How a flat image becomes a world with depth

1. Depth cues are the patterns in the flat image reaching each eye, and in the way your two eyes work together, that let you judge how far away things are.
2. Cues that work with one eye alone are monocular, and cues that need both eyes working together are binocular.

The monocular cues

1. Superimposition
   1. One object partly blocks your view of another, and the object doing the blocking is seen as the nearer of the two.
2. Relative size
   1. When you know two objects are roughly the same real size, the one casting the smaller image is judged to be further away.
3. Linear perspective
   1. Parallel lines such as rails or lane markings appear to converge as they recede, so the point where they meet reads as the most distant part of the scene.
4. Texture gradient
   1. A textured surface looks coarse and widely spaced close to you but finer and more tightly packed further off, and that gradient signals increasing distance.
5. Height in the plane
   1. Objects sitting higher in your visual field, nearer the horizon, are seen as further away than objects low down in the field.

The binocular cue

1. Stereopsis is the brain comparing the slightly different image each eye receives and reading how big the difference between them is.
2. A large difference means the object is close to you, while a small difference means it is far away.

Three kinds of visual illusion

1. A visual illusion is a perception that does not match the physical stimulus in front of you, and the same trick fools almost everyone in the same way.
2. In a distortion you recognise the object perfectly well but misjudge one of its features, such as its length, its size or its shape.
   1. The Muller-Lyer illusion
   2. Two lines of identical length look unequal, because the one with outward-spreading fins appears longer than the one with inward-pointing arrowheads.
3. In an ambiguous figure the image offers two equally good readings, and because you can hold only one at a time your perception flips between them.
   1. Rubin's vase
   2. The same black and white shape can be seen either as a vase or as two faces in profile, and it switches back and forth although the ink on the page never changes.
4. In a fiction you perceive an edge, a contour or a whole shape that is not physically present at all.
   1. The Kanizsa triangle
   2. Three notched circles are arranged so that you see a bright triangle with crisp edges, even though no triangle has been drawn.

Why these illusions happen

1. Distortions arise because the brain applies depth cues meant for a real three-dimensional scene to a flat figure, judges one part to be further away and scales it up, so size constancy is misapplied and the line looks longer.
2. Ambiguous figures arise because the flat image genuinely under-specifies the scene, leaving two readings equally well supported and neither of them settled by the input.
3. Fictions arise because the brain completes a familiar shape that the arrangement of parts strongly suggests, filling in edges the eye never actually received.

Keeping the world steady

1. The image an object casts on your retina keeps changing, yet perceptual constancy keeps your experience of the object steady, because the brain corrects the raw image using what it knows about distance, viewing angle and lighting.
2. Shape constancy
   1. A door swinging open throws a run of stretched, slanted shapes onto your retina, yet you go on seeing one rectangular door.
3. Colour constancy
   1. A red jumper still reads as red under a yellow streetlight, because the brain judges the object against its surroundings and discounts the lighting falling on the whole scene.
4. Size constancy
   1. A bus pulling away shrinks steadily on your retina, yet you see a full-size bus getting further off rather than a bus getting smaller, because the brain scales the image using how distant it judges the bus to be.

Turning this into marks

1. Name the cue and then say how it signals distance, because the name on its own takes one mark.
2. When a photograph or a described scene is given, quote the detail in it that shows the cue at work.
3. The mark most often lost is naming the family of an illusion without saying what the viewer actually misperceives.