OpenCVSharp特征点匹配及全景拼接

之前连续写了一些列关于OpenCV在Unity平台的入门基础，这次增加难度，来一个特征提取和拼接的案例。由于OpenCVForUnity没有实现Stitcher的功能，转而使用OpenCVSharp。可以去github的官方项目里找最新的OpenCVSharp.dll文件，不过官方提供的是.Net4.6版本（没有尝试是否可以自行编译成.Net3.5）。那我们就先把Unity项目设置为4.6的框架。

Player Settings -> Other Settings -> Configuration

OpenCVSharp 对 Mat 和 Texture2D 转换没有做专门的API，所以我们先把这个复杂的转换过程做成一个工具类。

using UnityEngine;
using OpenCvSharp;

public class Utils
{
    /// 

    /// Mat转Texture2D
    /// 
    /// 
    /// 
    public static Texture2D MatToTexture2D(Mat mat)
    {
        Texture2D t2d = new Texture2D(mat.Width, mat.Height);
        t2d.LoadImage(mat.ToBytes());
        t2d.Apply();
        //赋值完后为什么要Apply
        //因为在贴图更改像素时并不是直接对显存进行更改，而是在另外一个内存空间中更改，这时候GPU还会实时读取旧的贴图位置。
        //当Apply后，CPU会告诉GPU你要换个地方读贴图了。
        return t2d;
    }
}

Part.I 特征匹配

案例中我使用的是ORB特征提取，其他主流的算法还有SURF，SIFT，FAST、BRISK、FREAK等，当然数学层面上都已经通过OpenCV的函数实现了。写两个函数热热身，熟悉一下使用ORB进行特征点检测。

void Detect()
{
    var gray = new Mat(Application.streamingAssetsPath + "/Textures/p1.jpg", ImreadModes.GrayScale);
    KeyPoint[] keyPoints = null;
    using (var orb = ORB.Create(500))
    {
        keyPoints = orb.Detect(gray);
        Debug.Log($"KeyPoint has {keyPoints.Length} items.");
    }
}

void DetectAndCompute()
{
    var gray = new Mat(Application.streamingAssetsPath + "/Textures/p1.jpg", ImreadModes.GrayScale);
    KeyPoint[] keyPoints = null;
    using (var orb = ORB.Create(500))
    using (Mat descriptor = new Mat())
    {
        orb.DetectAndCompute(gray, new Mat(), out keyPoints, descriptor);
        Debug.Log($"keyPoints has {keyPoints.Length} items.");
        Debug.Log($"descriptor has {descriptor.Rows} items.");
    }
}

上述函数打印成功，即提取成功。导入2张测试图片，开始匹配工作。

/// 

/// Orb特征提取
/// 
void OnOrb()
{
    Mat image01 = Cv2.ImRead(Application.streamingAssetsPath + "/Textures/p1.jpg");
    Mat image02 = Cv2.ImRead(Application.streamingAssetsPath + "/Textures/p2.jpg");

    //灰度图转换
    Mat image1 = new Mat(), image2 = new Mat();
    Cv2.CvtColor(image01, image1, ColorConversionCodes.RGB2GRAY);
    Cv2.CvtColor(image02, image2, ColorConversionCodes.RGB2GRAY);

    KeyPoint[] keyPoint1 = null;
    KeyPoint[] keyPoint2 = null;
    using (ORB orb = ORB.Create(500))
    using (Mat descriptor1 = new Mat())
    using (Mat descriptor2 = new Mat())
    using (var matcher = new BFMatcher())
    {
        //特征点提取并计算
        orb.DetectAndCompute(image1, new Mat(), out keyPoint1, descriptor1);
        orb.DetectAndCompute(image2, new Mat(), out keyPoint2, descriptor2);
        Debug.Log($"keyPoints has {keyPoint1.Length},{keyPoint2.Length} items.");
        Debug.Log($"descriptor has {descriptor1.Rows},{descriptor2.Rows} items.");

        //特征点匹配
        DMatch[] matchePoints = null;
        matchePoints = matcher.Match(descriptor1, descriptor2);

        dstMat = new Mat();
        Cv2.DrawMatches(image01, keyPoint1, image02, keyPoint2, matchePoints, dstMat);
        t2d = Utils.MatToTexture2D(dstMat);
    }

    Sprite dst_sp = Sprite.Create(t2d, new UnityEngine.Rect(0, 0, t2d.width, t2d.height), Vector2.zero);
    m_srcImage.sprite = dst_sp;
    m_srcImage.preserveAspect = true;
}

匹配效果

Part.II 全景拼接

使用stitcher拼接其实比较容易，照stitcher.Stitch()函数的参数提示填值就可以了。花了好大功夫才实现，坑点主要在老版本的OpenCVSharp.dll内存奔溃（官方git上有说明，已经修复）。还有一开始一直在用手写算法去实现，这个暂时也写不出来了⁄(⁄ ⁄•⁄ω⁄•⁄ ⁄)⁄。有志在Unity中实现，可以参考这些文章。
http://www.pyimagesearch.com/2016/01/11/opencv-panorama-stitching/
https://github.com/tanaka0079/cpp/blob/master/opencv/image/panorama/orb.cpp

回到正题，开始拼接。

Mat p1Mat, p2Mat, dstMat;
bool tryUseGpu = true;
IEnumerable GenerateImages()
{
    p1Mat = Cv2.ImRead(Application.streamingAssetsPath + "/Textures/p1.jpg");
    p2Mat = Cv2.ImRead(Application.streamingAssetsPath + "/Textures/p2.jpg");
    yield return p1Mat;
    yield return p2Mat;
}

void OnStitch()
{
    IEnumerable images = GenerateImages();

    using (var stitcher = Stitcher.Create(tryUseGpu))
    using (var panoMat = new Mat())
    {
        Debug.Log("Stitcher start...");
        var status = stitcher.Stitch(images, panoMat);
        if (status != Stitcher.Status.OK)
        {
            Debug.Log("Can't stitch images, error code = " + (int)status);
            return;
        }
        stitcher.Dispose(); //处理掉
        t2d = Utils.MatToTexture2D(panoMat);
    }

    Sprite sp = Sprite.Create(t2d, new UnityEngine.Rect(0, 0, t2d.width, t2d.height), Vector2.zero);
    m_dstImage.sprite = sp;
    m_dstImage.preserveAspect = true;
}

拼接效果