Credits: Web Developers Virtual Library

There are hundreds of graphics formats. Almost every graphics program has its own. Web browsers can only display a few formats, and servers vary in the formats which they provide. The data is transferred in whatever seems to be the "best" format which the client and server have in common.

There are several kinds of graphics files:

  • Raster files (also called bitmapped files) contain graphics information represented as pixels, such as photographic images. These include GIF, PNG, TIFF and JPEG and are created in photo programs.
  • Animation formats are usually sequences of raster data.
  • Vector files contain data for mathematical operations and are typically used to store line art and CAD information and are created in draw programs.
  • Metafiles may contain either raster or vector graphics data.
  • Page Description Languages (PDL) are used to describe the layout of a printed page of graphics and text, e.g. Adobe PDF.
  • Multi-dimensional object formats store graphics data as a collection of objects (data and the code that manipulates it) that may be rendered as if seen from any viewpoint.
  • Virtual Reality Modeling Language (VRML) is a 3D, object-oriented language used for describing "virtual worlds" hyperlinked within the World Wide Web.
  • Multimedia file formats are capable of storing any of the above types of data, often including sound and/or video information and are created by programs such as Director.

Your choice will be determined by considerations such as

  • the image quality,
  • the data size (and therefore transfer time),
  • requirements for transparency, streaming, or progressive rendering,
  • whether the format is an open standard or a commercial one subject to copyright or patent fees.

GIF is a proprietary specification of Compuserve Information Services. GIF was introduced by CompuServe, and stands for Graphics Interchange Format. It was intended to be a platform-independent format with which users could transfer files over modem lines at low speeds.Filenames typically end in .gif It's a compressed format with 1 to 256 colors (8-bit). Typically the compression is 3:1 to 5:1. There are two standards: 87a, 89a (transparency). It allows 1 bit transparency (a pixel is either transparent or opaque). It allows a palette of a maximum of 256 colors, so representation of 24 bit color images in GIF involves loss.

GIF supports interlaced images: every 8th row in an image is displayed first, then every fourth, then every second... Interlacing was invented to allow users to visualize images being downloaded over slow modems. Most graphics programs can read interlaced GIFs but might not be able to create or write them.

JPEG stands for Joint Photographic Experts Group. It's a lossy compressed format supporting 24-bit, over 16 million colors. Use it for full-color and grey scale naturalistic images; use JPEG when the image has at least 16 colors. JPEG is not good for small, detailed text or images with hard edges. The JPEG (Joint Photographic Experts Group) standard is excellent for most realistic images (photos for example, but not line drawings or logos). It uses a powerful, though nominally "lossy", compression method. JPEG is best suited for truecolor original images; avoid using it on images that have already been forced into a 256-color palette, i.e. don't change a GIF into a JPEG.

PNG (Portable Network Graphics) is a lossless, portable, well-compressed RGB file format that and stands for Progressive display for on-line use. PNG uses better image compression technology than GIF, allowing for smaller files that download more quickly. It has higher bit depths (more colors) and  allows for 1, 2, 4, 8, 16, 24 and 32 bit images. PNG is being developed by the W3C, and will always be a free and open standard. PNG has built-in gamma correction. PNG can store gamma and chromaticity data for improved color matching on different (PC/MAC) platforms. Users can see an image the way it was intended to be seen by selecting the gamma level intended for their monitor.

When something looks good on your Mac, it will be too dark on your PC unless your image format is PNG. If the GIF is ultimately going to reside on the web, where it will be viewed by people with Mac's and PC's, you are out of luck. If you want everyone to view it correctly. The GIF file format (and JPEG too) can not encode the gamma correction used.  MORE

Up ] Image Quality ] Saving an Image ] Searching for Images ] Image Resources ]

 

 

 

 

 


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These pages authored and maintained by Marcella Dawson. Revised 07/13/99. Copyright 1995 CRPC GirlTECH. All rights reserved. . Email your comments. These pages were developed through GirlTECH '96, a teacher training and student technology council program sponsored by the Center for Research on Parallel Computation (CRPC), a National Science Foundation Science and Technology Center.