TABLE OF CONTENTS An image compression method is normally designed for a specific type of image, and this section lists various approaches to compressing images of different types. Only the general principles are discussed here; specific methods are described in the remainder of this chapter.
Approach 1 This is appropriate for bi-level images. A pixel in such an image is represented by one bit. Applying the principle of image compression to a bi-level image therefore means that the immediate neighbors of a pixel P tend to be identical to P. Thus, it makes sense to use run-length encoding (RLE) to compress such an image. A compression method for such an image may scan it in raster order (row by row) and compute the lengths of runs of black and white pixels. The lengths are encoded by variable-size (prefix) codes and are written on the compressed stream. An example of such a method is facsimile compression, Section 2.13.
It should be stressed that this is just an approach to bi-level image compression. The details of specific methods vary. For instance, a method may scan the image column by column or in zigzag (Figure 1.8b), it may convert the image to a quadtree (Section 4.30), or it may scan it region by region using a space-filling curve (Section 4.32).
Approach 2 Also for bi-level images. The principle of image compression tells us that the neighbors of a pixel tend to be similar to the pixel.
