SIFT特征简介
SIFT(Scale-Invariant Feature Transform)特征，即尺度不变特征变换，是一种计算机视觉的特征提取算法，用来侦测与描述图像中的局部性特征。 实质上，它是在不同的尺度空间上查找关键点(特征点)，并计算出关键点的方向。SIFT所查找到的关键点是一些十分突出、不会因光照、仿射变换和噪音等因素而变化的点，如角点、边缘点、暗区的亮点及亮区的暗点等。

SURF特征简介
SURF(Speeded Up Robust Features, 加速稳健特征) 是一种稳健的图像识别和描述算法。它是SIFT的高效变种，也是提取尺度不变特征，算法步骤与SIFT算法大致相同，但采用的方法不一样，要比SIFT算法更高效（正如其名）。SURF使用海森(Hesseian)矩阵的行列式值作特征点检测并用积分图加速运算；SURF 的描述子基于 2D 离散小波变换响应并且有效地利用了积分图。

SIFT匹配效果

SURF匹配效果

代码

using OpenCvSharp;
using OpenCvSharp.Extensions;
using System;
using System.Collections.Generic;
using System.Drawing;
using System.Linq;
using System.Text.RegularExpressions;
using System.Windows.Forms;
using static System.Net.Mime.MediaTypeNames;
 
namespace OpenCvSharp_Demo
{
    public partial class frmMain : Form
    {
        public frmMain()
        {
            InitializeComponent();
        }
 
        private void Form1_Load(object sender, EventArgs e)
        {
 
        }
 
        private void button2_Click(object sender, EventArgs e)
        {
 
            Mat matSrc = new Mat("1.jpg");
            Mat matTo = new Mat("2.jpg");
 
            var outMat = MatchPicBySift(matSrc, matTo);
 
            pictureBox2.Image = OpenCvSharp.Extensions.BitmapConverter.ToBitmap(outMat);
 
        }
 
        private void button1_Click(object sender, EventArgs e)
        {
            Mat matSrc = new Mat("1.jpg");
            Mat matTo = new Mat("2.jpg");
 
            var outMat = MatchPicBySurf(matSrc, matTo, 10);
 
            pictureBox2.Image = OpenCvSharp.Extensions.BitmapConverter.ToBitmap(outMat);
        }
 
        public Point2d Point2fToPoint2d(Point2f point) => new Point2d((double)point.X, (double)point.Y);
 
        public Mat MatchPicBySift(Mat matSrc, Mat matTo)
        {
            using (Mat matSrcRet = new Mat())
            using (Mat matToRet = new Mat())
            {
                KeyPoint[] keyPointsSrc, keyPointsTo;
                using (var sift = OpenCvSharp.Features2D.SIFT.Create())
                {
                    sift.DetectAndCompute(matSrc, null, out keyPointsSrc, matSrcRet);
                    sift.DetectAndCompute(matTo, null, out keyPointsTo, matToRet);
                }
                using (var bfMatcher = new OpenCvSharp.BFMatcher())
                {
                    var matches = bfMatcher.KnnMatch(matSrcRet, matToRet, k: 2);
 
                    var pointsSrc = new List<Point2f>();
                    var pointsDst = new List<Point2f>();
                    var goodMatches = new List<DMatch>();
                    foreach (DMatch[] items in matches.Where(x => x.Length > 1))
                    {
                        if (items[0].Distance < 0.5 * items[1].Distance)
                        {
                            pointsSrc.Add(keyPointsSrc[items[0].QueryIdx].Pt);
                            pointsDst.Add(keyPointsTo[items[0].TrainIdx].Pt);
                            goodMatches.Add(items[0]);
                            Console.WriteLine($"{keyPointsSrc[items[0].QueryIdx].Pt.X}, {keyPointsSrc[items[0].QueryIdx].Pt.Y}");
                        }
                    }
 
