Compression Ratio

The compression ratio is equal to the size of the original image divided by the size of the compressed image. This ratio gives an indication of how much compression is achieved for a particular image.

The compression ratio achieved usually indicates the picture quality. Generally, the higher the compression ratio, the poorer the quality of the resulting image. The trade-off between compression ratio and picture quality is an important one to consider when compressing images.

Furthermore, some compression schemes produce compression ratios that are highly dependent on the image content. This aspect of compression is called data dependency. Using an algorithm with a high degree of data dependency, an image of a crowd at a football game (which contains a lot of detail) may produce a very small compression ratio, whereas an image of a blue sky (which consists mostly of constant colors and intensities) may produce a very high compression ratio.

Compression Speed

Compression time and decompression time are defined as the amount of time required to compress and decompress a picture, respectively. Their value depends on the following considerations:

• the complexity of the compression algorithm

• the efficiency of the software or hardware implementation of the algorithm

• the speed of the utilized processor or auxiliary hardware

Generally, the faster that both operations can be performed, the better. Fast compression time increases the speed with which material can be created. Fast decompression time increases the speed with which the user can display and interact with images.

Image Quality

Image quality describes the fidelity with which an image-compression scheme recreates the source image data. Compression schemes can be characterized as being either lossy or lossless. Lossless schemes preserve all of the original data. Lossy compression does not preserve the data precisely; image data is lost, and it cannot be recovered after compression. Most lossy schemes try to compress the data as much as possible, without decreasing the image quality in a noticeable way. Some schemes may be either lossy or lossless, depending upon the quality level desired by the user.