How to use warpAffine function in OpenCV C++

Question

I need to warp an image by using an affine transformation. The error I get is that the "code has crashed".

Can anyone tell me what is wrong with my code?

Mat imgAffine, image, par;
image = imread("/media/sf_vbox_share/ubuntushare/board.jpg",1);

par.at<double>(0,0)=  1.01121;  //p1
par.at<double>(1,0)=  0.21067;  //p2;
par.at<double>(0,1)= -89.69693; //p3;
par.at<double>(1,1)= - 0.11557; //p4;
par.at<double>(0,2)= 1.44982;   //p5;
par.at<double>(1,2)= -193.66149;//p6;

imgAffine = Mat::zeros(image.rows, image.cols, image.type());
warpAffine(image,imgAffine,par, image.size(),INTER_LINEAR);
namedWindow("image",WINDOW_AUTOSIZE);
imshow("image",imgAffine);
cvWaitKey(0);

Answer 1

par is undefined in size so when trying to write elements to it, it is most likely giving you segmentation faults. Try allocating the memory for it first before setting its elements:

// Your code
Mat imgAffine, image, par;
image = imread ("/media/sf_vbox_share/ubuntushare/board.jpg",1);

// New code
par = Mat(2, 3, CV_64FC1); // Allocate memory

// Rest of your code
par.at<double>(0,0)=  1.01121;  //p1
par.at<double>(1,0)=  0.21067 ;  //p2;
par.at<double>(0,1)= -89.69693; //p3;
par.at<double>(1,1)= - 0.11557;  //p4;
par.at<double>(0,2)= 1.44982;   //p5;
par.at<double>(1,2)= -193.66149;//p6;
//
// ....
// ....

Alternatively, you can use the constructor of cv::Mat when declaring par , so at the beginning of the code:

Mat imgAffine, image;
Mat par(2, 3, CV_64FC1);

// Rest of your code
image = imread ("/media/sf_vbox_share/ubuntushare/board.jpg",1);

//
// ....
// ....