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Look into the function solvePnP which calculates the cameras perspective from given points, so long as you know the model space positions of these points. I assume since they are in the corners of a sheet of paper, you know the real world positions of the triangles?
With 4 points, you could use the P3P flag to solve for 3 points.
The process is very similar to obtaining the camera position relative to an Augmented Reality marker, so looking up tutorials/demo's of that would be a good start.