Automatic conversion from 2D to 3D takes all the factors mentioned above and derives the parallax from them, but this requires that the objects are at some distance and ideally the camera moves horizontally all the time. the different perspective as seen from each eye becomes what makes the brain see 3D. Only for very close objects the parallax, i.e. And since the eye always focuses on a specific distance, objects closer or further away seem blurry, giving another hint.
For objects farther away, the size in relation to other objects is the most important factor, occlusion (being partly covered by other objects) is another, different movement speeds (objects further away seem to move slower) a third. The problem is that human vision uses different cues to determine depth and 3D.