You track points in the scene across frames. This helps infer the 3D position though getting good accuracy is hard. Basically parallax gets created in time as

You track points in the scene across frames. This helps infer the 3D position though getting good accuracy is hard. Basically parallax gets created in time as the camera moves. I recall those childhood days when we could not explain why moon appeared to come along as our car moved. :-) Those differences in apparent speed betrays something about the distance of the object/point under consideration.