GLOBAL RATE CONTROL FOR MODEL-ASSISTED CODING OF LOW BIT RATE VIDEO

A method and apparatus for coding an image comprising (at least) two regions (e.g., a facial region and a background region) which are to be coded with different quantization error parameters (e.g., quantizer step sizes), wherein the respective quantization error parameters are determined with use of a feed-forward rate control technique. Initial quantization error parameters are assigned to each region, and the overall bit rate which would result for the coding of the given frame is computed based on these assigned quantization error parameters. The computed bit rate is compared to a given bit rate constraint which may, for example, represent a limitation of a transmission channel. Based on the result of this comparison, one or both of the assigned quantization error parameters are adjusted, thereby resulting in different quantization error parameters for the two regions. For example, if the computed bit rate exceeds the given bit rate constraint, at least one of the quantization error parameters is advantageously adjusted in a direction which results in the use of fewer coding bits (e.g., a coarser quantization). If the given bit rate constraint exceeds the computed bit rate, at least one of the quantization error parameters is advantageously adjusted in a direction which results in the use of more coding bits (e.g., a finer quantization resulting in a higher coding quality). Finally, each of the regions is coded based on the correspondingly assigned quantization error parameters.