All Terrain Thinking

A Compendium of things I think are Important

Generally Speaking, Think on this...

Graphic File Formats (2006)

QuickStudy: 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.

January 15, 2007 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.

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.

JPEG
The Joint Photographic Experts Group created the JPEG standard in 1990 for the efficient compression of photographic images. JPEG allows varying levels of lossy compression, letting you trade off quality against file size. Progressive JPEG is a way to rearrange the graphic data to permit a rough view of the entire image even when only a small portion of the file has been downloaded. The JPEG standard includes 29 distinct coding processes, but not all of them need to be used. If an image has flat areas of single color that transition sharply to contiguous areas, JPEG doesn’t work as well as GIF.

JPEG 2000 is a wavelet-based standard designed to supersede the original. It offers improved compression, including lossless compression, and supports multiple resolutions in a single file, but it has only limited support in current Web browsers.

GIF
The Graphic Interchange Format takes an image and re-creates it using a palette of no more than 256 colors. These palettes can be totally different for different images. GIF is a very efficient format that achieves very good compression for nonphotographic images. GIF also permits the creation of animated images by allowing a file to contain several different frames (each with its own palette) and to switch between them with a specified delay. In addition, GIF images are one of the few types that can have a transparent background, meaning that there’s no need to always display a rectangular area.

>GIF was once quite popular, but in 1995, it became the centerpiece of a patent dispute that clouded the issue of who could use what. The patent in question was for the LZW lossless compression algorithm; it expired in 2003.

PNG
Portable Network Graphics is a standard developed in 1996 as an alternative to and improvement on GIF, but without the patent issues and palette restrictions. PNG can compress an image more than GIF and supports improved background transparency/opacity but allows only single images, without animation.

Raw Data
As digital photography becomes ubiquitous and multimegapixel digital cameras grow more common, you may hear more about raw images. Most inexpensive, consumer-grade digicams store images as JPEG files (technically in EXIF format, a form of JPEG) that involve the loss of some detail. This was done initially to keep file sizes small, when flash memory storage was much more expensive than it is now.

Some higher-end cameras now offer the ability to save all image information as raw, unprocessed data in a nonstandardized format that takes more storage space but prevents the loss of subtle detail.

Raw images can be edited with professional-grade software and converted to JPEGs for printing or other forms of distribution.

Common Graphics File Formats Compared

Type File Extension Compression Methods Principal application/usage Patented? Originated by Graphics Interchange Format .gif Lempel-Ziv-Welch (LZW) algorithm Flat-color graphics, animation Expired Compuserve Joint Photographic Experts Group .jpg Loses some data Photographic images Disputed Joint Photographic Experts Group Portable Network Graphics .png Lossless Replacement for GIF No World Wide Web Consortium Raw negative Various None High-end digital cameras No Individual equipment makers Tagged Image File Format .tif Various or none Document imaging, scanning No Adobe Systems Inc. Windows bit map .bmp None On-screen display No Microsoft Corp. Source: Adapted from "Comparison of graphics file formats," at Wikipedia.org

 

Prior | Tell us what you think | Next

Add to Your Social Bookmarks: Del.icio.us Digg Furl Reddit Simpy Spurl Y! MyWeb - StumbleUpon