title: How can I use the output 3x3 matrix from getPerspectiveTransform in OpenCV?
body: <p>I'm now trying to analyze the perspective transform/homography matrix between two images capturing the same object (e.g., a rectangle) but at different perspectives/shooting angles. The perspective transform can be derived by using the function <code>getPerspectiveTransform</code> in OpenCV 2.3.1. I want to find the corresponding rotation and translation matrices.</p>
  
  <p>The output of <code>getPerspectiveTransform</code> is a 3x3 matrix which I can directly use it to warp the source image into the target image. But my question is that how I can find the rotation and translation matrices based on the obtained 3x3 matrix?</p>
  
  <p>I was looking into the funciton <code>decomposeProjectionMatrix</code> for the corresponding rotation and translation matrices. But the input is required to be a 3x4 projection matrix. How can I relate the perspective transformation (i.e., a 3x3 matrix) to the 3x4 projection matrix? Am I on the right track?</p>
  
  <p>Thank you very much.</p>