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POHow to find corners on a Image using OpenCv
 

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  1. POHow to find corners on a Image using OpenCv
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    <p>I´m trying to find the corners on a image, I don´t need the contours, only the 4 corners. I will change the perspective using 4 corners.</p> <p>I´m using Opencv, but I need to know the steps to find the corners and what function I will use.</p> <p>My images will be like this:(without red points, I will paint the points after) <img src="https://i.stack.imgur.com/9CtnW.jpg" alt="enter image description here"></p> <p>EDITED:</p> <p>After suggested steps, I writed the code: (Note: I´m not using pure OpenCv, I´m using javaCV, but the logic it´s the same).</p> <pre><code>// Load two images and allocate other structures (I´m using other image) IplImage colored = cvLoadImage( "res/scanteste.jpg", CV_LOAD_IMAGE_UNCHANGED); </code></pre> <p><img src="https://i.stack.imgur.com/ag0Sa.jpg" alt="enter image description here"></p> <pre><code> IplImage gray = cvCreateImage(cvGetSize(colored), IPL_DEPTH_8U, 1); IplImage smooth = cvCreateImage(cvGetSize(colored), IPL_DEPTH_8U, 1); //Step 1 - Convert from RGB to grayscale (cvCvtColor) cvCvtColor(colored, gray, CV_RGB2GRAY); </code></pre> <p><img src="https://i.stack.imgur.com/xCFht.png" alt="enter image description here"></p> <pre><code> //2 Smooth (cvSmooth) cvSmooth( gray, smooth, CV_BLUR, 9, 9, 2, 2); </code></pre> <p><img src="https://i.stack.imgur.com/sgBAi.png" alt="enter image description here"></p> <pre><code> //3 - cvThreshold - What values? cvThreshold(gray,gray, 155, 255, CV_THRESH_BINARY); </code></pre> <p><img src="https://i.stack.imgur.com/79WA4.png" alt="enter image description here"></p> <pre><code> //4 - Detect edges (cvCanny) -What values? int N = 7; int aperature_size = N; double lowThresh = 20; double highThresh = 40; cvCanny( gray, gray, lowThresh*N*N, highThresh*N*N, aperature_size ); </code></pre> <p><img src="https://i.stack.imgur.com/01VZ0.png" alt="enter image description here"></p> <pre><code> //5 - Find contours (cvFindContours) int total = 0; CvSeq contour2 = new CvSeq(null); CvMemStorage storage2 = cvCreateMemStorage(0); CvMemStorage storageHull = cvCreateMemStorage(0); total = cvFindContours(gray, storage2, contour2, Loader.sizeof(CvContour.class), CV_RETR_CCOMP, CV_CHAIN_APPROX_NONE); if(total &gt; 1){ while (contour2 != null &amp;&amp; !contour2.isNull()) { if (contour2.elem_size() &gt; 0) { //6 - Approximate contours with linear features (cvApproxPoly) CvSeq points = cvApproxPoly(contour2,Loader.sizeof(CvContour.class), storage2, CV_POLY_APPROX_DP,cvContourPerimeter(contour2)*0.005, 0); cvDrawContours(gray, points,CvScalar.BLUE, CvScalar.BLUE, -1, 1, CV_AA); } contour2 = contour2.h_next(); } } </code></pre> <p><img src="https://i.stack.imgur.com/Zj35W.png" alt="enter image description here"></p> <p>So, I want to find the cornes, but I don´t know how to use corners function like cvCornerHarris and others.</p>
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    1. COOpenCV's "corner" functions don't find corners in the way you're thinking of -- roughly speaking, they find areas with significant horizontal and vertical variation. The goal of the corner functions in OpenCV is to find distinctive parts of the image that will be useful for visual tracking, which is not necessarily what we commonly think of as corners.
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      1. USJosh Bleecher Snyder
    2. COThe exact code at http://stackoverflow.com/a/14368605/1832154 (except the resizing part, since your image is small enough already) gives http://i.imgur.com/hMdAlHX.png
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