如何将 Xamarin.android 与 OpenCV 集成

Question

我正在尝试将 Xamarin.android 与 Visual Studio 中的 OpenCV 连接起来，文档真的很差，谁能向我提供一些如何操作的步骤

Answer 1

在 Xamarin.Android 上使用 OpenCV 的方法不止一种：

1 - 使用 OpenCV4Android 的绑定：OpenCV4Android 是 OpenCV (C++) for Android (Java) 的包装器，使用 JNI。有了绑定，我们就有了 Java 和 C# 之间的接口（更多信息请参见https://docs.microsoft.com/en-us/xamarin/android/platform/binding-java-library/）。

它是在https://github.com/jeremy-ellis-tech/Xamarin.Android.OpenCV中实现的，它在 OpenCV 3.1.0 中使用。您可以按照安装说明和“减少 .dll 大小”来生成一个 dll 并引用它，或者您可以在您的 Visual Studio 解决方案中插入文件夹“/src/OpenCV.Binding”的 Visual Studio 项目并从您的项目到这个新项目。

如果您想使用更新的版本，您可以下载更新版本的 OpenCV4Android（OpenCV SourceForge 中名为 opencv-version-android-sdk.zip 的文件，例如：OpenCV4Android 4.1.0 链接https://sourceforge.net/ projects/opencvlibrary/files/4.1.0/ ) 并将上一个项目中文件夹“/src/OpenCV.Binding/Jars”的内容替换为文件夹“/sdk/native/libs”和“/sdk/native”的内容/3rdparty/libs” 提取的 OpenCV4Android。

这个项目还有一个fork的NuGet：https : //www.nuget.org/packages/Xamarin.OpenCV.Droid可以简化安装和使用，不过我没用过这个所以不好说有用。

由于此方法是 OpenCV4Android 的绑定而不是纯 OpenCV，因此您将使用 OpenCV4Android 的文档 ( https://opencv.org/android/ )。还值得一提的是，这样一来，我们有了三层编程语言（C#-Java-C++），所以我们在方法调用上有性能损失（JNI是一种负担）。因此，建议您尽可能少地使用呼叫。

2 - 使用 OpenCV C++ 的包装器：通过这种方式，我们将使用 C++ 共享库（.so）并从 C# 调用其方法（https://docs.microsoft.com/en-us/xamarin/android/platform /本机库）。为此，我们需要编写 OpenCV 方法的 PInvoke，这些方法很多，这意味着很多时间。因此，我们将使用某人已经制作的内容。

我们有 OpenCvSharp，它是 OpenCV 到 .NET 的包装器，显然运行良好。问题：它与 ARM 不兼容，因此无法在智能手机上运行。然而，一个好的灵魂将它改编为在 ARM 设备上运行：https : //github.com/Kawaian/OpenCvSharp。

如何快速使用它：在解决方案中插入文件夹“/src/OpenCvSharp”的项目并引用它。您将“/src/OpenCvSharp.Android/Native”的内容复制到项目的“lib”或“libs”文件夹中。然后，您将“.so”文件配置为“始终复制”到输出目录，并将其构建操作配置为“AndroidNativeLibrary”（如果您的项目是应用程序）或“嵌入式本机库”（如果您的项目是 Android 库）。

另一种方法是安装 NuGet ( https://www.nuget.org/packages/Kawaian.OpenCVSharp/ )，这会更容易一些，但也需要将“.so”文件复制到“lib " 或 "libs" 并配置它们。

这个包装器使用 OpenCV 3.2.0。我正在研究一种更新此项目的 OpenCV 版本的方法，但现在，它有效。

这种方式的巨大优势是性能（比较两种实现时，我的应用程序提高了约 30%）。但一个缺点是没有已经制作的 Android.Bitmap - OpenCV.Mat 转换方法。我通过改编 OpenCV4Android 的转换方法来实现它们：

// Method adapted from the original OpenCV convert at https://github.com/opencv/opencv/blob/b39cd06249213220e802bb64260727711d9fc98c/modules/java/generator/src/cpp/utils.cpp
///<summary>
///This function converts an image in the OpenCV Mat representation to the Android Bitmap.
///The input Mat object has to be of the types 'CV_8UC1' (gray-scale), 'CV_8UC3' (RGB) or 'CV_8UC4' (RGBA).
///The output Bitmap object has to be of the same size as the input Mat and of the types 'ARGB_8888' or 'RGB_565'.
///This function throws an exception if the conversion fails.
///</summary>
///<param name="srcImage">srcImage is a valid input Mat object of types 'CV_8UC1', 'CV_8UC3' or 'CV_8UC4'</param>
///<param name="dstImage">dstImage is a valid Bitmap object of the same size as the Mat and of type 'ARGB_8888' or 'RGB_565'</param>
///<param name="needPremultiplyAlpha">premultiplyAlpha is a flag, that determines, whether the Mat needs to be converted to alpha premultiplied format (like Android keeps 'ARGB_8888' bitmaps); the flag is ignored for 'RGB_565' bitmaps.</param>
public static void MatToBitmap(Mat srcImage, Bitmap dstImage, bool needPremultiplyAlpha = false)
{
    var bitmapInfo = dstImage.GetBitmapInfo();
    var bitmapPixels = dstImage.LockPixels();
 
