Affine and Perspective Transformation

In affine transformation (link, link2), all parallel lines in the original image will still be parallel in the output image. To find the transformation matrix, we need 3 points from input image and their corresponding locations in output image. Then cv2.getAffineTransform will create a 2×3 matrix which is to be passed to cv2.warpAffine. Affine transform can perform rotation, translation, resizing,

pts1 = np.float32([[50,50],[200,50],[50,200]])
pts2 = np.float32([[10,100],[200,50],[100,250]])

M = cv2.getAffineTransform(pts1,pts2)
dst = cv2.warpAffine(img,M,(cols,rows))

For perspective transformation (see links above), you need a 3×3 transformation matrix. Straight lines will remain straight even after the transformation. To find this transformation matrix, you need 4 points on the input image and corresponding points on the output image. Among these 4 points, 3 of them should not be collinear. Then transformation matrix can be found by the function cv2.getPerspectiveTransform. Then apply cv2.warpPerspective with this 3×3 transformation matrix.

pts1 = np.float32([[56,65],[368,52],[28,387],[389,390]])
pts2 = np.float32([[0,0],[300,0],[0,300],[300,300]])

M = cv2.getPerspectiveTransform(pts1,pts2)
dst = cv2.warpPerspective(img,M,(300,300))

In summary,

  • Affine transformation preserves lines and parallelism.
  • Perspective transformation preserves lines. Affine transform is a special case of perspective transformation.
  • PS, affine transformation does not preserve angle. Conformal transformation preserves angle.

 

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Affine and Perspective Transformation

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