Databases Reference
In-Depth Information
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F I GU R E 16 . 7
Data structure used in the EZW coder.
011
010
001
000
−
T
0
T
0
100
101
110
111
F I GU R E 16 . 8
A 3-bit midrise quantizer.
Before we describe the EZW algorithm, we need to introduce some terminology. Given
a threshold
T
, if a given coefficient has a magnitude greater than
T
,itiscalleda
significant
coefficient at level
T
. If the magnitude of the coefficient is less than
T
(it is insignificant) and
all of its descendants have magnitudes less than
T
, then the coefficient is called a
zerotree root
.
Finally, it might happen that the coefficient itself is less than
T
, but some of its descendants
have a value greater than
T
. Such a coefficient is called an
isolated zero
.
The EZW algorithm is a multiple-pass algorithm with each pass consisting of two steps:
significance map encoding
or the
dominant pass
, and
refinement
or the
subordinate pass
.If
c
max
is the value of the largest coefficient, the initial value of the threshold
T
0
is given by
