Hierarchical Encoding

The hierarchical mode provides a "pyramidal" encoding of an iamge at multiple resolutions, each differing in resolution from its adjacent encoding by a factor of two in either the horizontal or vertical dimension or both. The encoding procedure can be summarized as follows :
1.    Filter and down-sample the original image by the desired number of multiples of 2 in each dimension.

2.    Encode this reduced-size image using one of the sequential DCT, progressive DCT, or lossless
       encoders.

3.    Decode this reduced-size image and then interpolate and up-sample it by 2 horizontally and/or
       vertically, using the identical interpolation filter which the receiver must use.

4.    Use this up-sampled image as a prediction of the original at this resolution, and encode the difference
       image using one of the sequential DCT, progressive DCT, or lossless encoders.

5.    Repeat step 3 and step 4 until the full resolution of the image has been encoded.

The encoding in step 2 and step 4 must be done using only DCT-based processes, only lossless processes, or DCT-based processes with a final lossless process for each component.

Hierarchical encoding is useful in applications in which a very high resolution image must be accessed by a lower-resolution display. An example is an image scanned and compressed at high resolution for a very high-quality printer, where the image must also be displayed on a low-resolution PC video screen.

JPEG Introduction