1. An image processor comprising: a gradation value acquisition unit that acquires a gradation value of a pixel of interest which is a pixel sequentially selected as a target of a binarization process from input image data represented by pixels of M gradations, wherein M≧3; and a pattern determination unit that determines a filling pattern of a group of pixels of output image data corresponding to the pixel of interest according to a corrected gradation value acquired by adding, to the gradation value of the pixel of interest, an error value diffused from a pixel at a periphery of the pixel of interest, wherein the filling pattern includes at least a first pattern in which a predetermined plurality of pixels are filled and which forms a core of a dot and a third pattern in which substantially no pixel is filled and the pattern determination unit determines the filling pattern to be one of the first pattern and the third pattern according to a size relationship between the corrected gradation value and a predetermined threshold value which spatially varies in a periodic manner.