Jürgen Pandel

Variable bit-rate image sequence coding with adaptive quantization

Abstract In this paper a variable bit-rate image sequence coding scheme for ATM will be described. The code bit data structure is fully compatible to the CCITT H.261 recommendation for a p × 64 kbit/s constant bit-rate codec. In order to keep the average bit-rate as low as possible without quality degradation, the quantization of the DCT coefficients will be adapted according to the visibility of quantization errors. Such errors are most easily visible in homogeneous areas of medium brightness, especially around objects in front of a homogeneous background. According to the Weber/Fechner law and the γ-characteristic of the cathode ray tube, the visibility of errors decreases for very bright or dark areas. Structured areas are also less sensitive in this respect. A segmentation of the images into critical and less critical areas will be performed by the determination of the local activity and the average brightness of homogeneous regions. Some experimental results will be reported and compared with constant quantization using standard sequences and a sequence taken from regular PAL television. Obviously, the savings which can be obtained by this method depend on the content of the scene, but for most of the sequences the average bit-rate could be reduced by more than 30% without quality degradation.