Patent ID: 8717642

Claim:
A flatbed scanner, comprising: a transparent window on a top housing of the flatbed scanner for placing a sheet; an optical ruler on a back surface side of the top housing and close to the transparent window along a scanning direction, the optical ruler comprising at least one optical oblique line arranged thereon; a step motor; a scanner head having a scanning width larger than a width of the transparent window, the scanner head is configured to capture an image of the optical ruler along with the sheet while the scanner head is being progressively moved by the step motor; a processing unit configured to calculate a jag width of a captured oblique line in the captured image, wherein the jag width represents a distance from a front corner of one apparent step to another front corner of a next apparent step of the captured oblique line; and a controlling unit configured to control the step motor to adjust a speed of movement of the scanner head according to the jag width of the captured oblique line.