下面的代码选自Opencv2.4.9源码文件opencv\sources\modules\imgproc\src文件夹下的deriv.cpp文件，该cpp文件中的Laplacian(…)函数源码，下面只显示了ksize=1or3的情况，

void cv::Laplacian( InputArray _src, OutputArray _dst, int ddepth, int ksize,double scale, double delta, int borderType )
{Mat src = _src.getMat();if (ddepth < 0)ddepth = src.depth();_dst.create( src.size(), CV_MAKETYPE(ddepth, src.channels()) );Mat dst = _dst.getMat();if( ksize == 1 || ksize == 3 ){float K[2][9] ={{0, 1, 0, 1, -4, 1, 0, 1, 0},{2, 0, 2, 0, -8, 0, 2, 0, 2}};Mat kernel(3, 3, CV_32F, K[ksize == 3]);if( scale != 1 )kernel *= scale;filter2D( src, dst, ddepth, kernel, Point(-1,-1), delta, borderType );}else{//ksize等于其他值的情况}
}

关于拉普拉斯算子的介绍，请看下面的博客：
https://www.cnblogs.com/german-iris/p/4840647.html
https://blog.csdn.net/dcrmg/article/details/53677739