Compression Lesson

Compression Activity

In this exercise we will take an image and convert it to a variety of image file formats. We will then compress the image in each format using WinZip, to see what ratio of compression can be achieved in each format. We will also compress the entire folder in which they are stored, so as to determine a total compression ratio. If you want to check out the image first, click here

Step 1: Create two folders in your *whatever you call it* drive, or on your desktop. Name one folder Saturn and the second Saturn Zipped

Step 2: Copy the image below to the 'Saturn' Folder in your P Drive. To copy, click here

Step 3: Open this file in whichever program you are using to manipulate graphics, and convert to the following formats. Save each image to the Saturn Folder on your P Drive and name each image according to its file type, eg: satgif. / sattif.

NOTE: You can use another file name as long as you are consistent. The image itself is in JPEG format.

When you have saved it in the formats below you should then have seven ( 7) files.

A) TIFF (*.tif)

B) GIF (*.gif)

C) Windows Bitmaps (*.bmp)

D) Portable Networks Graphics (*.png)

E) PC Paintbrush (*.pcx)

F) Targa (*.tga)

Great! When you have done these you are one third of the way through!

Step 4: When you have completed this, use Windows Explorer or Macintosh equivalent to determine the file size of each individual file and note this on a 'Compression Exercise' work sheet. To determine file size, use the following steps:

A) In Windows Explorer (or Mac equiv) highlight the file. (eg sattif.)

B) Move your mouse to the task bar at the top of the page and click File so that a pop-down menu appears. (You can right-mouse-click)

C) Continue to hold your mouse button while moving down the list to Properties and release your mouse button. A list of file properties will appear including size.

Step 5: Use Windows Explorer to determine the size of the folder containing all images. Follow the steps as above, but highlight the folder rather than an individual file. Write this on your handout sheet.

OK, Now for the interesting bit...

Step 6: Open WinZip. Click on New and type in a file name for one of your files, but add zip as a subfix:

eg satgif. would become satgifzip.

or sattif. would become sattifzip. etc.

Make sure that you are saving to your Saturn Zipped Folder. When you have typed in this name and are sure that you will be saving to the correct folder, click ok.

Step 7: A new dialogue box will appear asking you which file do you want to compress. Choose the file of the same name as the file you just added the subfix zip to.

eg: if you are compressing the tif file you may have typed in satgifzip. as the name you wished your new file to be. Now you would choose satgif. as the file to compress, to create the new file.

When you have done all of the above and you are sure that you are saving the correct file to the correct folder, click ok.

Repeat the above for each file, so that each is now also saved as a zip file and stored in the Saturn Zipped Folder.

You have done it!

Step 8: When you have zipped each file individually, determine the file size for each new zip file using windows explorer (for PC). Write the new file size on your worksheet.

Step 9: When all of the above is done, compress the 'Saturn Folder', saving it as saturnzip in your Saturn Zipped Folder. Determine it's new file size and write this on your work sheet.

Step 10: Delete all of the files you have created after: a) completing the worksheet. or b) before you leave the lab.

Here is the image. It is a NASA Hubble Telescope Photograph, downloaded in TIFF format.

1

SaturnTIFF50%2.tif (542882 bytes)

WinZip: A popular compression utility

Compression: Reduction of file size using algorithms of varying complexity

Format: The name given to 'file type'