                    var outMat = new Mat();
                    // 堆代码 duidaima.com
                    // 算法RANSAC对匹配的结果做过滤
                    var pSrc = pointsSrc.ConvertAll(Point2fToPoint2d);
                    var pDst = pointsDst.ConvertAll(Point2fToPoint2d);
                    var outMask = new Mat();
                    // 如果原始的匹配结果为空, 则跳过过滤步骤
                    if (pSrc.Count > 0 && pDst.Count > 0)
                        Cv2.FindHomography(pSrc, pDst, HomographyMethods.Ransac, mask: outMask);
                    // 如果通过RANSAC处理后的匹配点大于10个,才应用过滤. 否则使用原始的匹配点结果(匹配点过少的时候通过RANSAC处理后,可能会得到0个匹配点的结果).
                    if (outMask.Rows > 10)
                    {
                        byte[] maskBytes = new byte[outMask.Rows * outMask.Cols];
                        outMask.GetArray(out maskBytes);
                        Cv2.DrawMatches(matSrc, keyPointsSrc, matTo, keyPointsTo, goodMatches, outMat, matchesMask: maskBytes, flags: DrawMatchesFlags.NotDrawSinglePoints);
                    }
                    else
                        Cv2.DrawMatches(matSrc, keyPointsSrc, matTo, keyPointsTo, goodMatches, outMat, flags: DrawMatchesFlags.NotDrawSinglePoints);
                    return outMat;
                }
            }
        }
 
        public Mat MatchPicBySurf(Mat matSrc, Mat matTo, double threshold = 400)
        {
            using (Mat matSrcRet = new Mat())
            using (Mat matToRet = new Mat())
            {
                KeyPoint[] keyPointsSrc, keyPointsTo;
                using (var surf = OpenCvSharp.XFeatures2D.SURF.Create(threshold, 4, 3, true, true))
                {
                    surf.DetectAndCompute(matSrc, null, out keyPointsSrc, matSrcRet);
                    surf.DetectAndCompute(matTo, null, out keyPointsTo, matToRet);
                }
 
                using (var flnMatcher = new OpenCvSharp.FlannBasedMatcher())
                {
                    var matches = flnMatcher.Match(matSrcRet, matToRet);
                    //求最小最大距离
                    double minDistance = 1000;//反向逼近
                    double maxDistance = 0;
                    for (int i = 0; i < matSrcRet.Rows; i++)
                    {
                        double distance = matches[i].Distance;
                        if (distance > maxDistance)
                        {
                            maxDistance = distance;
                        }
                        if (distance < minDistance)
                        {
                            minDistance = distance;
                        }
                    }
                    Console.WriteLine($"max distance : {maxDistance}");
                    Console.WriteLine($"min distance : {minDistance}");
 
                    var pointsSrc = new List<Point2f>();
                    var pointsDst = new List<Point2f>();
                    //筛选较好的匹配点
                    var goodMatches = new List<DMatch>();
                    for (int i = 0; i < matSrcRet.Rows; i++)
                    {
                        double distance = matches[i].Distance;
                        if (distance < Math.Max(minDistance * 2, 0.02))
                        {
                            pointsSrc.Add(keyPointsSrc[matches[i].QueryIdx].Pt);
                            pointsDst.Add(keyPointsTo[matches[i].TrainIdx].Pt);
                            //距离小于范围的压入新的DMatch
                            goodMatches.Add(matches[i]);
                        }
                    }
 
                    var outMat = new Mat();
 
                    // 算法RANSAC对匹配的结果做过滤
                    var pSrc = pointsSrc.ConvertAll(Point2fToPoint2d);
                    var pDst = pointsDst.ConvertAll(Point2fToPoint2d);
                    var outMask = new Mat();
                    // 如果原始的匹配结果为空, 则跳过过滤步骤
                    if (pSrc.Count > 0 && pDst.Count > 0)
                        Cv2.FindHomography(pSrc, pDst, HomographyMethods.Ransac, mask: outMask);
                    // 如果通过RANSAC处理后的匹配点大于10个,才应用过滤. 否则使用原始的匹配点结果(匹配点过少的时候通过RANSAC处理后,可能会得到0个匹配点的结果).
                    if (outMask.Rows > 10)
                    {
                        byte[] maskBytes = new byte[outMask.Rows * outMask.Cols];
                        outMask.GetArray(out maskBytes);
                        Cv2.DrawMatches(matSrc, keyPointsSrc, matTo, keyPointsTo, goodMatches, outMat, matchesMask: maskBytes, flags: DrawMatchesFlags.NotDrawSinglePoints);
                    }
                    else
                        Cv2.DrawMatches(matSrc, keyPointsSrc, matTo, keyPointsTo, goodMatches, outMat, flags: DrawMatchesFlags.NotDrawSinglePoints);
                    return outMat;
                }
            }
        }
 
    }
}