Jae S. Lim

Multidimensional bit-rate control for video communication

In conventional bit rate control schemes, the buffer level is controlled by adjusting the quantization step size while the frame rate and spatial resolution chosen for coding are fixed throughout the coding process. In this paper, we consider a more general Multidimensional (M-D) bit rate control where the frame rate, spatial resolution and quantization step size are jointly adapted for buffer control. In the M-D bit rate control setting, the problem is to decide which frames to code (and which frames to skip) along with the spatial resolution and the quantization step size to use for each coded frame. Given a finite set of operating points on a M-D grid, we formulate the optimal solution of the M-D buffer-constrained allocation problem. The formulation allows a skipped frame to be reconstructed from one coded frame using any temporal interpolation method. A dynamic programming algorithm is presented to obtain an optimal solution for the case of intraframe coding which is a special case of dependent coding. We experiment with both zero-order hold and motion-compensated temporal interpolation. Operational rate-distortion (R-D) bounds are illustrated for both the M-D and conventional bit rate control approaches. Our focus is one very low bit rate applications where a significant delay is tolerable.

Video Compression and Noise Reduction Using Transform/Subband Coding and Adaptive Amplitude Modulation

This article presents the basic ideas behind transform and subband representations of a signal. Although transform and subband representations appear to be different, in fact, they are equivalent. This theoretical relationship gives useful insights into the design of video processing and compression systems. An application of this result to the design of an adaptive amplitude modulation/demodulation (AM/DM) system for noise reduction of images is described. Also discussed is the AM noise-reduction technique for additive noise, and a method is presented to reduce the AM side information by exploiting the transform representation of an image.

Optimal transform coefficient selection for images

An algorithm for methodically deriving rate-distortion points for transform coefficient selection schemes for images is presented. The idea is to iteratively generate a set of convex hulls from which a composite operational rate- distortion curve is derived. Although this approach can be used to generate optimal interior rate-distortion points, the complexity is high. A fast suboptimal approach is then proposed which is based upon a modified version of threshold selection. In the modified threshold selection algorithm, each transform block operates on a point along a non-convex rate-distortion curve which is generated from rank ordering of coefficients in the block. Simulations of this fast algorithm using finely quantized DCT coefficients from an image with separate coding of amplitudes and runlengths show that very good rate-distortion performance can be obtained. These simulations also suggest that the modified threshold selection curve tends to lie within the first few convex hulls generated from the composite shell method. The modified threshold selection algorithm provides a fast way for achieving good rate-distortion performance in transform coding systems.