title: How to measure ratio between lines in a photo
body: <p>I'm working with OpenCV for a task on measuring the solar angle in a photo (without any camera parameter). In the photo there is a straight stick with the height of 3 meters standing in the middle of the field. The shadow it casts, however, lies obliquely on the ground (not in the same projection plane as the stick). I can obtain the pixel length of the stick and shadow, but I don't know if the ratio should be directly calculated with the two numbers, since only lines within the same projection plane have the same scale. </p>
  
  <p>This is more like a graphic issue rather than algorithm. Can anyone shed some light on me about how to determine the height-shadow ratio?</p>