File Formats: GIF and JPEG
The three major file formats for images on the Web are GIF, JPEG, and PNG. Each format has a specific use when saving images for the Web. Web developers and designers need to know the differences between these formats to optimize download times and user compat-ibility.
The Graphics Interchange Format (GIF), developed by CompuServe, is based on a 256-color palette. GIF is best used for screen captures, line drawings, graphics with sharp edges and images with transparency. When reducing colors to the 256 available in the GIF file format, Photoshop Elements performs dithering on the image. Dithering simulates the desired color with a color from the GIF palette. GIF is a lossless format, meaning that the picture quality is not reduced by the compression algorithm. The compression algorithm is the formula that a file format uses to store file information.
Dithering may be effective, but often it destroys the quality of an image that has color complexity. Such richness is characteristic of real-world images such as photographs, scanned images and computer art created with 3-D rendering programs. Images that are “color complex” are better-suited to the JPEG format. However, this format is not without limitations. JPEG is a lossy format (i.e., saving an image in this format gradually reduces the quality of the image due to loss of color information). The JPEG compression algorithm handles sharp edges and abrupt changes poorly.
One feature that GIF and JPEG share is interlacing (in GIF terminology) or progres-sive encoding (in JPEG terminology). Interlacing or progressive encoding creates a rough image preview at the beginning of the download process. The image clarity then gradually increases as the image loads. This behavior often keeps the user’s attention while a page loads. Interlacing is specified in the Save for Web dialog. Non-interlaced images down-load at the highest quality and are ideal for images that have small file sizes.
A newer image standard is making its mark on the Web. The Portable Network Graphics (PNG, pronounced ping) format was developed in response to a decision by the UniSys corporation to start charging royalties on the GIF format, on which UniSys holds a patent. PNG is a suitable replacement for both GIF and JPEG because it has the better qual-ities of both formats. For example, PNG can encode in RGBA—the A stands for alpha transparency, which makes images transparent against any background, similar to opacity. The PNG file format solves many problems that previously existed with transparency. An image with both color complexity and transparency could not be saved as a transparent GIF or a JPEG. The PNG file format supports millions of colors as well as transparency. This makes it a great alternative for both GIF and JPEG. Photoshop Elements supports the PNG format, as do the latest versions of both Netscape Communicator and Internet Explorer. Web developers increasingly are using the PNG file format. For more information on the PNG format, visit www.w3.org/Graphics/PNG.
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