DCT Progressive Encoding

The DCT progressive mode of operation consists of the same DCT and Quantization steps that are used by DCT sequential mode. The key difference is that each image component is encoded in multiple scans rather than in a single scan.The first scan(s) encode a rough but recognizable version of the image which can be transmitted quickly in comparison to the total transmission time, and are refined by succeeding scans until reaching a level of picture quality that was established by the quantization tables.

To achieve this requires the addition of an image-sized buffer memory at the output of the quantizer, before the input to entropy encoder. The buffer memory must be of sufficient size to store the image as quantized DCT coefficients, each of which is 3 bits larger than the source image samples. After each block of DCT coefficients is quantized, it is stored in the coefficient buffer memory. The buffered coefficients are then partially encoded in each of multiple scans.

There are two complementary methods by which a block of quantized DCT coefficients may be partially encoded :
 

Spectral Selection Method
  Only a specified "band" of coefficients from the zig-zag sequence need be encoded within a given scan. This procedure is called "spectral selection" because each band typically contains coefficients which occupy a lower or higher part of the spatial-frequency spectrum for that 8x8 block.


The coefficients within the current band need not be encoded to their full (quantized) accuracy in a given scan. Upon a coefficient's first encoding, the N most significant bits can be encoded first, where N is specifiable. In subsequent scans, the less significant bits can then be encoded.

Successive Approximation Method
Both procefures can be used separately, or mixed in flexible combinations.
 

JPEG Intruduction