How Web Image Formats Work

Module D23c

Contents
Audience and Objectives
About this document...

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Overview

Two kinds of images are widely used on the Web, and are likely to remain popular for many years:

GIF

Name means: Graphic Image Format
Developed by: CompuServe
Made available in: 1987
Type of compression: Potentially lossy + Lossless (LZW)
Best used for: Diagrams, typography, simple color representations

JPEG

Name means: Joint Photographic Experts Group
Developed by: Joint Photographic Experts Group members
Made available in: 1991
Type of compression: Lossy + Lossless (Huffman)
Best used for: Photographs, complex color shading

GIF Images

GIF images are produced by putting bitmap images through a six-step process:

1. Color listing

2. Sort by frequnecy

3. Apply color limit

4. Adust non-represented colors

5. Represent pixels by frequency

6. Apply LZW (lossless compression)

JPEG Images

This is the process by which bitmap images are converted to JPEG format:

1. Divide image into squares 8 x 8 pixels on a side (64 pixels total)

2. Apply Discrete Cosine Transformation (DCT), a lossy compression technique

3. Quantize the result to find the most frequent colors

4. Throw away redundant colors according to the degree of compression required

5. Apply Huffman coding (lossless compression)

Audience:

This is for people who know in general how computer graphics work, and want to know more about Web image formats.

Objectives

When you successfully complete this lesson, you will be able to...

Discuss the general process by which an image is rendered as a GIF
Discuss the general process by which an image is rendered as a JPEG

About this document...

Module D23c: How Web Image Formats Work

This document is part of a modular instruction series in Computer Information Systems. For more information, see the overview or the list of modules in this series, D: Desktop Publishing and Computer Graphics. This document has been used in the following classes: INP 152

Author:: Laurence J. Krieg
Institution:: Department of Computer Information Systems, Washtenaw Community College
History:: Original: September 2001; This version posted: [an error occurred while processing this directive]