    if (bitmapInfo.Format != Format.Rgba8888 && bitmapInfo.Format != Format.Rgb565)
        throw new Exception("Invalid Bitmap Format: It is of format " + bitmapInfo.Format.ToString() + " and is expected Rgba8888 or Rgb565");
    if (srcImage.Dims() != 2)
        throw new Exception("The source image has " + srcImage.Dims() + " dimensions, while it is expected 2");
    if (srcImage.Cols != bitmapInfo.Width || srcImage.Rows != bitmapInfo.Height)
        throw new Exception("The source image and the output Bitmap don't have the same amount of rows and columns");
    if (srcImage.Type() != MatType.CV_8UC1 && srcImage.Type() != MatType.CV_8UC3 && srcImage.Type() != MatType.CV_8UC4)
        throw new Exception("The source image has the type " + srcImage.Type().ToString() + ", while it is expected CV_8UC1, CV_8UC3 or CV_8UC4");
    if (bitmapPixels == null)
        throw new Exception("Can't lock the output bitmap");
 
    if (bitmapInfo.Format == Format.Rgba8888)
    {
        Mat tmp = new Mat((int)bitmapInfo.Height, (int)bitmapInfo.Width, MatType.CV_8UC4, bitmapPixels);
        if (srcImage.Type() == MatType.CV_8UC1)
        {
            Android.Util.Log.Info("MatToBitmap", "CV_8UC1 -> RGBA_8888");
            Cv2.CvtColor(srcImage, tmp, ColorConversionCodes.GRAY2RGBA);
        }
        else if (srcImage.Type() == MatType.CV_8UC3)
        {
            Android.Util.Log.Info("MatToBitmap", "CV_8UC3 -> RGBA_8888");
            Cv2.CvtColor(srcImage, tmp, ColorConversionCodes.RGB2RGBA);
        }
        else if (srcImage.Type() == MatType.CV_8UC4)
        {
            Android.Util.Log.Info("MatToBitmap", "CV_8UC4 -> RGBA_8888");
            if (needPremultiplyAlpha)
                Cv2.CvtColor(srcImage, tmp, ColorConversionCodes.RGBA2mRGBA);
            else
                srcImage.CopyTo(tmp);
        }
    }
    else
    {
        // info.format == ANDROID_BITMAP_FORMAT_RGB_565
        Mat tmp = new Mat((int)bitmapInfo.Height, (int)bitmapInfo.Width, MatType.CV_8UC2, bitmapPixels);
        if (srcImage.Type() == MatType.CV_8UC1)
        {
            Android.Util.Log.Info("MatToBitmap", "CV_8UC1 -> RGB_565");
            Cv2.CvtColor(srcImage, tmp, ColorConversionCodes.GRAY2BGR565);
        }
        else if (srcImage.Type() == MatType.CV_8UC3)
        {
            Android.Util.Log.Info("MatToBitmap", "CV_8UC3 -> RGB_565");
            Cv2.CvtColor(srcImage, tmp, ColorConversionCodes.RGB2BGR565);
        }
        else if (srcImage.Type() == MatType.CV_8UC4)
        {
            Android.Util.Log.Info("MatToBitmap", "CV_8UC4 -> RGB_565");
            Cv2.CvtColor(srcImage, tmp, ColorConversionCodes.RGBA2BGR565);
        }
    }
    dstImage.UnlockPixels();
    return;
}

// Method adapted from the original OpenCV convert at https://github.com/opencv/opencv/blob/b39cd06249213220e802bb64260727711d9fc98c/modules/java/generator/src/cpp/utils.cpp
///<summary>
///This function converts an Android Bitmap image to the OpenCV Mat.
///'ARGB_8888' and 'RGB_565' input Bitmap formats are supported.
///The output Mat is always created of the same size as the input Bitmap and of the 'CV_8UC4' type,it keeps the image in RGBA format.
///This function throws an exception if the conversion fails.
///</summary>
///<param name="srcImage">srcImage is a valid input Bitmap object of the type 'ARGB_8888' or 'RGB_565'</param>
///<param name="dstImage">dstImage is a valid output Mat object, it will be reallocated if needed, so it may be empty.</param>
///<param name="needUnPremultiplyAlpha">unPremultiplyAlpha is a flag, that determines, whether the bitmap needs to be converted from alpha premultiplied format (like Android keeps 'ARGB_8888' ones) to regular one; this flag is ignored for 'RGB_565' bitmaps.</param>
public static void BitmapToMat(Bitmap srcImage, Mat dstImage, bool needUnPremultiplyAlpha = false)
{
    var bitmapInfo = srcImage.GetBitmapInfo();
    var bitmapPixels = srcImage.LockPixels();
 
    if (bitmapInfo.Format != Format.Rgba8888 && bitmapInfo.Format != Format.Rgb565)
        throw new Exception("Invalid Bitmap Format: It is of format " + bitmapInfo.Format.ToString() + " and is expected Rgba8888 or Rgb565");
    if (bitmapPixels == null)
        throw new Exception("Can't lock the source bitmap");
 
    dstImage.Create((int)bitmapInfo.Height, (int)bitmapInfo.Width, MatType.CV_8UC4);
 
    if (bitmapInfo.Format == Format.Rgba8888)
    {
        Android.Util.Log.Info("nBitmapToMat", "RGBA_8888 -> CV_8UC4");
        Mat tmp = new Mat((int)bitmapInfo.Height, (int)bitmapInfo.Width, MatType.CV_8UC4, bitmapPixels);
        if (needUnPremultiplyAlpha)
            Cv2.CvtColor(tmp, dstImage, ColorConversionCodes.mRGBA2RGBA);
        else
            tmp.CopyTo(dstImage);
    }
    else
    {
        // info.format == ANDROID_BITMAP_FORMAT_RGB_565
        Android.Util.Log.Info("nBitmapToMat", "RGB_565 -> CV_8UC4");
        Mat tmp = new Mat((int)bitmapInfo.Height, (int)bitmapInfo.Width, MatType.CV_8UC2, bitmapPixels);
        Cv2.CvtColor(tmp, dstImage, ColorConversionCodes.BGR5652RGBA);
    }
 
    srcImage.UnlockPixels();
    return;
}