Graphic File Formats

Definition: Graphic images are stored digitally using a small number of standardized graphic file formats, including bit map, TIFF, JPEG, GIF, PNG; they can also be stored as raw, unprocessed data.

There are likely billions of graphic images available on the World Wide Web, and with few exceptions, almost any user can view any of them with no difficulty. This is because all those images are stored in what amounts to a handful of file formats. Before discussing the principal graphics file formats, however, we need to review the two fundamental types of graphics: raster and vector.

A raster image is like a photo in your newspaper. Look closely and you’ll see it’s made up of equally spaced round dots in several distinct colors. But if you look at an ad featuring a line drawing or, better yet, a banner headline, you won’t see an interrupted line of dots but a solid image bounded by smooth curves. Those are vector graphics. Many graphics are created as vector graphics and then published as raster images.

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Most graphics that we see on-screen, and many that are printed on paper, are actually structured as rectangular grids of pixels or colored dots. A full-color image requires more color information than a black-and-white image. Some types of graphics use geometric functions that allow them to be scaled up or down in size.

One final distinction should be made between how an image is stored (its graphic file format) and how it is generated for viewing by the end user.

Most devices that output images, whether they be monitors, TVs or ink-jet printers, actually produce raster output. They create successive minuscule lines, each consisting of a line of dots of different colors (and perhaps sizes) that end up on the final page as both images and letters. Before the advent of modern high-resolution displays, there were CRT devices that actually produced true vector output, but those are mainly history now. So we need to provide our monitors or printers with sequences of all those colored dots. A graphic that is already rasterized will save time and electrons because it doesn’t need further processing by the computer.

BMP
The simplest way to define a raster graphic image is by using color-coded information for each pixel on each row. This is the basic bit-map format used by Microsoft Windows. The disadvantage of this type of image is that it can waste large amounts of storage. Where there’s an area with a solid color, for example, we don’t need to repeat that color information for every new contiguous pixel. Instead, we can instruct the computer to repeat the current color until we change it. This type of space-saving trick is the basis of compression, which allows us to store the graphic using fewer bytes. Most Web graphics today are compressed so that they can be transmitted more quickly. Some compression techniques will save space yet preserve all the information that’s in the image. That’s called “lossless” compression. Other types of compression can save a lot more space, but the price you pay is degraded image quality. This is known as “lossy” compression.

TIFF
Most graphics file formats were created with a particular use in mind, although most can be used for a wide variety of image types. Another common bit-mapped image type is Tagged Image File Format, which is used in faxing, desktop publishing and medical imaging. TIFF is actually a “container” that can hold bit maps and JPEGs and allows (but doesn’t require) various types of compression.

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