understanding compression

The goal of compression is to reduce the amount of space a file takes up, while keeping it as close as possible to its original form. Some compression techniques permanently shrink the file. Others shrink it temporarily while in storage or transit and expand it once delivered. (Think of stuffing big items in a small suitcase and unpacking them on the other end.)

2 kinds of compression:

  1. Lossless compression squashes a file in a way that allows it to completely recover, with no information or quality lost.

  2. Lossy compression also squashes a file down to size, but some information is lost in the process. Once lost, it can't be restored.

JPEG compression produces different results depending on the level of compression. The more you compress, the smaller and rougher your image will be. The original image (a TIFF file) was 53K. Image 1 is 35K. This image was saved at level 12 for "Maximum" quality. It's nearly identical to the original. Image 2 is 15.8K. This JPEG was saved at level "8." It lost a bit of detail, but looks good, considering it's half the size. Image 3 is 7.5K. This JPEG was saved at level "1" for "Low" quality. And it shows.

graphics/204fig01.gif

So, lossless compression re-creates the original file exactly as it was. And lossy compression reinterprets it, producing an approximation of the original.

Lossless compression is the ideal we're striving toward. But right now, lossy compression is often the only choice. For video production in particular, there's just no way to deliver (or even work with) the extra-extra-large files without it.



The Unusually Useful Web Book
The Unusually Useful Web Book
ISBN: 0735712069
EAN: 2147483647
Year: 2006
Pages: 195
Authors: June Cohen

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