A novel solution to PnP problem for a camera with unknown focal length

Aiming at PnP problem for an camera with unknown focal length which the reference points number greater then six, propose an algorithm with high accuracy and speed. The method consist of three steps: 1) four non-coplanar virtual control points are selected and the linear relationship between those virtual control points and space point are solved; 2) Based on above result, linear equations according to the relationship between space points and virtual control points and the image points are setup, and then image coordinates of virtual control points and camera internal parameters are solved by these equations; 3) rotation matrix and translation vector are solved by OI algorithm according to virtual control points and their image points. Experimental results show that our algorithm has a higher accuracy than currently accepted algorithms for PnP problem with uncalibrated camera and even better than state-of-the-art algorithm for calibrated camera. Further more, our algorithm can converge to correct result in a short runtime, and is of better robustness.