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Other Still Image Compression Methods

There are many other methods you can use to compress images. In addition to other lossless compression schemes like LZW which is part of the GIF file format and can also be applied to TIFF images, there are several lossy compressors, too.

As you might have guessed, lossy compression does not decode exactly the same image as it encodes. Lossy compression takes advantage of the difference between the amount of data that an image can contain, and the amount of data the human eye can see. It turns out that you can remove quite a bit of data from an image and the human eye will not detect the change. Throwing out this invisible data can dramatically decrease a file’s size. There is, of course, much more to it than this.

The most common lossy compression method is JPEG. We’ve used JPEG compressed images throughout this online class. They are extremely effective when compressing photographic images and the amount of data (and thereby image) loss can be adjusted. For example, we started with an uncompressed image of some daffodils whose file size is 232KB and compressed it twice.

Here we set the amount of loss very low and got a file that was only 60KB. That’s 26% of the original file size and it looks great:

1

Whereas with this one we introduced a great deal of loss into this JPEG and produced a 5KB file. That’s 2% of the original file size:

2

You can see that a huge amount of data can be thrown away and have little perceptible effect on the image, but if you go too far you will eventually degrade the image. Again, all compression is a balance between file size and quality.

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Next Page: Intraframe vs. Interframe Compression