Information Technology Reference
In-Depth Information
Figure 5.71:
Zerotree coding relies on the redundancy in wavelet coefficients from the lowest resolution upwards in
a tree structure.
Zerotree coding is efficient because in real images the energy is concentrated at low frequency and most trees do
not need to be coded as far as the leaf node.
It is important to appreciate that the scanning order in which zerotree coding takes place must go from the lowest
AC sub-band upwards. However, when the coefficients have been designated by the zerotree coder into the four
categories above, they can be transmitted in any order which is convenient. MPEG-4 supports either tree depth
scanning or band-by-band scanning. Figure 5.72(a) shows that in tree-depth scanning the coefficients are
transmitted one tree at a time from the lowest spatial frequency to the highest. The coding is efficient but the
picture will not be available until all the data have been received. Figure 5.72(b) shows that in band-by-band
scanning the coefficients are transmitted in order of ascending spatial frequency. At the decoder a soft picture
becomes available first, and the resolution gradually increases as more sub-bands are decoded. This coding
scheme is less efficient but the decoding latency is reduced which may be important in image database browsing
applications.
Figure 5.72:
(a) In tree-depth scanning the coefficients are scanned up each tree in turn. (b) In band-by-band
scanning the coefficients are transmitted in order of increasing resolution.
Figure 5.73(a) shows a simple wavelet coder operating in single quant mode. After the DWT, the DC or root
coefficients are sent to a predictive coder where the values of a given coefficient are predicted from adjacent
coefficients. The AC coefficients are zerotree coded. Coefficients are then quantized and arithmetic coded. The
decoder is shown in (b). After arithmetic decoding, the DC coefficients are obtained by inverse prediction and the
AC coefficients are re-created by zerotree decoding. After inverse quantizing the coefficients control an inverse
DWT to re- create the texture.
