Scalable direction representation for image compression with direction-adaptive discrete wavelet transform

The direction-adaptive discrete wavelet transform (DA-DWT) locally adapts the filtering direction to the geometric flow in the image. DA-DWT image coders have been shown to achieve a rate-distortion performance superior to non-adaptive wavelet coders. However, since the direction information must always be signalled regardless of total bit-rate, performance at very low bit-rates might be worse. In this paper, we propose two scalable direction representations: the layered scheme which is similar to the scalable motion vector representation in scalable video coding and the level-unit scheme which provides finer granularity upon the layered scheme. Experimental results indicate that we can achieve the desirable performance at both low and high bit rates with our proposed level-unit scheme. Significant improvement in image quality (about 3-5 dB) is observed at very low bit rate, relative to non-scalable coding of the direction information.