In this thesis work, three problems for the improvement of conventional transform coders are investigated.
By an accurate representation of the parametric distortion function in the block quantization, new normalized variance levels for bit allocation are obtained. It has been shown that the performance of the new scheme is about 15-25 percent better than that of a conventional one.
In the adaptive two-dimensional transform coding with activity classification, several classification measures are investigated in view of adaptation and hardware simplicity. It has been found that the partial sum of the absolute coefficient values of the first quarter, scanned by a zig-zag pattern in each transformed block, is the most efficient measure.
Lastly, for a direct processing without a large amount of buffer, the first-order Markov modelling is investigated. It has been found that it can be suitably used in the one-dimensional transform coding, but not in the two-dimensional transform coding. In addition, the recursive algorithm for a direct processing in the two-dimensional transform coding and two types of hybrid coding, transform-DPCM and DPCM-transform, have been studied.
Simulations have been made with two 8-bit resolution 256×256 size images called GIRL and COUPLE and the reconstructed images are taken from a displayer of 4-bit resolution for 128×128 size, after averaging the adjacent four pixels.
본 논문에서는 기존 transform coder의 개선에 관한 세가지 문제가 연구되었다.
비트 수에 따라 변화하는 parametric distortion function의 근사식을 이용함으로써, 최적의 비트 할당을 위한 새로운 variance level들이 제시되었으며, simulation결과 normalized mean-square error가 약 15~25% 개선됨이 밝혀졌다.
Adaptive two-dimensional transform coding에 있어서, 각 block의 activity 를 구분하기 위한 여러 measure들이, adaptation 및 hardware 간소화의 측면에서 고려 되었으며, simalation결과, zig-zag scanning한 coefficient들 중에서 처음의 $\frac{1}{4}$부분의(16 × 16 block에서 약 70개) 절대치의 합을 이용하는 것이 가장 효과적임이 밝혀졌다.
마지막으로, 기존의 transform coder가 대량의 buffer를 필요로 하는데 반해, 거의 buffer가 필요없는 direct processing을 위한 1차 Markov modelling이 고려 되었다. Simulation결과 one-dimensional transform coding에는 적합하나, two-dimensional transform coding에는 부적합하며, Tescher 및 Cox에 의해 제안된 recursive방식이 효과적임이 밝혀졌다. 그외, transform-DPCM, DPCM-transform등 두 종류의 hybrid coding 방식이, 실제적인 측면에서 검토되었다.
