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    copied!<p>This is a recurring subject in Stackoverflow and since I was unable to find a relevant implementation I decided to accept the challenge.</p> <p>I made some modifications to the squares demo present in OpenCV and the resulting C++ code below is able to detect a sheet of paper in the image:</p> <pre><code>void find_squares(Mat&amp; image, vector&lt;vector&lt;Point&gt; &gt;&amp; squares) { // blur will enhance edge detection Mat blurred(image); medianBlur(image, blurred, 9); Mat gray0(blurred.size(), CV_8U), gray; vector&lt;vector&lt;Point&gt; &gt; contours; // find squares in every color plane of the image for (int c = 0; c &lt; 3; c++) { int ch[] = {c, 0}; mixChannels(&amp;blurred, 1, &amp;gray0, 1, ch, 1); // try several threshold levels const int threshold_level = 2; for (int l = 0; l &lt; threshold_level; l++) { // Use Canny instead of zero threshold level! // Canny helps to catch squares with gradient shading if (l == 0) { Canny(gray0, gray, 10, 20, 3); // // Dilate helps to remove potential holes between edge segments dilate(gray, gray, Mat(), Point(-1,-1)); } else { gray = gray0 &gt;= (l+1) * 255 / threshold_level; } // Find contours and store them in a list findContours(gray, contours, CV_RETR_LIST, CV_CHAIN_APPROX_SIMPLE); // Test contours vector&lt;Point&gt; approx; for (size_t i = 0; i &lt; contours.size(); i++) { // approximate contour with accuracy proportional // to the contour perimeter approxPolyDP(Mat(contours[i]), approx, arcLength(Mat(contours[i]), true)*0.02, true); // Note: absolute value of an area is used because // area may be positive or negative - in accordance with the // contour orientation if (approx.size() == 4 &amp;&amp; fabs(contourArea(Mat(approx))) &gt; 1000 &amp;&amp; isContourConvex(Mat(approx))) { double maxCosine = 0; for (int j = 2; j &lt; 5; j++) { double cosine = fabs(angle(approx[j%4], approx[j-2], approx[j-1])); maxCosine = MAX(maxCosine, cosine); } if (maxCosine &lt; 0.3) squares.push_back(approx); } } } } } </code></pre> <p>After this procedure is executed, the sheet of paper will be the largest square in <code>vector&lt;vector&lt;Point&gt; &gt;</code>:</p> <p><img src="https://i.stack.imgur.com/JQ0DH.jpg" alt="opencv paper sheet detection"></p> <p>I'm letting you write the function to find the largest square. ;)</p>
 

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