The invention relates to a method of locally adjusting a quantization step for each region of an image in a sequence of n images. The method comprises the following steps: calculating, for each image of the sequence, a first difference of bits saved on the potential masking region when quantizing it with a first quantization step and a second difference of bits required by the region of interest when quantizing it with a second quantization step; and in each image of the sequence, assigning to the potential masking region the first quantization step and if the sum of the n first differences of bits is higher than the sum of the n second differences of bits, assigning said second quantization step to said region of interest and otherwise assigning a third quantization step such that the sum of said n first differences of bits is distributed between the regions of interest according to a reconstruction quality criterion.