Data compression Image compression

The explosive growth in the use of digital data poses problems of transmission time and/or storage volume. Technological advances, while undeniable, are often not enough, either because they are not yet mature, or because they are too expensive. Hence the importance of chasing unnecessary bits. And, if you look closely, there are more of them than you might think. With a few tricks and a bit of method, it's possible to save - sometimes substantially - on the number of binary digits used to digitally encode data. This is known as compression.

Today, there are few fields that have not succumbed to digital technology. These include, of course, sound and images. However, in addition to their ever-increasing use, the latter - and this is particularly true of images - are characterized by the fact that :

• the number of bits they require is considerable;

• their use often imposes real-time constraints.

This explains the particular importance of compressing these signals. We'll be giving them pride of place.

We'll see that there are many different techniques available for compressing digital data. They all have different advantages and disadvantages, and making a choice is a tricky business. This is particularly true for images. Here are a few criteria.