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Project Summary

Difficulty	5
Time required	Short (several days)
Prerequisites	Good computer skills; Project costs lower depending on printing method and number of images printed.
Material Availability	Readily available
Cost	Very Low (under $20) to Low ($20 - $50)
Safety	No issues

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Abstract

Objective

The objective of this project is to compare at least two different image compression algorithms and rate them for:

• image quality, e.g.:
  • subjective quality ranking (1–10 scale after careful comparison),
  • resolution (what is the size of the smallest detail you can distinguish in the image?), or
  • come up with your own quality measure.
• compression ratio (size of original image file/size of compressed image file)

Introduction

What's so important about image compression algorithms? With today's computers, DVDs and digital cameras, digital images are everywhere. For example, on websites images are used for advertising, and as illustrations and diagrams for articles. Image files can easily be huge (more on this later). Image compression is essential for DVDs and speedy downloading of webpages over a network connection.

What makes image files so big? Digital images such as photographs are generally encoded as rows and columns of pixels (from picture elements). This type of image format is called a raster image. The more pixels in each row and column, the better the resolution of the image. For example, an image that is 1280 pixels wide by 1024 pixels has better resolution than an image that is 640 pixels wide by 512 pixels tall. It will appear smoother, less "pixellated". But the higher resolution comes at the cost of four times as many pixels. In addition to spatial resolution, there is also the problem of color encoding. An image with 24 bits of color information for each pixel will generally look better than an image with only 16 bits of color information for each pixel. Again, the better the color resolution, the bigger the image file.

In order to make the most efficient use of images, it is important to match the spatial and color resolution of the image to the spatial and color resolution of the intended output device. For example, a typical computer screen has a spatial resolution of about 75 pixels per inch (also referred to as dots per inch or "dpi"), and color resolution of 24 bits per pixel. Printers, on the other hand, have much higher spatial resolution, anywhere from 300 dpi to as much as 9600 dpi horizontally by 2400 dpi vertically. Obviously, a high-quality printed image is going to be a lot bigger than the same size image displayed on a computer screen.

Image compression algorithms are designed to minimize image file size in order to speed up image data transmission. There are "lossless" algorithms, which have the property that all of the original information in the image can be recreated from the compressed image, and there are "lossy" algorithms, which sacrifice some of the original image information in order to compress the image further.

How much can an image be compressed with a "lossy" algorithm before you start to notice a difference in image quality? Do some types of images lose image quality more quickly than others? This project will help you to answer these questions, and more.

Terms, Concepts and Questions to Start Background Research

To do an experiment in image compression, you should do research that enables you to understand the following terms and concepts:

• image file formats: JPG (also known as JPEG), GIF, PNG, TIFF
• lossless compression vs. lossy compression,
• pixel,
• printer resolution vs. screen resolution,
• 8-bit vs. 16-bit vs. 24-bit color.

• What are the advantages of lossy image compression over lossless image compression?
• What are the advantages of lossless image compression over lossy image compression?
• Does JPEG image compression work equally well with all types of images? (For example, consider color vs. grayscale images, or line drawings vs. continuous-tone images, or natural landscape vs. manmade cityscape images, etc.)
  • If not, what types of images compress "better"?
  • What types of images compress "worse"?
  • How about other compression algorithms?
• What should you look for when assessing image quality?

Bibliography

• digital image basics (including a gallery of sample test images):
  • http://en.wikipedia.org/wiki/Digital_image
  • http://en.wikipedia.org/wiki/Image_compression
  • http://en.wikipedia.org/wiki/Graphics_file_formats
  • http://en.wikipedia.org/wiki/Standard_test_image
• download Irfan view: http://www.irfanview.com/main_download_engl.htm
• JPEG compression:
  • http://www.prepressure.com/library/file-formats/jpeg
  • FAQ part 1: http://www.faqs.org/faqs/jpeg-faq/part1/
  • FAQ part 2: http://www.faqs.org/faqs/jpeg-faq/part2/
• PNG compression:
  • Basic Introduction: http://www.libpng.org/pub/png/pngintro.html
  • home page: http://www.libpng.org/pub/png/

Materials and Equipment

• computer (with Internet connection),
• image writing software (e.g., Irfan view),
• digital image files:
  • use your own digital camera (optional, but helpful), or
  • download images from the Web (see Bibliography for one possible source)
• printer, for comparing images,
• an assistant to secretly label the print-outs, so you can rate the images "blind".

