Abstract: A color image capturing device provided with groups of two rows of photoelectric conversion elements for each color of R, G and B is mounted on a carriage and planar color images are read by moving the carriage in a sub scanning direction. As each first row of photoelectric conversion elements and each second row of photoelectric conversion elements are offset from each other by an amount equivalent to a half of the width of an individual element in a main scanning direction, and resolution in the main scanning direction is enhanced. As each row of photoelectric conversion elements is arranged at pitch equivalent to the height of four rows, all the rows of photoelectric conversion elements can read the same line, even if the carriage is moved at speed twice or four times as fast as that in reading at 600 dpi to read at the resolution of 300 dpi or 150 dpi in the sub scanning direction at high speed in case the reading resolution of each row of photoelectric conversion elements is 600 dpi.

Abstract: An image to be read is divided into blocks, and pixels included in each block are consecutively binarized on the basis of a threshold. At this time, the threshold is determined in accordance with a maximum value of an image signal of an area including a block adjacent to an attention-focused pixel. If the threshold is smaller than a reference threshold which is determined in accordance with the maximum value of the image signal of a particular reference pixel of a preceding block, the area is changed, and the threshold is redetermined in accordance with the maximum value of the image signal of the new area.