Patent ID: 8139881

Claim:
Method for adjusting locally a quantization step in an image relative to a setpoint quantization step, said image comprising at least one region of interest and one potential masking region, each of said regions possibly comprising a number of disjoint groups of pixels, said region of interest and potential masking region being encoded in a first and a second number of bits respectively when they are quantized with said setpoint quantization step, wherein it comprises the following steps: assigning, to said region of interest, a first quantization step, lower than or equal to said setpoint quantization step, said region being encoded in a third number of bits when it is quantized with said first quantization step; determining a second quantization step higher than or equal to said setpoint quantization step and lower than or equal to a third quantization step such that the difference between said third and first numbers of bits is lower than or equal to the difference between the second number of bits and a fourth number of bits equal to the number of bits used to encode said potential masking region quantized with said second quantization step; and assigning when such a second quantization step exists, said second quantization step to said potential masking region, otherwise assigning said third quantization step to said potential masking region.