代码：

Bitmap bitmapNew = bitmap.copy(Bitmap.Config.ARGB_8888, true);

//opencvプリプロセッサ

if (!OpenCVLoader.initDebug()) {

Log.d("--------opencv--------", "OpenCVLoader error");

}

//Mat変換

Mat src = new Mat(bitmapNew.getHeight(), bitmapNew.getWidth(), CvType.CV_8UC4);

Utils.bitmapToMat(bitmapNew, src);

//グレー化

Imgproc.cvtColor(src, src, Imgproc.COLOR_RGB2GRAY);

//Gaussian Filters

Imgproc.GaussianBlur(src, src, new Size(1, 1), 0, 0);

//二値化

Imgproc.threshold(src, src, 0.0, 255.0,

Imgproc.THRESH_BINARY | Imgproc.THRESH_OTSU);

//ビット反転

Mat hierarchy = Mat.zeros(new Size(1, 1), CvType.CV_8UC1);

Core.bitwise_not(src, src);

// 輪郭抽出

Listcontours = new ArrayList();

Imgproc.findContours(src, contours, hierarchy, Imgproc.RETR_EXTERNAL, Imgproc.CHAIN_APPROX_TC89_L1);

Mat dst = Mat.zeros(new Size(src.width(), src.height()), CvType.CV_8UC3);

// 輪郭を描画 (Yellow)

Scalar color = new Scalar(255, 255, 0);

Imgproc.drawContours(dst, contours, -1, color, 1);

Listlist = new ArrayList();

Rect box = null;

for (int i = 0; i < contours.size(); i++) {

// 小さい輪郭を除去 (ノイズ除去)

if (Imgproc.contourArea(contours.get(i)) < dst.size().area() / (1000)) {

continue;

}

MatOfPoint ptmat = contours.get(i);

// 輪郭の重心を描画 (Red)

color = new Scalar(255, 0, 0);

MatOfPoint2f ptmat2 = new MatOfPoint2f(ptmat.toArray());

RotatedRect bbox = Imgproc.minAreaRect(ptmat2);

box = bbox.boundingRect();

Imgproc.circle(dst, bbox.center, 5, color, -1);

list.add(bbox.center);

// 輪郭を囲む四角形を描画 (Green)

color = new Scalar(0, 255, 0);

Imgproc.rectangle(dst, box.tl(), box.br(), color, 2);

}

Utils.matToBitmap(dst, bitmapNew);

如果想使用上述代码，还需要在build.gradle中的dependencies中添加

implementation 'com.quickbirdstudios:opencv:3.4.1'