A method of image-distance transformation using bi-directional scans is provided. The method includes the steps of: performing a first scan on each pixel of an input image using a first mask in a first order to generate an intermediate image; and performing a second scan on each pixel of the intermediate image using a second mask in a second order to obtain distance information of each pixel in the input image. A first current pixel in the input image that is not compared with prior pixels in the first order and in a first current segment is used in the first comparison process in the first scan, and a second current pixel that is compared with prior pixels in the second order and in a second segment is used in the second comparison process in the second scan.