OpenCV中的分段错误

Question

我最近开始使用 C++ 做一些图像处理的东西，并面临分割错误的问题。希望有人能帮我弄清楚发生了什么？谢谢！！问题是下面附加的代码可以正常工作，但是如果我的“vector roi_corners(4);”的声明改为“vector roi_corners;”然后使用“push_back()”和“clear()”更新向量，它会得到分段错误。谁能帮我澄清这个问题的原因？谢谢！！

OpenCV 版本：4.4.0

MacOS 版本：10.14.5

可行的代码

#include <opencv2/opencv.hpp>
#include <iostream>
#include <math.h>
#include <vector>  

#define PI 3.14159265
#define WINDOW "Image Correction"

using namespace std;
using namespace cv;

vector< Point2f> roi_corners(4); 
vector< Point2f> dst_corners(4); 
Mat img1;
Mat cache;
int roi_id;

void On_mouse(int event, int x, int y, int flags, void*);


int main(int argc, char const *argv[])
{
    
    //import image
    roi_id = 0;
    img1 = imread("Board.jpg");
    if ( img1.empty() )
    {
        cerr << "Please Import an Image!!" << endl;
    }
    int factor = 60;//for pixel adjustment
    cache = img1.clone();//copy for retake points
    imshow(WINDOW, img1);
    
    
    /*Run the point taking procedure*/
    while(true){
     setMouseCallback(WINDOW, On_mouse, 0);
     char c = (char)waitKey( 10 );
     if(c=='n') break;//press 'n' when determine the four point you want 
     if(c=='e') {roi_id=0; img1 = cache.clone();} //press 'e' to retake the foru point

    }
    

    /*For adjustment point estimation*/
    dst_corners[0].x = roi_corners[0].x;
    dst_corners[0].y = roi_corners[0].y;
    dst_corners[1].x = roi_corners[0].x+factor*1;
    dst_corners[1].y = roi_corners[0].y;
    dst_corners[2].x = roi_corners[0].x+factor*1;
    dst_corners[2].y = roi_corners[0].y+factor*1;
    dst_corners[3].x = roi_corners[0].x;
    dst_corners[3].y = roi_corners[0].y+factor*1;

    Mat M = getPerspectiveTransform(roi_corners, dst_corners);
    Mat warped_image;
    

    /*Print the corrected picture*/
    Size sz = cache.size();
    warpPerspective(cache, warped_image, M, Size(sz.width, sz.height)); // do perspective transformation
    imshow("Corrected Image", warped_image);
    waitKey(0);
    cout<<"complete"<<endl;
    return 0;
}


void On_mouse(int event, int x, int y, int flags, void*)
{   imshow(WINDOW, img1);
    if(roi_id<4){
        if (event == EVENT_LBUTTONDOWN){
            roi_corners[roi_id].x=x; 
            roi_corners[roi_id].y=y;
            cout<<"The Point You Take is: "<<x<<' '<<y<<endl;
            roi_id++;
            circle(img1, Point(x,y), 2, Scalar(0, 0, 255), LINE_8 ,0);
            imshow(WINDOW, img1);
        }

    }

}

如果我这样修改代码，它会显示段错误

#include <opencv2/opencv.hpp>
#include <iostream>
#include <math.h>
#include <vector>  

#define PI 3.14159265
#define WINDOW "Image Correction"

using namespace std;
using namespace cv;

vector< Point2f> roi_corners; 
vector< Point2f> dst_corners; 
Mat img1;
Mat cache;


void On_mouse(int event, int x, int y, int flags, void*);


int main(int argc, char const *argv[])
{
    
    //import image
    roi_id = 0;
    img1 = imread("Board.jpg");
    if ( img1.empty() )
    {
        cerr << "Please Import an Image!!" << endl;
    }
    int factor = 60;//for pixel adjustment
    cache = img1.clone();//copy for retake points
    imshow(WINDOW, img1);
    
    
    /*Run the point taking procedure*/
    while(true){
     setMouseCallback(WINDOW, On_mouse, 0);
     char c = (char)waitKey( 10 );
     if(c=='n') break;//press 'n' when determine the four point you want 
     if(c=='e') {roi_corner.clear(); img1 = cache.clone();} //press 'e' to retake the foru point

    }
    

    /*For adjustment point estimation*/
    dst_corners[0].x = roi_corners[0].x;
    dst_corners[0].y = roi_corners[0].y;
    dst_corners[1].x = roi_corners[0].x+factor*1;
    dst_corners[1].y = roi_corners[0].y;
    dst_corners[2].x = roi_corners[0].x+factor*1;
    dst_corners[2].y = roi_corners[0].y+factor*1;
    dst_corners[3].x = roi_corners[0].x;
    dst_corners[3].y = roi_corners[0].y+factor*1;

    Mat M = getPerspectiveTransform(roi_corners, dst_corners);
    Mat warped_image;
    

    /*Print the corrected picture*/
    Size sz = cache.size();
    warpPerspective(cache, warped_image, M, Size(sz.width, sz.height)); // do perspective transformation
    imshow("Corrected Image", warped_image);
    waitKey(0);
    cout<<"complete"<<endl;
    return 0;
}


void On_mouse(int event, int x, int y, int flags, void*)
{   imshow(WINDOW, img1);
    if(roi_corners.size()<4){
        if (event == EVENT_LBUTTONDOWN){
            roi_corners.push_back(Point2f(x,y)); 
            cout<<"The Point You Take is: "<<x<<' '<<y<<endl;
            circle(img1, Point(x,y), 2, Scalar(0, 0, 255), LINE_8 ,0);
            imshow(WINDOW, img1);
        }

    }

}

Answer 1

与您的代码有关的几件事。您声明了一个包含 4 个元素的向量，但不对其进行初始化。根据您的平台和数据类型，您可能会看到不良行为。尝试在同一行声明和初始化你的向量：

std::vector<cv::Point2f> roi_corners( 4, cv::Point2f(0.0, 0.0) );

当然，如果你用初始容量（size）和初始值声明向量，你可以使用std::vector<>::operator[]索引每个元素：

roi_corners[0] = cv::Point2f( 1.0, 2.0 );
roi_corners[1] = cv::Point2f( 3.0, 4.0 );
roi_corners[2] = cv::Point2f( 5.0, 6.0 );
roi_corners[3] = cv::Point2f( 7.0, 8.0 );

通过使用初始大小声明向量，您分配了内存，这些内存将用于store 和load 向量的各个元素。现在，假设您没有使用初始大小声明向量并使用push_back 随时添加元素：

//vector declaration with no initial size:
std::vector<cv::Point2f> roi_corners;

//store a new element into the vector:
roi_corners.push_back( cv::Point2f(1.0, 2.0) );

太棒了，您的向量已经存储了一个新元素，并且只显示了一项的容量。但是，您仍然这样做：

roi_corners[0] = cv::Point2f( 1.0, 2.0 ); // data overwrite in position 0
roi_corners[1] = cv::Point2f( 3.0, 4.0 ); // you haven't allocated memory for this yet!

Result: seg fault

推论：如果您有一个大小为N 的预定义向量，您可以通过std::vector<>::operator[] 对从0 到N 的元素进行索引，因为已分配内存来保存所有N 元素。如果您尝试处理超出此范围的元素，您将收到seg fault。