Instructional Module D22c

Images for the Web 1

Contents

 

 
About this Document Audience and Objectives Q & A about Imaging on the Web

Q & A about Imaging on the Web
This module presents information as a series of questions and answers...

Questions

Answers/More Information
1. Why is it important to achieve small file size in Web imaging?
1. To reduce download time and storage space

 
2. What are the three types of image files supported by Web browsers?
2. GIF, JPEG, PNG
3. What does the abbreviation JPEG stand for?

3. Joint Photographic Experts Group
http://www.jpeg.org/public/jpeghomepage.htm

 
4. How did the JPEG image format become a standard?
4. Need to combine more than 256 colors and small file size led a group of engineers from the graphics industry to get together in the late 1980s. This group sought and received recognition from ISO (International Organization for Standards), and ITU (Interneational Telecommuniations Union), which eventually approved the JPEG specifications as a standard.
5. How does the JPEG format compresses images?
5. Each group of 64 pixels (8x8) is analyzed for the most significant color contrasts. These are represented digitally, and the resulting code is then compressed using the Huffman encoding compression technique.
http://www.faqs.org/faqs/compression-faq/part2/section-6.html
6. What degrees of compression are generally available with JPEG?
6. 0 to 100%
7. What effect does high compression have on JPEG image quality?
7. The image becomes blurred with seemingly random spots of color surrounding objects of contrasting color.
8. What are the two ways in which JPEG images can be rendered on a screen?
8. "Standard" (picture is built top-to-bottom) and "progressive" (entire picture comes gradually into focus).
9. What types of image is JPEG best suited for?
9. Photographs of people and landscapes; graphics with broad- spectrum color gradients.
10. What does the abbreviation GIF stand for? 10. Graphic Interchange Format
11. What is the origin of GIF as an imaging standard? 11. It was developed by CompuServe in the mid-1980s to speed image download times for their on-line customers.
12. In computer imaging, what is a color palette? 12. A list of the colors available for the pixels in an image.
13. How does GIF use color palettes to compress images? 13. Pixels are indexed to a particular palette color. The smaller the palette, the fewer the bits needed to represent a pixel. http://www.oreilly.com/centers/gff/gff-faq/gff-faq3.htm#49
14. What is the mathematical compression algorithm used by GIF images? 14. LZW (Lempel-Ziv-Welch) compression
15. What is "transparency" in computer imaging? 15. Pixels in an image take on the color of the pixels in the layer below them.
16. To what extent can transparency be implemented in the GIF standard? 16. One level of transparency is available: complete transparency.
17. How are images animated? 17. A series of images is created, and each is shown in turn to provide the illusion of motion.
18. How is animation implemented in GIF images? 18. The GIF file contains a series of images together with information about how frequently they should be changed, how many times to loop through the set, etc.
19. What is the simplest way to reduce the file size of a GIF image without changing the image's apparent size? 19. Reduce the number of colors.
20. What licensing and patent issues are involved in using GIF images? 20. GIF technology is patented by CompuServe; LZW compression is patented by UniSys. It is legally required that designers of images stored as GIFs pay a royalty; this is done through purchase of the graphics program that was used to convert the image to GIF. http://www.oreilly.com/centers/gff/gff-faq/gff-faq3.htm#49
21. What does the abbreviation PNG stand for? 21. Portable Network Graphics http://www.libpng.org/pub/png/pngintro.html
22. What is the origin of PNG as a graphic standard? 22. In early 1995, Thomas Boutell drafted an portable bitmap image format and made the draft available on the Internet. A group of interested people quickly revised and refined the specifications, which were approved by W3C and IETF late in 1996. http://www.libpng.org/pub/png/pnghist.html
23. What features are implemented in the PNG standard that are not available in GIF or JPEG?

23. PNG incorporates image layers, gamma correction (to compensate for different monitor settings), and multiple levels of transparency. Also, it uses a lossless compression algorithm, so it can be used for saving work-in-progress.

On the other hand, PNG does not implment animation, and for images with 24-bit color, JPEG or GIF is usually more compact.

http://www.libpng.org/pub/png/pngintro.html

 

About this document...

Review:

 Click here Reviewfor review format.

Audience:

 This is for people who know the basics of computer graphics (see module D20c) and want to begin learning how images are stored on the Web.

Objectives:

 When you successfully complete this lesson, you will be able to demonstrate knowledge of the various image compression formats for the Web.

Module D22c:

 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 143.

Author:

 Laurence J. Krieg

Institution:
Internet Professional Department, Washtenaw Community College
History: Original: 10 December 2000
Last modification: Monday, 31-Aug-2009 11:47:56 EDT
Copyright: Copyright © 2001, Laurence J. Krieg.
Instructors: You may point to this file in your Web-based materials.
Students: you may make a copy for your personal use.
All other uses: contact the author, Laurence J. Krieg for permission. Email krieg@ieee.org