OpenCV：minMaxLoc 没有给出最小/最大值

Question

我正在尝试关注 OpenCV tutorial on image template matching using javacpp-presets。我想放弃分数太低的比赛，但 minMaxLoc 似乎不是 return minVal 和 maxVal.

import org.bytedeco.javacpp.DoublePointer;
import org.bytedeco.javacpp.opencv_core.Mat;
import org.bytedeco.javacpp.opencv_core.Point;

import static org.bytedeco.javacpp.opencv_core.CV_32FC1;
import static org.bytedeco.javacpp.opencv_core.NORM_MINMAX;
import static org.bytedeco.javacpp.opencv_core.minMaxLoc;
import static org.bytedeco.javacpp.opencv_core.normalize;
import static org.bytedeco.javacpp.opencv_imgcodecs.IMREAD_COLOR;
import static org.bytedeco.javacpp.opencv_imgcodecs.imread;
import static org.bytedeco.javacpp.opencv_imgproc.TM_CCOEFF_NORMED;
import static org.bytedeco.javacpp.opencv_imgproc.matchTemplate;

public class MatchTemp {

    public static void match(String img, String temp) {

        Mat image = imread(img, IMREAD_COLOR);
        Mat template = imread(temp, IMREAD_COLOR);

        int resultRows = image.rows() - template.rows() + 1;
        int resultCols = image.cols() - template.cols() + 1;
        Mat result = new Mat(resultRows, resultCols, CV_32FC1);

        matchTemplate(image, template, result, TM_CCOEFF_NORMED);

        Mat normalizedResult = new Mat();
        normalize(result, normalizedResult, 0, 1, NORM_MINMAX, -1, new Mat());

        DoublePointer maxVal = new DoublePointer();
        DoublePointer minVal = new DoublePointer();
        Point minLoc = new Point();
        Point maxLoc = new Point();

        minMaxLoc(normalizedResult, minVal, maxVal, minLoc, maxLoc, null);

        System.out.println("min: "+minLoc.x()+", "+minLoc.y());
        System.out.println("max: "+maxLoc.x()+", "+maxLoc.y());

        minVal.get(); // this throws exception
        maxVal.get(); // this throws exception
    }
}

例外情况是：

Exception in thread "main" java.lang.NullPointerException: This pointer address is NULL.
    at org.bytedeco.javacpp.DoublePointer.get(Native Method)
    at org.bytedeco.javacpp.DoublePointer.get(DoublePointer.java:101)
    ...

我做错了什么？我怎样才能得到分数？我是否必须手动检查 Mat 的内容？

Answer 1

该错误告诉您它正在尝试访问 Null 引用。阅读 DoublePointer constructor which tells to look at Pointer.Pointer() 的文档，它基本上告诉您以下内容：

Default constructor that does nothing.

这意味着它只是一个类似C++的指针：

double *p;

它没有指向任何 double，但是文档告诉你还有另一个 DoublePointer 的构造函数

public DoublePointer(long size)

表示它分配了一个这样大小的数组。如果大小等于 1，则相当于在 C++ 中说这相当于：

double p[1];

或者基本上

double *p = new double;

我从未使用过 JavaCpp，所以如果我遗漏了什么，请发表评论。

OpenCV：minMaxLoc 没有给出最小/最大值

OpenCV: minMaxLoc gives no min / max value

java

opencv

image-processing

template-matching

javacpp