JNI将Android Bitmap转为OpenCV的Mat

导读

在封装so库的时候，底层c++代码的实现使用了OpenCV对图片进行操作，而Android中从摄像头中获取到的图片数据类型是Bitmap数据类型的，所以这里就避免不了做数据类型的转换。转换的方案主要有两种，一种是封装的JNI接口方法直接接收cv::Mat数据类型的参数，第二种是JNI方法的接口接收Bitmap数据类型的参数，在JNI中实现Bitmap到cv::Mat的转换。

使用OpenCV的SDK实现数据类型的转换

第一种方法，如果想在Android中使用cv::Mat的数据类型，我们可以直接通过导入OpenCV的SDK，然后通过opencv的Utils实现bitmap到cv::Mat的转换。缺点在于需要添加opencv的库文件，会导致最终的apk文件变大。

Android studio中添加OpenCV SDK参考：https://blog.csdn.net/qq_34123324/article/details/85330720

import org.opencv.android.Utils;

Mat mat = new Mat();    
Bitmap bmp32 = bmp.copy(Bitmap.Config.ARGB_8888, true);
Utils.bitmapToMat(bmp32, mat);

在JNI中接收Mat参数的时候，使用Jobject类型，然后再做个强制转换就行了

JNI实现Bitmap到Mat的转换

下面的方法实现了Bitmap到cv::Mat和cv::Mat到Bitmap的转换

#include 

#include 
#include 
#include 

#define ASSERT(status, ret)     if (!(status)) { return ret; }
#define ASSERT_FALSE(status)    ASSERT(status, false)

bool BitmapToMatrix(JNIEnv * env, jobject obj_bitmap, cv::Mat & matrix) {
    void * bitmapPixels;                                            // Save picture pixel data
    AndroidBitmapInfo bitmapInfo;                                   // Save picture parameters

    ASSERT_FALSE( AndroidBitmap_getInfo(env, obj_bitmap, &bitmapInfo) >= 0);        // Get picture parameters
    ASSERT_FALSE( bitmapInfo.format == ANDROID_BITMAP_FORMAT_RGBA_8888
                  || bitmapInfo.format == ANDROID_BITMAP_FORMAT_RGB_565 );          // Only ARGB? 8888 and RGB? 565 are supported
    ASSERT_FALSE( AndroidBitmap_lockPixels(env, obj_bitmap, &bitmapPixels) >= 0 );  // Get picture pixels (lock memory block)
    ASSERT_FALSE( bitmapPixels );

    if (bitmapInfo.format == ANDROID_BITMAP_FORMAT_RGBA_8888) {
        cv::Mat tmp(bitmapInfo.height, bitmapInfo.width, CV_8UC4, bitmapPixels);    // Establish temporary mat
        tmp.copyTo(matrix);                                                         // Copy to target matrix
    } else {
        cv::Mat tmp(bitmapInfo.height, bitmapInfo.width, CV_8UC2, bitmapPixels);
        cv::cvtColor(tmp, matrix, cv::COLOR_BGR5652RGB);
    }
    
    //convert RGB to BGR
    cv::cvtColor(matrix,matrix,cv::COLOR_RGB2BGR);

    AndroidBitmap_unlockPixels(env, obj_bitmap);            // Unlock
    return true;
}



bool MatrixToBitmap(JNIEnv * env, cv::Mat & matrix, jobject obj_bitmap) {
    void * bitmapPixels;                                            // Save picture pixel data
    AndroidBitmapInfo bitmapInfo;                                   // Save picture parameters

    ASSERT_FALSE( AndroidBitmap_getInfo(env, obj_bitmap, &bitmapInfo) >= 0);        // Get picture parameters
    ASSERT_FALSE( bitmapInfo.format == ANDROID_BITMAP_FORMAT_RGBA_8888
                  || bitmapInfo.format == ANDROID_BITMAP_FORMAT_RGB_565 );          // Only ARGB? 8888 and RGB? 565 are supported
    ASSERT_FALSE( matrix.dims == 2
                  && bitmapInfo.height == (uint32_t)matrix.rows
                  && bitmapInfo.width == (uint32_t)matrix.cols );                   // It must be a 2-dimensional matrix with the same length and width
    ASSERT_FALSE( matrix.type() == CV_8UC1 || matrix.type() == CV_8UC3 || matrix.type() == CV_8UC4 );
    ASSERT_FALSE( AndroidBitmap_lockPixels(env, obj_bitmap, &bitmapPixels) >= 0 );  // Get picture pixels (lock memory block)
    ASSERT_FALSE( bitmapPixels );

    if (bitmapInfo.format == ANDROID_BITMAP_FORMAT_RGBA_8888) {
        cv::Mat tmp(bitmapInfo.height, bitmapInfo.width, CV_8UC4, bitmapPixels);
        switch (matrix.type()) {
            case CV_8UC1:   cv::cvtColor(matrix, tmp, cv::COLOR_GRAY2RGBA);     break;
            case CV_8UC3:   cv::cvtColor(matrix, tmp, cv::COLOR_RGB2RGBA);      break;
            case CV_8UC4:   matrix.copyTo(tmp);                                 break;
            default:        AndroidBitmap_unlockPixels(env, obj_bitmap);        return false;
        }
    } else {
        cv::Mat tmp(bitmapInfo.height, bitmapInfo.width, CV_8UC2, bitmapPixels);
        switch (matrix.type()) {
            case CV_8UC1:   cv::cvtColor(matrix, tmp, cv::COLOR_GRAY2BGR565);   break;
            case CV_8UC3:   cv::cvtColor(matrix, tmp, cv::COLOR_RGB2BGR565);    break;
            case CV_8UC4:   cv::cvtColor(matrix, tmp, cv::COLOR_RGBA2BGR565);   break;
            default:        AndroidBitmap_unlockPixels(env, obj_bitmap);        return false;
        }
    }
    AndroidBitmap_unlockPixels(env, obj_bitmap);                // Unlock
    return true;
}

JNIEXPORT void JNICALL
Java_com_example_MainActivity_JniBitmapExec(JNIEnv * env, jobject /* this */,
        jobject obj_bitmap, jobject obj_bitmapOut)
{
    cv::Mat matBitmap;
    bool ret = BitmapToMatrix(env, obj_bitmap, matBitmap);          // Bitmap to cv::Mat
    if (ret == false) {
        return;
    }

    // opencv processing of mat

    ret = MatrixToBitmap(env, matBitmap, obj_bitmapOut);       // Bitmap to cv::Mat
    if (ret == false) {
        return;
    }
}

注意在CMakelist文件中添加以下代码，不然编译的时候会报错

target_link_libraries( # Specifies the target library.
        #在target_link_libraries中添加下面的依赖项
        jnigraphics
        )