a bitmap image is this collection of squares, it doesn't hold up well to expansion. Designers, when talking about vector and bitmap images, often talk about the resolution of those images. That is how many dots per inch an image has. The more dots per inch, the better the resolution of the image. The higher the resolution, the better detail the image generally has; conversely, the lower the resolution, the less detail the image has. A bitmap image holds up especially well when a designer reduces the dots per inch of an existing high-resolution image. However, going in the opposite direction doesn't work. This is because it's physically impossible to add more squares to the bitmap; the result is a blurry image. It is possible, however, to make them more dense, going from a higher resolution to a lower one.

The other half of the vector and bitmap equation is the vector image. This kind of image is much more sophisticated because it has more moving parts. Each vector image is made up of a handful of smaller images, just like the bitmap, except that the vector image's parts are scalable. This means that designers can change the resolution of a vector image in both directions, making a high-res image lower or making a low-res image higher. If designers are looking for an image that can be easily scaled in both directions, then their choice between vector and bitmap should be an easy one.

Something else to keep in mind in any discussion of vector and bitmap images is that the vector images are not restricted to a rectangular shape, as are the bitmap images. A designer who places a vector image on top of another image won't cover up any of the detail of the image underneath. A bitmap image does this because it is entirely made up of square pixels, even the white ones in the background that we don't realize are there. This is another point to keep in mind when choosing between vector and bitmap.

A key point in the discussion of vector and bitmap, however, is that most widely used graphics file formats are of the bitmap variety, like GIFs, JPGs, PICTs, and TIFFs. This is because vector images lack the consistency of bitmap images in terms of photo-realism. If you're trying to decide between vector and bitmap for digital photos or photo-like images on a web page, the better choice for a more photo-realistic image is the bitmap. Many vector images and are commonly converted to bitmap format for use on web pages. The widely known exception to this is the PDF file, which is a vector image and looks rather good when unconverted.