ConvexHull Android ndk和Opencv中的参数无效

Question

I have this C++ OpenCV code in my jni folder of android application hello-jni.cpp . I just want to find and draw convexhull but cause of hull[i] the convexhull method generate an error " invalid arguments ". If I cast (vector<point>(hull[i])) the program runs and generates this error:

libc "Fatal signal 11 (SIGSEGV) at 0x00000004 (code=1)"

Any help really really appreciated.

#include <jni.h>
#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <android/log.h>
#include <opencv/cv.h>
#include <vector>
#include <cmath>
#include <opencv2/opencv.hpp>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>

#define  LOG_TAG    "hellojni"
#define  LOGI(...)    __android_log_print(ANDROID_LOG_INFO,LOG_TAG,__VA_ARGS__)
#define  LOGE(...)   __android_log_print(ANDROID_LOG_ERROR,LOG_TAG,__VA_ARGS__)
#define ORIGCOL2ANDROIDORGCOL CV_BGR2BGRA

using namespace std;
using namespace cv;

extern "C" {

    JNIEXPORT jint JNICALL      Java_com_elmira_getconvexhull_MainActivity_convertNativeGray(
        JNIEnv*, jobject, jlong addrRgba, jlong addrGray);
    JNIEXPORT jint JNICALL     Java_com_elmira_getconvexhull_MainActivity_convertNativeGray(
        JNIEnv*, jobject, jlong addrRgba, jlong addrGray) {

        Mat& mRgb = *(Mat*)addrRgba;
        Mat& mGray = *(Mat*)addrGray;

        int conv = 0;
        jint retVal;

        Mat src; Mat src_gray;
        src = mRgb;

        cvtColor(src, src_gray, CV_BGR2GRAY);
        blur(src_gray, src_gray, Size(3, 3));

        Mat src_copy = src.clone();
        Mat threshold_output;
        vector<vector<Point> > contours;
        vector<vector<Point> > hull(contours.size());
        vector<Vec4i> hierarchy;
        int thresh = 100;

        threshold(src_gray, threshold_output, thresh, 255, THRESH_BINARY || CV_THRESH_OTSU);

        /// Find contours
        findContours(threshold_output, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, Point(0, 0));


        __android_log_print(ANDROID_LOG_INFO, "inmethod", "size %d *** %d", contours.size(), threshold_output.cols);

        for (int i = 0; i < contours.size(); i++)
        {
            convexHull(Mat(contours[i]), hull[i], false, false);
        }
        retVal = (jint)conv;
        return retVal;
    }
}

Answer 1

When you initialize hull

vector<vector<Point> > contours;
vector<vector<Point> > hull(contours.size());

hull size is 0 , since contours size is 0. So when you access hull[i] you are accessing out of bounds.

Declare hull after findContours :

 findContours(threshold_output, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, Point(0, 0));
 vector<vector<Point> > hull(contours.size());

---

You can also simply call convexHull as:

 convexHull(contours[i], hull[i]);

since 3rd argument orientation is false by default, and 4th argument returnPoints is ignored when 2nd argument is a std::vector .

---

thresh value is ignored when you use THRESH_OTSU .

---

UPDATE

It seems that there are some problems with Android NDK. A simple workaround is:

• disable the error for invalid arguments, or
• use the following

Code:

Mat mHull;
convexHull(Mat(contours[i]), mHull, false, true);
hull[i].assign(mHull.begin<Point>(), mHull.end<Point>());