The usual assumption is that where the base of a shape is higher than that of a similar shape, then the one with the higher base is further away. This cue relates objects to the horizon. When one object is interpreted as obscuring part of another one, the one which seems to be obscured is seen as being further away. This cue is stronger with familiar shapes, where a familiar outline is broken by the other shape which is seen as in front of it. I have already alluded briefly to familiar size .
Familiar size is itself a distance cue.
Our previous experience with objects leads us to assume that where some appear smaller than others those which appear to be smaller are likely to be further away. In a well-known experiment on 'Size as a cue to distance' published in the American Journal of Psychology in 1951, W H Ittelson presented observers with three playing cards in a darkened room with all other depth cues removed. The observers tended to judge the larger cards as being closer than the others. Shadow is also an important cue for depth. Look at the illustration below.