Abstract: Fingerprint periodicity/frequency is an important parameter for filtering，segmentation，ridge line tracking and matching in Automated Fingerprint Identification System（AFIS）.Due to the influence of image definition，even a half-pixel error may cause periodicity/frequency calculation error，resulting in a subversive result in the post-processing.With the sub-pixel calculation，the fingerprint periodicity matrix and normal direction image can be gotten accurately and the sharp corner of fingerprint image center line in existing direction image method can be avoided，the AFIS performance also can be improved significantly.

摘要： 指纹的周期/频率是AFIS中的滤波、分割、脊线跟踪和匹配的重要参数。受图像分辨率的影响，半个像素单位的误差都可能造成周期/频率的计算错误，导致后处理过程中出现颠覆性结果。通过亚像素的计算，可以精确地输出指纹的周期矩阵和法向图，避免了方向图算法中无法计算指纹中心线急转弯的问题，使整个AFIS的性能得到了很大的提高。