Subband coding and matrix quantization of images

This paper describes a method of splitting an image into subbands using quadrature mirror filter (QMF) banks followed by a 2:1 decimation. Linear phase FIR filters are used for the QMF filter bank. Both full and octave band splitting are performed up to three levels. It is found that among linear phase FIR filters of different tap length, transition width/ and reconstruction error, a twelve-tap filter gives the least distortion due to aliasing error caused by decimation and interpolation. The subband images are encoded using vector or matrix quantizer to achieve bit rates less than 0.5 bit per pixel. For good visual quality, higher bit rates are assigned to low bands since it has been observed that these bands carry all the essential details in the original image in a compressed form whereas the high bands are much less significant and, therefore, assigned lower bit rates. Further investigation reveals that an appropriate level of splitting should be three. Moreover, octave band splitting is preferable to full-tree coding even though the latter results in higher SNR value. This is due to the fact that octave band coding requires fewer codewords and codebooks which are important from implementation point of view.