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For image and video coding under transmission constraints,
determination of quantization steps at encoder plays an
important role in controlling the generated bit-rate and coding
performance. In this thesis, we analyze the quantization
information from some Video CD titles and compare them with
several known rate control algorithms. In addition, we
investigate how to determine quantization steps for good coding
quality. We approach this by improving on a well-known quantizer
control algorithm, namely, Test Model of MPEG2. We know that
the rate-distortion curve of a macroblock fully characterize the
relationship between rate and distortion and can thus be used in
determination of good quantization steps. Becasue of the heavy
computational load required for calculating the real R-D curves,
we develop a piecewise approximation model to predict the real
curves. In deciding an appropriate quantization step, we first
calculate a reference step on the buffer status to prevent the
buffer from overflow and underflow. Next, the human vision
factor and the slope of the estimated rate-distortion curve are
incorporated into adjustment of this quantization step to obtain
a suboptimal quantization step. Simulation reesults show that,
we can attain not only higher PSNR values but also higher human
visual quality in coded video.
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