File Formats and How Each One is Used

By Kenny Williams

One of the most confusing aspects of digital imaging is the variety of file formats and to try and remember what each one is used for. The thing you should understand is that not every format is equal in terms of image quality.

Cameras often compress the image down because of storage space limitations on your digital camera. Data compression is the process of squeezing information into a smaller size, so the data occupies less space and takes less time to transmit or download. For images, there are two main types of compression: lossless and lossy.

With lossless compression, file size is reduced, but no data is lost in the process. When you open the compressed file, the image retains the quality of the original.

Lossy compression results in much smaller file sizes because it throws away redundant information. When you open the compressed image, the tossed-out information is re-created. For example, in an image of a blue sky, thousands of pixels are the same color of blue. To reduce the file size, when this image is compressed, most of those similarly colored pixels are thrown out and replaced with an algorithm stating how many times the one blue pixel left should be repeated when the image is opened.

There are two types of images: bit map (also known as raster) and vector. Bit maps are composed of pixels, or picture elements, in a grid. Pixels identify where each individual square of color is located in the image. The number of pixels per square inch (ppi), or dots per inch (dpi), determines the image resolution. Bit maps are resolution-dependent: the higher the resolution, the higher the image quality.

Here are the most common file types:

BMP. BMPs (bit maps) are part of the Windows OS (operating system) and are used in Windows background images. BMPs can use as many as 24 bits of color per pixel, which means you can produce monochrome, grayscale, or full-color BMP images. Lossless compression, typically RLE (Run-Length Encoded), is used to reduce the BMPís large file size. Among bit-mapped formats, BMP is the least sophisticated and thus is not as well suited for complex images.  BMPs are best suited for line drawings and simple color images, rather than photographic images.

TIFF. Like BMPs, TIFFs (Tagged Image File Format) can use as many as 24 bits of color per pixel, which translates to more than 16.7 million color choices. For high-resolution color images, TIFF is a good choice because it supports several color depths. However, high resolution leads to larger file sizes. Because of the larger file size, TIFF images are preferred for the storing and printing of high-resolution color images. The TIFF file format is widely accepted, and most imaging software supports it, but it is rarely found on the Web.

JPEG. Pronounced jay-peg, the JPEG (Joint Photographic Experts Group) image file format is one of three main image formats found on the Web. A big reason for its use is its high compression rate: a JPEG (which Windows often shortens to JPG) can be reduced to as little as 5 percent of its original size, which translates to faster download times. However, when you compress a JPEG, the compression is lossy, and this loss of information can introduce artifacts (missing pixels, abrupt color changes) into the image. The smaller the JPEG becomes, the more the quality degrades. This makes JPEGs the wrong choice for images with sharp edges, such as lettering and line art. If your image has sharp edges and you save it as a JPEG, you will see jagged edges, or visible pixel steps along the image edges, when you enlarge the image.

On the plus side, JPEGs support 24 bits of color per pixel. By taking advantage of the full color spectrum, JPEG is best for saving realistic images, such as photographs and naturalistic images. In addition, you can create progressive JPEG files, which are a series of image scans. During download, the first scan that appears is the lowest quality image, which is followed by higher quality scans until the highest quality image appears. JPEGs are used only for still images. Another file format, MPEG (Moving Picture Experts Group), is used for video.

GIF. This acronym is most commonly pronounced jiff (like the peanut butter), but it isnít unusual to hear it pronounced with a hard G No matter how you say it; GIFs (Graphics Interchange Format) are also used for Web graphics. However, unlike JPEGs, GIFs only support 8 bits of color per pixel, only 256 colors. This color limitation dictates the kind of images GIFs should be used for, such as images with solid colors or images with areas of uniform color.

Other advantages that GIFs offer include support for one-color transparency and simple animation capabilities where multiple images embedded into a single file are displayed in a slideshow fashion, creating the look of animation. In addition, GIFs support interlaced images, which are similar to progressive JPEGs.

PNG. As a reaction to the attempt to charge royalties for LZW compression, several image developers created the PNG (Portable Network Graphics; pronounced "ping") file format. PNG is a patent-free format that supports 48 bits of color per pixel and, using lossless compression, compresses to sizes 10 percent to 30 percent smaller than GIF files. However, compressed PNG files are still five to 10 times larger than JPEGs.

Other advantages of PNGs include 8-bit alpha transparency, the ability to create effects such as glows, interlacing that is twice as fast as GIFs, and gamma correction, where brightness and contrast levels are measured and adjusted for consistent viewing across platforms. In addition, you can display and print the same PNG image at different resolutions. However, PNG images donít support animation.

PNG was envisioned as a replacement for GIF. But with little support from major browsers, such as Netscape and Internet Explorer, PNG is taking longer to gain widespread support.

If you would like to read more about image formats go to http://www.image-formats.com

If you have any questions, comments or suggestions, or are in need of computer lessons, feel free to e-mail me at:  WebServerColumn@yahoo.com.

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