We all know that the figures in this kind of image seem to get larger as they near the top right-hand corner, although we also know from previous encounters with such images that the three figures are in fact all the same size. We find it hard to avoid interpreting the converging lines as indicating linear perspective, which requires us to accept that the figure is breaking the rules by seeming to be larger rather than smaller as it gets further away. For reference, it may be useful to know that the simplest version of such illusions deriving from linear perspective is called the 'Ponzo illusion'. In this basic version, there are two lines converging towards the top of the image, together with two horizontal lines. Linear perspective is only one kind of depth cue in a static two-dimensional image such as a painting, drawing or photograph.
Relative size is another depth cue.
Where an image features several objects of similar shape, the tendency is to assume that the smaller objects are further away. 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.