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POOpenCV Mat image changes size after function call by reference
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2012-10-15T20:32:11.183
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i am writing an application using OpenCV. Recently i've noticed strange things sometimes happen to cv::Mat object when passed to function by reference. Here's the code: <pre><code>cv::Mat img1=cv::imread(argv[1]); cv::Mat img2=cv::imread(argv[2]); double scale=0.4; int thr=52; //resize image cv::Mat tmp; img1.copyTo(tmp); cv::resize(tmp,img1,cv::Size(),scale,scale); img2.copyTo(tmp); cv::resize(tmp,img2,cv::Size(),scale,scale); cv::Mat im1,im2; try { cv::Sobel(img1,im1,CV_8U , 0/*x order*/, 1/*y order*/,5/*kernel size*/,1/*scale*/); cv::Sobel(img2,im2,CV_8U , 0/*x order*/, 1/*y order*/,5/*kernel size*/,1/*scale*/); } catch(...) { std::cout << "Something went wrong calculating Sobel!" << std::endl; } cv::imshow("im1",im1); cv::imshow("im2",im2); //create empty mat with the same size as im1, im2 cv::Mat bin1(im1.size().height, im1.size().width, CV_8UC1); cv::Mat bin2(im2.size().height, im2.size().width, CV_8UC1); std::cout << "_________OUT_________" << std::endl; std::cout << im1.size().width << " " << bin1.size().height << std::endl; std::cout << im2.size().width << " " << bin2.size().height << std::endl; getExtreme(im1,bin1,thr); getExtreme(im2,bin2,thr); </code></pre> The getExtreme function is defined like this: <pre><code> void getExtreme(cv::Mat &img, cv::Mat &dst,int prag) { int x=img.size().width; int y=img.size().height; std::cout << "_________________IN______________________" << std::endl; std::cout << x << " " << dst.size().width << std::endl; std::cout << y << " " << dst.size().height << std::endl; int i,j; for (i=0;i<x;i++) { for (j=0;j<y;j++) { if(img.data[img.channels()*img.cols*j+i]>prag) dst.data[dst.channels()*dst.cols*j+i]=255; } } } </code></pre> while the images which size i am printing are the same, the output of the program is: __OUT__ 819 819 819 819 ________IN__________ 819 819 435 435 _______IN___________ 819 819 435 435 so for some reason the bin1 and bin2 images (cv::Mat) change size. however, if i set the double scale=0.4; to double scale=1;, thus not resizing the image but retaining its original size, the size of the image matches outside and inside the function: __OUT__ 2048 1088 2048 1088 ________IN__________ 2048 2048 1088 1088 _______IN___________ 2048 2048 1088 1088 What am i doing wrong? How does resizing influence the mat objects? why is it different at all, if it is passed by reference i should have the same object inside the function. thanks
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OpenCV Mat image changes size after function call by reference
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1. COBy the way with your raw data access to pixel, if the image have more than 1 channel, you set only the first channel to 255 in your loop. Consider using OpenCV thresholding functions as shown in my answer
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