Experimental Procedure

1. Do your background research, and test your knowledge by explaining the terms and concepts, and answering the questions in the section above.
2. Select a set of images for your experiment.
   • From your background research, you should be able to form a hypothesis about which types of images will work best with each compression algorithm you test. Try to select images for both best- and worst-case for each algorithm.
   • If you take your own digital photographs, you will not have to worry about copyright issues.
   • If you download images from the Web, make sure that you have permission to use the image from the copyright holder (if any).
3. Learn how to use your image-writing program to save images in compressed format.
   • Read about the different options in your program's help file.
   • Select "low", "medium" and "high" image quality compression settings for each algorithm you are using.
4. Use your image-writing program to save each image with the "low", "medium", and "high" image quality settings you selected for each compression algorithm. Name each file so that you can tell which settings and algorithm were used to create it, and which original image it came from.
5. Have your assistant help you to print the images, labeling them on the back with a coded name. Your assistant should be able to match the coded names to the original ones, but you should not know the code. All of the images should be printed with the same paper type, printer and printer settings.
6. Study the images, and arrange them in order of image quality from best to worst. Then assign a "quality score" to the images, for example, on a scale of 1–10. If two or more images appear indistinguishable, rank them equally. Take notes on what features you used to rank the images as you did. (This is called a "blind" test, because you do not know which algorithm and settings produced which image.)
7. After all of the images have been rated, have your assistant reveal the codes. Now you can add the compression information to your ratings, and make comparisons.
8. Compare the trade-offs in file size vs. image quality for different image types. For example, you might want to make a graph of compression ratio vs. your image quality ranking. Can you draw any conclusions about when image compression is worthwhile and when it is not? Or how much compression is acceptable for a particular purpose (say, sending a photo album of your summer vacation to your cousin by email).

Variations

• Use a resolution target as one of your test images. For example, here is a US Air Force bar target. The bars are arranged in groups of six (three horizontal and three vertical) at progressively smaller sizes. Make a set of images with "low", "medium" and "high" compression settings. Print the images exactly the same way (same size, printer resolution, paper, etc.) Study the image print-outs with a magnifying glass to see the smallest set of features you can distinguish, and compare this to the compression ratio. Click to download high-resolution image:
  
• If you print out your images at "thumbnail" size, is a higher compression ratio more or less noticeable than when the image is printed at normal size? Why? How does this compare to images intended for viewing on the computer screen vs. images intended for printing?
• "zip" files: another common file compression method is the "zip" file, a lossless compression method. Can compressed image files be further compressed by "zipping" them?

• For more science project ideas in this area of science, see Computer Science Project Ideas.

Credits

Andrew Olson, Ph.D., Science Buddies

Last edit date: 2006-01-08 16:49:48

Career Focus

If you like this project, you might enjoy exploring careers in Computer Science.

	Computer Programmer Computers are essential tools in the modern world, handling everything from traffic control, car welding, movie animation, shipping, aircraft design, and social networking to book publishing, business management, music mixing, health care, agriculture, and online shopping. Computer programmers are the people who write the instructions that tell computers what to do.			Computer Software Engineer Are you interested in developing cool video game software for computers? Would you like to learn how to make software run faster and more reliably on different kinds of computers and operating systems? Do you like to apply your computer science skills to solve problems? If so, then you might be interested in the career of a computer software engineer.
	Network Systems and Data Communications Analyst Computers are an important part of our lives. We use computers to hold and process data, to control manufacturing factories, and to surf the Internet. We are all part of many different kinds of computer networks that are continually sharing information. The role of the network systems and data communications analyst is to design, model, and evaluate computer networks so that they can share information seamlessly. This is an exciting career for those people who enjoy working with rapidly changing technology.			Software Quality Assurance Engineer and Tester Software quality assurance engineers and testers oversee the quality of a piece of software's development over its entire life cycle. Their goal is to see to it that the final product meets the customer's requirements and expectations in both performance and value. During the software life cycle, they verify (officially state) that it is possible for the software to accomplish certain tasks. They detect problems that exist in the process of developing the software, or in the product itself. They try and make things not work (try to "break" the software) by creating errors or combinations of errors that a user might make. For example, if a user enters a period or a pound sign for a password, will that break the software? They seek to anticipate potential issues with the software before they become visible. At the end of the life cycle, they reflect upon how problems or bugs arose, and figure out ways to make the software development process better in the future.
	Computer Hardware Engineer Whether you are playing video games, surfing the Internet, or writing a term paper, computers are an integral part of our daily lives. Computer hardware engineers work to make computers faster, more robust, and more cost-effective. They design the microprocessor chips that make your computer function, along with the equipment that makes computing easy and fun to do.

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