The invention relates to a method of locally adjusting a quantization step for each region of an image with respect to a setpoint quantization step. The method comprises the steps of: assigning, to said region of interest, a first quantization step, lower than or equal to said setpoint 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 number of bits required by the region of interest is lower than the number of bits saved on the potential masking region; 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.