Why is pointPolygonTest() method of OpenCV4Android returning -1 for every pixel?

asked 2016-01-13 13:46:45 -0500

Solace gravatar image

In the following code, I have carried out the following steps:

  1. Loaded an image from sdcard.

enter image description here

  1. Converted it to HSV format.

  2. Used inRange function to mask out the red color.

enter image description here

  1. Used findContours to find the contours.

  2. Find the largest contour from those contours.

  3. Created an ROI around the largest contour using boundingRect and submat functions.

enter image description here

  1. Converted this ROI Mat to HSV format.

enter image description here

  1. Iterated through the ROI Mat, and check for each pixel if it lies within the largest contour. I used the method pointPolygonTest to find this out, but it returns -1 for every pixel, as can be seen from the Log.i output I have pasted here. The question is why? How can I correct this.

    private Scalar detectColoredBlob() {
        rgbaFrame = Highgui.imread("/mnt/sdcard/DCIM/rgbaMat4Mask.bmp");
        Mat hsvImage = new Mat();
        Imgproc.cvtColor(rgbaFrame, hsvImage, Imgproc.COLOR_BGR2HSV);
        Highgui.imwrite("/mnt/sdcard/DCIM/hsvImage.bmp", hsvImage);// check
        Mat maskedImage = new Mat();
        Core.inRange(hsvImage, new Scalar(0, 100, 100), new Scalar(10, 255, 255), maskedImage);
        Highgui.imwrite("/mnt/sdcard/DCIM/maskedImage.bmp", maskedImage);// check
        List<MatOfPoint> contours = new ArrayList<MatOfPoint>();
        Imgproc.findContours(maskedImage, contours, new Mat(), Imgproc.RETR_LIST, Imgproc.CHAIN_APPROX_SIMPLE);
        // \/ We will use only the largest contour. Other contours (any other possible blobs of this color range) will be ignored.
        MatOfPoint largestContour = contours.get(0);
        double largestContourArea = Imgproc.contourArea(largestContour);
        for (int i = 1; i < contours.size(); ++i) {// NB Notice the prefix increment.
            MatOfPoint currentContour = contours.get(i);
            double currentContourArea = Imgproc.contourArea(currentContour);
            if (currentContourArea > largestContourArea) {
                largestContourArea = currentContourArea;
                largestContour = currentContour;
        MatOfPoint2f largestContour2f = new MatOfPoint2f(largestContour.toArray());// Required on Line 289. See http://stackoverflow.com/questions/11273588/how-to-convert-matofpoint-to-matofpoint2f-in-opencv-java-api
        Rect detectedBlobRoi = Imgproc.boundingRect(largestContour);
        Mat detectedBlobRgba = rgbaFrame.submat(detectedBlobRoi);
        Highgui.imwrite("/mnt/sdcard/DCIM/detectedBlobRgba.bmp", detectedBlobRgba);// check
        Mat detectedBlobHsv = new Mat();
        Imgproc.cvtColor(detectedBlobRgba, detectedBlobHsv, Imgproc.COLOR_BGR2HSV);
        Highgui.imwrite("/mnt/sdcard/DCIM/roiHsv.bmp", detectedBlobHsv);// check
        for (int firstCoordinate = 0; firstCoordinate < detectedBlobHsv.rows(); firstCoordinate++) {
            for (int secondCoordinate = 0; secondCoordinate < detectedBlobHsv.cols(); secondCoordinate++) {
                Log.i(TAG, "HAPPY " + Arrays.toString(detectedBlobHsv.get(firstCoordinate, secondCoordinate)));
                if (Imgproc.pointPolygonTest(largestContour2f, new Point(firstCoordinate, secondCoordinate), false) == -1) {
                    Log.i(TAG, "HAPPY ....................... OUTSIDE");
        Highgui.imwrite("/mnt/sdcard/DCIM/processedcontoured.bmp", detectedBlobHsv);// check
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