JPEG is widely used image compression technique. It is used in image processing systems such as copiers, scanners and digital camera’s. These devices ofte3n require high-speed image compression techniques.

JPEG Encoder:

In the figure DCT stands for the Discrete Cosine Transform and IDCT stands for Inverse Discrete Cosine Transform. The input image is partitioned into a 8x8 sub-block. The DCT is computed on each of the 8x8 blocks of pixels. The coefficients with zero frequency in both the directions is called the “DC coefficient’ and the remaining 63 coefficients are called the ‘AC coefficients’. The DCT processing step lays the foundation for achieving data compression by concentrating most of the signal in the lower spatial frequencies. The 64 DCT coefficients are scalar quantized using the uniform quantisation tables based upon psychovisual experiments. After the DCT coefficients are quantised, the coefficients are ordered according to the zigzag scan as shown in the below figure. The purpose of the zigzag scanning is based upon the observation that most of the high-frequency coefficients are zero after quantization.

JPEG Decoder:

The JPEG standard is a broad standard encompassing several compression and transmission modes. In order to facilitate future expansion to other modes, this implementation has a very modular construction. A single thread of control code handles all individual routines (kernels) which are called multiple times as required by the application. This control code will always be in ‘C’to facilitate the changes in the control architecture. The quantizes and the Huffman encodes the DC coefficients obtained from the DCT module. In JPEG the DC component is differentially encoded i.e. a difference between the present and the preceding DC component is computed and this difference is quantized and encoded. Quantization involves an inherent division operation with an element from the quantizer table. In this implementation a reciprocal quantizer table, pre-computed from the quantizer table, is used.