Image-reading device detecting edge of sheet by dynamically setting mask region

Abstract

An image-reading device reads an image of a sheet while conveying the sheet. The image-reading device sets target lines in the image for searching an edge pixel representing a sheet edge in the image. The target lines include first and second lines that are sequentially arranged. When the first target line includes an edge pixel, a mask region is set on the image. A detection unit searches an edge pixel by sequentially referring to the target lines except the first line according to a line order from the second line and detects a positional coordinate of the edge pixel from among pixels outside the mask. Once the edge pixel is included in the first target line, the mask region is set continuously and dynamically from the first target line to a target line on which detection of the edge pixel ends.