在Android中应用OpenCV方法时出现java.lang.IndexOutOfBoundsException错误

Question

该代码是关于使用 OpenCV 查找 Android 设备相机中最大的矩形。应用程序总是强行关闭，但我找不到问题所在。

该方法的输入是CvCameraViewFrame.rgba()得到的一个Mat。

private Mat findLargestRectangle(Mat original_image)
{
    Mat imgSource = original_image;

    // convert the image to black and white
    Imgproc.cvtColor(imgSource, imgSource, Imgproc.COLOR_BGR2GRAY);

    // convert the image to black and white does (8 bit)
    Imgproc.Canny(imgSource, imgSource, 50, 50);

    // apply gaussian blur to smoothen lines of dots
    Imgproc.GaussianBlur(imgSource, imgSource, new Size(5, 5), 5);

    // find the contours
    List<MatOfPoint> contours = new ArrayList<MatOfPoint>();
    Imgproc.findContours(imgSource, contours, new Mat(), Imgproc.RETR_LIST, Imgproc.CHAIN_APPROX_SIMPLE);

    double maxArea = -1;
    int maxAreaIdx = -1;
    MatOfPoint temp_contour = contours.get(0); // the largest is at the
                                                // index 0 for starting
                                                // point
    MatOfPoint2f approxCurve = new MatOfPoint2f();
    Mat largest_contour = contours.get(0);
    List<MatOfPoint> largest_contours = new ArrayList<MatOfPoint>();
    for (int idx = 0; idx < contours.size(); idx++)
    {
        temp_contour = contours.get(idx);
        double contourarea = Imgproc.contourArea(temp_contour);
        // compare this contour to the previous largest contour found
        if (contourarea > maxArea)
        {
            // check if this contour is a square
            MatOfPoint2f new_mat = new MatOfPoint2f(temp_contour.toArray());
            int contourSize = (int) temp_contour.total();
            Imgproc.approxPolyDP(new_mat, approxCurve, contourSize * 0.05, true);
            if (approxCurve.total() == 4)
            {
                maxArea = contourarea;
                maxAreaIdx = idx;
                largest_contours.add(temp_contour);
                largest_contour = temp_contour;
            }
        }
    }
    MatOfPoint temp_largest = largest_contours.get(largest_contours.size() - 1);
    largest_contours = new ArrayList<MatOfPoint>();
    largest_contours.add(temp_largest);

    Imgproc.cvtColor(imgSource, imgSource, Imgproc.COLOR_BayerBG2RGB);
    Imgproc.drawContours(imgSource, largest_contours, -1, new Scalar(0, 255, 0), 1);

    // create the new image here using the largest detected square

    Toast.makeText(getApplicationContext(), "Largest Contour: ", Toast.LENGTH_LONG).show();

    return imgSource;
}

以下是LogCat中的错误信息:

error opening trace file: No such file or directory (2)
Tegra Version detected: 0
FATAL EXCEPTION: Thread-14346
java.lang.IndexOutOfBoundsException: Invalid index 0, size is 0
at java.util.ArrayList.throwIndexOutOfBoundsException(ArrayList.java:251)
at java.util.ArrayList.get(ArrayList.java:304)
at org.opencv.samples.tutorial2.Tutorial2Activity.findLargestRectangle(Tutorial2Activity.java:221)
at org.opencv.samples.tutorial2.Tutorial2Activity.onCameraFrame(Tutorial2Activity.java:169)
at org.opencv.android.CameraBridgeViewBase.deliverAndDrawFrame(CameraBridgeViewBase.java:387)
at org.opencv.android.JavaCameraView$CameraWorker.run(JavaCameraView.java:328)
at java.lang.Thread.run(Thread.java:856)

Tutorial2Avctivity 的第 221 行是:

MatOfPoint temp_contour = contours.get(0);

请告诉我错误是什么。非常感谢!

Answer 1

这仅仅意味着轮廓的 ArrayList 的大小为空。

我建议添加一个条件 block ，以使您的应用程序能够应对在输入图像中找不到轮廓的情况。

如果无论输入图像如何，您始终没有得到轮廓，您可能需要 review the documentation (或Javadoc):

image – Source, an 8-bit single-channel image. Non-zero pixels are treated as 1’s. Zero pixels remain 0’s, so the image is treated as binary . You can use compare() , inRange() , threshold() , adaptiveThreshold() , Canny() , and others to create a binary image out of a grayscale or color one.

也许 Canny() 函数中的阈值没有产生任何边缘。

如果您仍然没有运气，也许您可​​以使用输入图像和 this blog post 中的值进行测试.