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PORotate cv::Mat using cv::warpAffine offsets destination image
 

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  1. PORotate cv::Mat using cv::warpAffine offsets destination image
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    <p>I'm trying to <strong>rotate</strong> a <strong>1296x968</strong> image by <strong>90 degrees</strong> using the <strong>C++ API</strong> of OpenCV and I'm facing a few problems.</p> <p><strong>Input</strong>: <img src="https://i.stack.imgur.com/Hyel9.jpg" alt="input"></p> <p><strong>Rotated</strong>: <img src="https://i.stack.imgur.com/NDSFw.jpg" alt="output"></p> <p>As you can see, the rotated image has a few problems. First, it has the same size of the original, even though I specifically create the destination <code>Mat</code> with the inverted size of the original. As a result, the destination image gets cropped. </p> <p>I suspect this is happening because I'm calling <code>warpAffine()</code> and passing the size of the original <code>Mat</code> instead of the size of destination <code>Mat</code>. But I'm doing this because <strong>I followed <a href="https://stackoverflow.com/questions/2289690/opencv-how-to-rotate-iplimage/2300459#2300459">this answer</a></strong>, but now I suspect that the answer may be wrong. So this is my first doubt/problem.</p> <p>The second, is that <code>warpAffine()</code> is <strong>writing to the destination at a certain offset</strong> (probably to copy the rotated data to the middle of the image) and this operation leaves a horrible and <strong>large black border</strong> around the image. </p> <p><strong>How do I fix these issues?</strong></p> <p>I'm sharing the source code below:</p> <pre><code>#include &lt;cv.h&gt; #include &lt;highgui.h&gt; #include &lt;iostream&gt; using namespace cv; using namespace std; void rotate(Mat&amp; image, double angle) { Point2f src_center(image.cols/2.0F, image.rows/2.0F); Mat rot_matrix = getRotationMatrix2D(src_center, angle, 1.0); Mat rotated_img(Size(image.size().height, image.size().width), image.type()); warpAffine(image, rotated_img, rot_matrix, image.size()); imwrite("rotated.jpg", rotated_img); } int main(int argc, char* argv[]) { Mat orig_image = imread(argv[1], 1); if (orig_image.empty()) { cout &lt;&lt; "!!! Couldn't load " &lt;&lt; argv[1] &lt;&lt; endl; return -1; } rotate(orig_image, 90); return 0; } </code></pre>
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    1. COFrom the documentation of `cv::warpAffine`: dst – Destination image; will have size dsize and the same type as src
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    2. COI understand that, thanks @krynr. If you feel like you have the solution to the problems feel free to post an answer.
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