Character string envelope correlation based vehicle license plate tilt correction in vertical direction

Obtaining vehicle license plate (VLP) character strings from VLP image is an essential technology in the intelligent transportation systems. The accuracy of character string segmentation would be greatly impacted by character string tilt resulted from different imaging angle. A character string envelope based vertical tilt correction method is proposed in this paper according to VLP's symmetry characteristic of character string and character space. Firstly, top envelope and foot envelope are calculated from VLP character string. Then, based on the cross-correlation function of top envelope and foot envelope, the coordinate of maximum value of the cross-correlation function which is corresponding to the horizontal axis offset is obtained to calculate the VLP's incline angle in vertical direction. The experimental results show that the proposed method performs better in both time complexity and evaluation error of the vertical tilt angle compared with other methods.