Patent ID: 7286714
Filing Date: 2007-10-23
Classification: H04N

Abstract:
1. A method of compressing a digital image signal having coefficients allocated to the pixels of an image, the method comprising: forming spatial blocks of N×M coefficients allocated to the pixels of the image, where N and M are non-zero integers; transforming each spatial block in a frequency domain in order to form a frequency block of N×M coefficients; grouping a plurality of frequency blocks in macroblocks and of a plurality of macroblocks in segments, together with the following steps for each segment: first quantization of the coefficients of each frequency block, with a first quantization step set, defined for each frequency block by an index group including an index which is identical for all the frequency blocks of a macroblock, and second quantization of the coefficients of at least some of the frequency blocks, with a second quantization step set substituted for the first quantization step set, the first and the second quantization step sets being chosen according to a target value of the amount of data needed to represent the quantized coefficients of the segment and according to gain factors respectively calculated for the frequency blocks of the segment, the gain factor relating to one block representing a reduction, on passing from one quantization step set to the other for the said block, of the amount of data necessary to represent the quantized coefficients, in which the gain factor calculated for a frequency block is the difference between the amounts of data needed to represent the quantized coefficients of the frequency block with the first set and with the second set, respectively, divided by the amount of data needed to represent the quantized coefficients of the said frequency block with the first set.