Abstract: Center extraction is a primary stage in the extraction of Placido image. Therefore, a curve fitting method is proposed to detect the center. Firstly, Hough transform was used to locate the approximate center, and Placido image was located in the polar coordinate. Then the Gaussian kernel function was convolved with the image, and the points were detected in the polaraxis. Finally, based on the coordinates of these points, the accurate center was obtained by the ellipse fitting method. The precision can be brought to the order of sub-pixels by this algorithm and the mean error of diopter is less than 0.25D. Extracting results show that proposed method has higher accuracy and better robustness and provides guarantee on the precise corneal topography.

Key words: corneal topography, center extraction, curve fitting, Gaussian kernel function

摘要： 中心提取是Placido图像特征提取的关键环节，提出了一种基于曲线拟合的中心定位方法。利用Hough变换定位图像的像素级中心，建立图像的极坐标系；在极坐标轴方向，通过亮环灰度变化曲线的导数特征检测亮环中心线；将亮环中心线进行曲线拟合，确定亮环中心位置。中心提取精度达到亚像素级，利用标准球验证算法精度，屈光度误差小于0.25D。与常规算法进行对比，分析结果表明，曲线拟合法在准确性和稳定性上更优，为绘制精确的角膜地形图提供了保证。

关键词: 角膜地形图仪, 中心提取, 曲线拟合, 高斯核函数