US Patent 8483441 Automatic recognition of scale marks on level ruler

Image recognition and distance calculation methods and devices are provided. First, an image corresponding to a level ruler is obtained, wherein the image includes a first predefined scale and a second predefined scale projected on the level ruler. Then, a baseline height of a baseline mark in the level ruler is determined according to the image. The heights of the first predefined scale and the second predefined scale are calculated according to the baseline height of the baseline mark, the position of the baseline mark in the image, and the positions of the first predefined scale and the second predefined scale in the image. Thereafter, a distance to the level ruler is calculated according to a height difference between the heights of the first predefined scale and the second predefined scale.