Direction alignment algorithm for direction-adaptive discrete wavelet transform

2-D discrete wavelet transform (2-D DWT) represents non-vertical or non-horizontal edges inefficiently. Direction-adaptive discrete wavelet transform (DA-DWT) solves this problem by filtering along the directions of textures. DA-DWT partitions images into many blocks and finds the most suitable direction of each block. The block partition and direction information need to be transmitted as side information. A conventional DA-DWT often produces inconsistent directions of neighboring blocks and thus results in large amount of side information. In this paper, we propose a bottom-up direction alignment algorithm to align the block directions in local areas. Our algorithm can reduce 50% or more in side-information bits at the cost of negligible prediction error increase.