Image and Video Compression

91Image Compression Using Vector Quantization

simple VQ and gain-shaped VQ when coded at 0.25 bpp with a 4 × 4 block

size (Bovik 2005).

5.7 Classified Vector Quantizer

As discussed in Section 5.2.1, the major drawback of VQ-based compression

is blockiness in decompressed images. Edge information is lost or degraded

in small-size codebooks. In order to preserve the edges, it is necessary to

have such codevectors present in the codebook. Gersho and Ramamurthi

(1984) worked on this problem by classifying each image block vector into

different classes or categories. Some of the classes are as follows:

1. Shade with smooth variation of gray levels ...