Lab 6
Part 4

Image Compression

In Lab 4 we saw that images are saved in compressed format. In particular, Paint Shop Pro allows you to choose the compression rate for the JPG format. The higher the compression rate, the smaller the file but the poorer the picture quality. In Lab 4 we warned you not to use too high a compression level so that your images would not have poor quality. Now we are going to experiment with the effect of higher compression levels.

When saving images that are going to be sent over a network, for example images on Web pages, understanding the effects of compression becomes important. Web designers want high quality images, but the file size must be kept to a minimum so that their pages will load quickly. The largest amount of compression that gives acceptable picture quality is desired. We would like you to find what you believe is that right amount of compression for a particular file of your choosing.

Review the discussion "Image Formats and Compression" on Page 2 of Lab 4. Then choose one of the images you used for Lab 4 or a new image and experiment with the various picture qualities by saving the picture as different JPG files, each with a different compression level and picture quality. Here's how: when saving the file, after clicking on Save As, and selecting Jpeg/Jif as the file type, there is an Options button in the lower right of that window. Clicking on that brings up the compression options. (You will have to give each of the pictures a different file name when saving them.) Close all pictures and reopen each version. Notice how the quality varies. Also note the file sizes of your pictures. Find the compression factor which you believe is the maximum compression that gives an acceptable picture quality. Save in your public_html directory for submission the original file, the file at the minimum acceptable compression rate, and a file at an unacceptable (for quality) compression rate. If you think the original file is the file at the minimum acceptable compression rate, still save files at three different compression rates.