Patent ID: 7623727

Claim:
An image binarization apparatus comprising: a block division unit which divides a multi-valued image into blocks; a luminance value output unit which outputs a luminance value of each pixel forming the multi-valued image; a binarization threshold value setting unit which sets a binarization threshold value to be used when binarizing the multi-valued image; a low luminance threshold value setting unit which sets a low luminance threshold value to be used when removing low luminance values; a low luminance value removal unit which inputs luminance values of each pixel from among luminance values output by said luminance value output unit, removes luminance values that are lower than the low luminance threshold value set by said low luminance threshold value setting unit, and outputs only those luminance values which exceed the low luminance threshold value; and a mean luminance value calculation unit which calculates a mean luminance value of the luminance values output by said low luminance value removal unit, wherein said low luminance threshold value setting unit sets the low luminance threshold value based on mean luminance values of at least selected of the blocks; and said binarization threshold value setting unit sets the binarization threshold values based on mean luminance values of the blocks.