In this paper, we give geometric constructive solutions to the 3D vision problems such as -- how to back project an image point and how to project a space point in monocular vision, -- how to determine the epipolar geometry in stereo vision, -- how to locate the optical center of the camera. No direct calibration is needed, the necessary reference points are directly included in the computation. The method involves only simple geometric computation. The appropriate mathematical tool, projective geometry, is introduced. Results on preliminary experiments are discussed and some future research directions are indicated. This work is supported by Bra First project from Cec DG XIII and the Greco Prc Communication Homme-Machine under the project Orasis.