Suboptimal quantization control employing approximate distortion-rate relations for motion video coding

In typical image and video coding techniques, the choice of quantizer scales at the encoder plays a key role in determining the generated bit-rate and the coding performance. The distortion-rate (D-R) curve of a video unit fully characterizes the relation between quantizing distortion and encoder output rates and can thus be used in the choice of good quanitizer scales. However, obtainment of the D-R curves is a heavy computational load. We develop a piecewise-linear/exponential model to approximate the true curves for macroblocks. Based on this D-R model, we devise a quantizer control method which is a modification of that in Test Model 5 (TM5) for MPEG2. A reference quanitizer scale is first calculated from past bit usages and the buffer fullness as in TM5. Then the slope of each approximated macroblock D-R curve at this quanitizer scale is calculated. In line with human vision characteristics, an adjusted slope value is computed according to a macroblock activity measure. The quantizer scale yielding the adjusted slope on the D-R curve is found and used. Simulation results show that this method of quanitizer choice can attain not only higher PSNR values but also higher visual quality in coded video.