In a case where a dot forming condition is not satisfied, whether to form a dot for a target pixel is determined according to a predetermined method and dependently on a colorant-corresponding gradation value for the target pixel. The colorant-corresponding gradation value for each pixel is defined for a color component corresponding to colorant and representing density of the color component in color of the each pixel. The dot forming condition includes that the target pixel is an edge pixel and that a density index value for the target pixel, which is related to density of the color component in the color of the target pixel that is represented by the colorant-corresponding gradation value, is greater than a threshold value. In a case where a dot forming condition is satisfied, it is determined that a dot is to be formed for the target pixel, irrespective of the colorant-corresponding gradation value.

In a case where a dot forming condition is not satisfied, whether to form a dot for a target pixel is determined according to a predetermined method and dependently on a colorant-corresponding gradation value for the target pixel. The colorant-corresponding gradation value for each pixel is defined for a color component corresponding to colorant and representing density of the color component in color of the each pixel. The dot forming condition includes that the target pixel is an edge pixel and that a density index value for the target pixel, which is related to density of the color component in the color of the target pixel that is represented by the colorant-corresponding gradation value, is greater than a threshold value. In a case where a dot forming condition is satisfied, it is determined that a dot is to be formed for the target pixel, irrespective of the colorant-corresponding gradation value.

Provided is a scanner having sixteen or fewer sensor chips capable of scanning an A4 document; an optical unit that reduces and focuses an image of an A4 document on the sensor chips; and a generator that synthesizes images of the A4 document read by the sensor chips, and generates image data.

A read image processing apparatus is provided for processing a read image read by line image sensors arranged such that one end parts of which adjacent to each other are overlapped in a sub-scanning direction. The read image processing apparatus includes a density variation detector, a seam processor, and a line image generator. The density variation detector detects whether a density variation occurs for not less than a predetermined number of times or not in each of a plurality of regions in a seam-read image. The seam processor generates a seam image by synthesizing the seam-read images read by the line image sensors adjacent to each other in accordance with a detection result made by the density variation detector. The line image generator generates a read image corresponding to one line based on read images read by the line image sensors and the seam image generated by the seam processor.