For example, suppose in an image, there is a rectangular wall of which four of its corners are clearly visible, this would form a plane which looks like a trapezoid when viewed from an oblique angle.
From the image, the four points in 2-D space can be obtained with respect to the frame. How could you calculate the angle between the wall and the line of projection from the camera?
You may be able to use vanishing points/line for your purpose. The two pairs of parallel lines on the wall determine the vanishing points/line. Have a read at this file which may be useful.
https://inst.eecs.berkeley.edu/~ee290t/fa11/lectures/October%2025%20vanishing%20points%20and%20line.pptx
Ebenezer, is this a case where the camera is mobile? First, I believe you need to have the camera intrinsic parameters to be able to transform between camera coordinates and image coordinates. Then, for the wall corner points, do you know their relative locations in the "wall coordinate system". In that case, you can use PnP (solvePnP in OpenCV) to find the camera pose relative to the wall. That should get you the information you are looking for. I am not sure whether this is the easiest method.
The simplest solution was to find the angle between the two lines using the tangent and slopes, and find the arc cosine of the angle between them to get the estimate of the angle with respect to the camera perspective.
Thank you very much for the insights Mr. Wolfgang, Dr. Sun and Dr. Beek. All your answers really helped me a lot to come up with the solution.
Dear Ebenezer R.H.P. Isaac
Thank you for the quastion and your answer.
Regards
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