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I need to find the size or coordinates of a rectangle that is displayed as a quadrilateral in a 3D image. The quadrilateral is on a plane that lines up with 3d world vanishing points. To clarify, the quadrilateral IS a rectangle in the 3D world, and that's the rectangle I want the size of.

I do not need to get all the textures and make a new image. I also do not know the coordinates of the target rectangle as required by the homography (perspective transformation) solutions I've seen, because I don't know the aspect ratio it's supposed to have.

I've read through this thread: proportions of a perspective-deformed rectangle and the guy seemed to find an algorithm that works. However I've read other research papers that claim to calculate a homography yet they don't say how they did it. Also it seems such a basic function there would be something in the existing openCV library.

Thanks.

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  • Please clarify your question. Are you looking to determine the aspect ratio? Or trying to figure out how to calculate a homography using OpenCV? – David Nilosek Nov 18 '13 at 03:56
  • I actually need to compare the ratio of overlap between 2 rectangles on the same plane from a 3d perspective. Aspect ratios would be fine if they were both on the same scale. Otherwise I need sizes relative to the 3d space. – user3003140 Nov 18 '13 at 04:19
  • Here's another example: http://stackoverflow.com/questions/2992264/extracting-a-quadrilateral-image-to-a-rectangle . The person posting indicates they already know the coordinates of the rectangle in the destination. That's my problem - I don't know how they got it or if they are just guessing? – user3003140 Nov 18 '13 at 18:58
  • Here also: http://stackoverflow.com/questions/13324768/projective-geometry-how-do-i-turn-a-projection-of-a-rectangle-in-3d-into-a-2d-v?rq=1 . The person answering says you must know the size of the sheet of paper in both 3d and 2d. This doesn't seem right to me. Surely we must be able to learn the size of something relative to its environment given the vanishing points. – user3003140 Nov 18 '13 at 19:15

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