代码：

//定义smoothed算子边缘检测函数
public static Bitmap Smoothed(Bitmap a)
        {
            int w = a.Width;
            int h = a.Height;
            try
            {
                Bitmap dstBitmap = new Bitmap(w, h, System.Drawing.Imaging.PixelFormat.Format24bppRgb);
                System.Drawing.Imaging.BitmapData srcData = a.LockBits(new Rectangle
                 (0, 0, w, h), System.Drawing.Imaging.ImageLockMode.ReadOnly, System.Drawing.Imaging.PixelFormat.Format24bppRgb);
                System.Drawing.Imaging.BitmapData dstData = dstBitmap.LockBits(new Rectangle
                 (0, 0, w, h), System.Drawing.Imaging.ImageLockMode.WriteOnly, System.Drawing.Imaging.PixelFormat.Format24bppRgb);
                unsafe
                {
                    byte* pIn = (byte*)srcData.Scan0.ToPointer();
                    byte* pOut = (byte*)dstData.Scan0.ToPointer();
                    byte* p;
                    int stride = srcData.Stride;
                    for (int y = 0; y < h; y++)
                    {
                        for (int x = 0; x < w; x++)
                        {
                            //边缘八个点像素不变
                            if (x == 0 || x == w - 1 || y == 0 || y == h - 1)
                            {
                                pOut[0] = pIn[0];
                                pOut[1] = pIn[1];
                                pOut[2] = pIn[2];
                            }
                            else
                            {
                                int r0, r1, r2, r3, r4, r5, r6, r7, r8;
                                int g1, g2, g3, g4, g5, g6, g7, g8, g0;
                                int b1, b2, b3, b4, b5, b6, b7, b8, b0;
                                double vR, vG, vB;
                                //左上
                                p = pIn - stride - 3;
                                r1 = p[2];
                                g1 = p[1];
                                b1 = p[0];
                                //正上
                                p = pIn - stride;
                                r2 = p[2];
                                g2 = p[1];
                                b2 = p[0];
                                //右上
                                p = pIn - stride + 3;
                                r3 = p[2];
                                g3 = p[1];
                                b3 = p[0];
                                //左
                                p = pIn - 3;
                                r4 = p[2];
                                g4 = p[1];
                                b4 = p[0];
                                //右
                                p = pIn + 3;
                                r5 = p[2];
                                g5 = p[1];
                                b5 = p[0];
                                //左下
                                p = pIn + stride - 3;
                                r6 = p[2];
                                g6 = p[1];
                                b6 = p[0];
                                //正下
                                p = pIn + stride;
                                r7 = p[2];
                                g7 = p[1];
                                b7 = p[0];
                                // 右下 
                                p = pIn + stride + 3;
                                r8 = p[2];
                                g8 = p[1];
                                b8 = p[0];
                                //中心点
                                p = pIn;
                                r0 = p[2];
                                g0 = p[1];
                                b0 = p[0];
                                //使用模板
                                vR = (double)(Math.Abs(r3 + r5 + r8 - r1 - r4 - r6) + Math.Abs(r1 + r2 + r3 - r6 - r7 - r8));
                                vG = (double)(Math.Abs(g3 + g5 + g8 - g1 - g4 - g6) + Math.Abs(g1 + g2 + g3 - g6 - g7 - g8));
                                vB = (double)(Math.Abs(b3 + b5 + b8 - b1 - b4 - b6) + Math.Abs(b1 + b2 + b3 - b6 - b7 - b8));
                                if (vR > 0)
                                {
                                    vR = Math.Min(255, vR);
                                }
                                else
                                {
                                    vR = Math.Max(0, vR);
                                }
                                if (vG > 0)
                                {
                                    vG = Math.Min(255, vG);
                                }
                                else
                                {
                                    vG = Math.Max(0, vG);
                                }
                                if (vB > 0)
                                {
                                    vB = Math.Min(255, vB);
                                }
                                else
                                {
                                    vB = Math.Max(0, vB);
                                }
                                pOut[0] = (byte)vB;
                                pOut[1] = (byte)vG;
                                pOut[2] = (byte)vR;
                            }
                            pIn += 3;
                            pOut += 3;
                        }
                        pIn += srcData.Stride - w * 3;
                        pOut += srcData.Stride - w * 3;
                    }
                }
                a.UnlockBits(srcData);
                dstBitmap.UnlockBits(dstData);
                return dstBitmap;
            }
            catch
            {
                return null;
            }
        }

效果：