Introduction#
How many bytes are needed to code one second of Full HD video?
Solution
A Full HD video is characterized by images of size 1920 × 1080, each of the three channels (RGB) is coded with 1 byte, and there are 50 frames per second.
One second contains 1920 × 1080 × 3 × 50 = 311 040 000 bytes, or around 311 Mb. Thus, a DVD (4,7 Gb) can store only 15 seconds of a movie… 😲 You would need more than 600 DVDs to watch a movie…
This example shows the need for image compression! The purpose of image compression is to encode the information contained in the image with fewer bytes than the original image.
OK, but how to quantify information? In 1948, Claude Shannon proposed that an image or a message is very informative if it is unlikely. It is still the mathematical definition of information today.
We distinguish two kinds of compression:
With Lossless compression, no information is lost during compression, i.e. the original image and the compressed image are perfectly identical. It is possible to reach a compression ratio up to 1/10 (the size in bytes of the compressed image is 1/10 of the original one).
In Lossy compression, we accept to lose information during compression (while ensuring that the original image remains close to the original one). The compression ratio can reach up to 1/100!