Adaptive hierarchical algorithms of vector quantization (VQ) for image
coding are proposed. First the basic codebook is generated adaptively
using adaptive VQ, then the quadruplets of codes/indices in the so-ca
lled zigzag order are coded into higher level (second and third levels
) codes by creating the second- and third-level index codebooks to red
uce the redundancy presented in the codes. Partially matched quadruple
ts are also encoded in the second and third layers using the index cod
ebooks along with corresponding correction schemes. The third-layer en
coding achieves a better compression ratio than a two-layer encoding s
cheme, which was shown to be optimal when partial encoding was not ado
pted. This three-layer coding scheme achieves better compression with
no extra distortion and little extra computation. Experiments show enc
ouraging results.