JPEG (.JPG)

Joint Photographic Experts Group (ISO/ITU-T). It has defined many types of image compression. JPEG is a DCT-based data compression standard for individual pictures (intra-frame). It offers compression of between two and 100 times and has three levels of processing which are defined as: baseline, extended and lossless encoding.

JPEG baseline compression coding, which is overwhelmingly the most common in both the broadcast and computer environments, starts with applying DCT to 8 x 8 pixel blocks of the picture, transforming them into frequency and amplitude data. This itself may not reduce data but then the generally less visible high frequencies can be divided by a high ‘quantizing’ factor (reducing many to zero), and the more visible low frequencies by a much lower factor. The ‘quantizing’ factor can be set according to data size (for constant bit rate) or picture quality (constant quality) requirements – effectively adjusting the compression ratio. The final stage is Huffman coding which is lossless but can further reduce data by 2:1 or more.

Baseline JPEG coding creates .jpg files and it is very similar to the I-frames of MPEG, the main difference being they use slightly dissimilar Huffman tables.

See also: Motion JPEG