Patent ID: 8135226

Claim:
An image encoder comprising: filtering means which hierarchically performs a filtering process on an input image, to generate a plurality of sub-bands; quantization means which quantizes coefficients in each sub-band after the filtering processing, wherein each sub-band has an assigned quantization step size; code block generation means which divides the sub-bands, to generate a plurality of code blocks each having a predetermined size; bit plane generation means which generates bit planes from a most significant bit to a least significant bit, for each code block; encoding means which arithmetically encodes a coding pass generated for each bit plane; and format means which formats by adding a header to an arithmetic code generated by the encoding means, to generate an encoded code stream, wherein, the quantization means has a capability to vary a width of a dead zone corresponding to the assigned quantization step size by (1) dividing each of the coefficients in each of the sub-bands by the assigned quantization step size corresponding to the sub-band to which the coefficient being divided belongs and (2) adding a first constant value to the result of the division before rounding down the result, the dead zone being a quantization zone where the coefficients are quantized to a value of 0, and the format means sets a predetermined parameter included in the header with the quantization step size, the predetermined parameter being used to determine a second constant value for use with the quantization step size to calculate a wavelet transformation coefficient when an inverse quantization corresponding to the quantization of the quantization means is performed; and wherein when the width of a dead zone has been varied, then the second constant value is 0; otherwise the second constant value is determined as one of a plurality of values between and including zero and one, and said plurality of values include values other than 0 and 0.5.