JPEG Decompression Algorithm

JPEG Decompression Algorithm

Decompression is roughly the inverse process from compression, but there are some additional steps needed to produce a good output image.

The logical steps needed in JPEG decompression are:

Step 1: Scanning of the JPEG file, decoding of headers/markers etc.

Step 2: Huffman entropy decoding of the coefficient sequence.

( Uses Huffman table from DHT marker).

Step 3: Zigzag reordering of the elements in each 8x8 block.

Step 4: De-quantization of the elements in each 8x8 block.

(Uses Quantization table from DQT marker).

Step 5: Inverse DCT transformation of each 8x8 block.

Step 6: Re-subsampling. At this point a pixel image of the original dimensions

has been recreated.

Step 7: Color space reconversion (e.g., YCbCr to RGB). This is a null step for

greyscale.

Repeat the steps 2-7 for each 8x8 block until all the MCUs are processed.