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POHow Can I Perform a Perspective Transform Using a Homography Matrix (3x3) in JavaCV?
 

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  1. POHow Can I Perform a Perspective Transform Using a Homography Matrix (3x3) in JavaCV?
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    <p>I have been trying for some time now to take an image of a pool table from an arbitrary angle, locate four coordinate pairs (x,y) of the corners, and rotate/warp the image such that the resulting image contains only the table from a bird's eye view. I am using JavaCV to accomplish this task. So far, I have written code which successfully calculates the four corners of the table as shown below (I have coded red cvCircles of radius 25 on the four corners).</p> <p><img src="https://i.stack.imgur.com/Zmrwk.jpg" alt="enter image description here"></p> <p>After finding my four corners:<br> Corner 1 (Bottom Right) X: 3234 Y: 1858<br> Corner 2 (Bottom Left) X: 0 Y: 1801<br> Corner 3 (Top Right) X: 2722 Y: 1069<br> Corner 4 (Top Left) X: 523 Y: 1030<br></p> <h2>I attempted to use cvFindHomography and then cvWarpPerspectve to accomplish this. My code can be found below:<br></h2> <pre><code>// Initialize Table Corners as Image Coordinates float[] aImg = { corners.get(0).x(), corners.get(0).y(), // BR X: 3234 Y: 1858 corners.get(1).x(), corners.get(1).y(), // BL X: 0 Y: 1801 corners.get(2).x(), corners.get(2).y(), // TR X: 2722 Y: 1069 corners.get(3).x(), corners.get(3).y() // TL X: 523 Y: 1030 }; // Initialize World Coordinates (Are these even correct? How do I determine these?) float[] aWorld = { 0.0f, 0.0f, 0.0f, 1.0f, 1.0f, 0.0f, 1.0f,1.0f }; // Create 3x3 Homography Matrix CvMat homography = cvCreateMat(3, 3, opencv_core.CV_32FC1); opencv_imgproc.cvGetPerspectiveTransform(aImg, aWorld, homography); System.out.println(homography.toString()); // Create imgWarped and Warp Perspective IplImage imgWarped = cvCreateImage(cvGetSize(img), 8, 3); opencv_imgproc.cvWarpPerspective(img, imgWarped, homography, opencv_imgproc.CV_INTER_LINEAR, CvScalar.ZERO); // Create Canvas and Display Image CanvasFrame canvas = new CanvasFrame("Warped Image"); canvas.showImage(imgWarped); canvas.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); </code></pre> <p><br> The 3x3 Homography Matrix I'm obtaining is:<br> [ 0.0011688289, 7.928632E-4, -1.4279466<br> -8.698455E-5, 0.0049045905, -5.006235<br> -2.8205379E-5, 0.0015696458, 1.0 ]<br> <br></p> <h2>**Unfortunately, the result is a purely black image containing <em>nothing</em>. Am I approaching this incorrectly? Any advice, code, pseudocode, etc. would be GREATLY APPRECIATED!<br></h2> <h2>Thank you so much for reading.**</h2>
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    1. USChuck D
    1. USMatthew
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      1. POHow Can I Perform a Perspective Transform Using a Homography Matrix (3x3) in JavaCV?
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    1. COHave you tried using the 4 original corners of your `img` as coordinates for `aWorld`?
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      1. POHow Can I Perform a Perspective Transform Using a Homography Matrix (3x3) in JavaCV?
      1. USSamuel Audet
    2. COYeah but unfortunately this did not work. However, I did find a solution. By altering aWorld parameters 1.0f to the img.width()/img.height() values I was able to successfully warp the perspective. Thank you for your help Sam! And also for developing JavaCV :)
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      1. USMatthew
 

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