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.

An image reading device includes a reading member and a skew detector. The reading member reads an image of a passing document at a read position preset in a document transport path. The skew detector detects a skew of the document by selecting one of a first skew detection mode and a second skew detection mode based on information that specifies a method for detecting the skew of the document. The first skew detection mode is a mode in which the skew of the document relative to a transport direction is detected based on an image of a leading-edge section, in the transport direction, read by the reading member. The second skew detection mode is a mode in which the skew of the document relative to the transport direction is detected based on an entire image read by the reading member.

A scanner is disclosed. The scanner has a background pattern that is captured as part of the scanned image. The scanner may be a flatbed scanner, a sheet-feed scanner, or a flatbed scanner using an automatic document feeder (ADF).