JPEG: Coding Steps
The aim of the most common mode of JPEG is to provide the best possible quality at a given compression ratio. Therefore the original data must be brought into a suitable form allowing to differentiate between important (relevant) and less important (irrelevant) contents.
The JPEG standard specifies a multi-level coding process:
1. Image Preparation (Separation into Components)
The image will be separated into components, which are adapted to the different quality requirements.
2. Transformation (DCT: Discrete Cosine Transform)
Each individual component is transferred into a format, that allows to make conclusions about the structure of the contents. This offers the option to distinguish between basic and more complex contents.
3. Quantization (Weighting of the Contents)
The transformed data are weighted according to their meaning for the image contents.
4. Entropy Coding (Huffman Coding)
Elimination of redundant information.
The outlined procedure is the common mode of operation. Additionally several other modes are specified, for example a version for lossless compression.