Lossy Compression
Lossy compression schemes save the most important data with little or no noticeable quality depreciations. Lossy schemes save data size by making sacrifices and removing un-noticeable information. Once uncompressed, the original data that was compressed will not be exactly the same as the original. This is ok for some media, such as sound or video, but it is impossible to use on data.

Most lossy compression schemes are able to change the quality of their output by sacrificing their storage capacity. A good example is the JPEG lossy compression scheme.

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Uncompressed 32450 bytes 24 bits/pixel

JPEG (setting 1)	JPEG (setting 50)	JPEG (setting 99)
15810 bytes	3483 bytes	1025 bytes
11.7 bits/pixel	2.6 bits/pixel	0.76 bits/pixel

As you can see, the compression setting 1 looks vary similiar to the original, and a setting that lies somewhere in between setting 1 and setting 50 would be the best size/quality trade off.

Lossless Compression
With lossless compression, no data is lost. Information is compressed without making compromises in quality. This is why any information that needs to remain perfect is compressed using lossless compression. All data compression formats like RAR and GZip are lossless, because information has to be exact.

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Uncompressed	Lossless	Lossy
7200 bytes	902 bytes	6523 bytes
4 bits/pixel	0.5 bits/pixel	3.6 bits/pixel

Lossless compression can be smaller in size, and a lot higher in quality over lossy compression. This is when few colours are used, there is a lot of repetition, and there are straight lines. The JPEG image to the right has severe problems in quality because it compresses using blending and averages. The straight lines in the pattern are smudged. The end result is that the original colours look smudged. Lossless does not suffer from these limitations, because the original information remains intact. Lossless compression is built more around compressing patterns then random images, so for this example it resulted in a smaller file size because of its greater compression ratio.

With lossy compression, data is lost every time the image is encoded so it is bad to edit from a lossy format. Instead, an original copy should be saved in a lossless format, and every time that the image needs to be edited, the lossless image should be used.

Quality Loss of Level 50 JPEG Compression

Original

Compressed

Difference

There is obviously some image quality loss. The difference was calculated by using a difference filter in a graphics editing application, and then creating a negative image to make the difference easier to see.

Lossless Versus Lossy | Data Compression | Image Compression | Audio Video Compression