java - OpenCV 4.1 Java -contourArea() 断言深度 == CV_32F || 失败船体矩阵的深度 == CV_32S

Question

我尝试根据生成的船体点计算船体面积凸包()。

我关注了OpenCV Python tutorial (因为没有 Java 教程)并摆弄代码完成。

这是代码:

Imgproc.findContours(patternEdges, patternContours, new Mat(), Imgproc.RETR_EXTERNAL, Imgproc.CHAIN_APPROX_SIMPLE);
MatOfInt patternHull = new MatOfInt();
Imgproc.convexHull(patternContours.get(0), patternHull);
Imgproc.contourArea(pickPoints(patternContours.get(0), patternHull)); // fails here

但这会引发以下异常:

Exception in thread "main" CvException [org.opencv.core.CvException: cv::Exception: OpenCV(4.1.2) /home/build/git/opencv/modules/imgproc/src/shapedescr.cpp:274: error: (-215:Assertion failed) npoints >= 0 && (depth == CV_32F || depth == CV_32S) in function 'contourArea']
  at org.opencv.imgproc.Imgproc.contourArea_1(Native Method)
  at org.opencv.imgproc.Imgproc.contourArea(Imgproc.java:1607)
  at com.acme.opencv.Test.main(Test.java:94)

显然矩阵内部数据类型是错误的。但为什么，以及如何转换它？

我使用的是 OpenCV 4.1.2。

Answer 1

问题是，在Java中，API只实现了填充MatOfInt与原始点矩阵的索引。

这就是Javadoc (直接从 C++ 文档生成)说:

hull Output convex hull. It is either an integer vector of indices or vector of points. In the first case, the hull elements are 0-based indices of the convex hull points in the original array (since the set of convex hull points is a subset of the original point set). In the second case, hull elements are the convex hull points themselves.

(Emphasis mine)

没有“或”。只有 MatOfInt，它是原始点矩阵。

您可以通过这样的辅助函数使用选取的点创建一个新矩阵:

private static MatOfPoint2f pickPoints(MatOfPoint points, MatOfInt indices) {
    Point[] pickedPoints = new Point[indices.rows()];
    int newRow = 0;
    for (int index : indices.toArray()) {
        pickedPoints[newRow++] = new Point(points.get(index, 0));
    }
    return new MatOfPoint2f(pickedPoints);
}

然后使用它:

Imgproc.findContours(patternEdges, patternContours, new Mat(), Imgproc.RETR_EXTERNAL, Imgproc.CHAIN_APPROX_SIMPLE);
MatOfInt patternHullIndices = new MatOfInt();
Imgproc.convexHull(patternContours.get(0), patternHullIndices);
Imgproc.contourArea(pickPoints(patternContours.get(0), patternHull));