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Depth cues

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The image on the back of your eye is flat, yet you see a 3D world. Sensation is receiving information through the senses; perception is making sense of it. Depth cues help the brain judge distance.

Monocular depth cues

Monocular cues work with one eye, and artists use them to show depth in flat pictures.
Superimposition (overlap): an object that blocks part of another is seen as closer.
Relative size: of two similar objects, the smaller one looks further away.
Linear perspective: parallel lines, such as a road or railway, seem to meet in the distance.
Texture gradient: surfaces look more detailed close up and smoother further away (like a pebbly beach).
Height in the plane: objects higher up in the picture, nearer the horizon, look further away.

Binocular depth cue: stereopsis

Binocular cues need both eyes.
Our eyes are a few centimetres apart, so each eye sees a slightly different image (retinal disparity). The brain combines the two images, and the difference tells it how far away things are. This is stereopsis.
The closer an object, the bigger the difference between the two images. Try holding a finger near your face and closing each eye in turn: it seems to jump.

Spotting cues in a picture

Describe a picture of a street: the road narrows to a point (linear perspective), the people further down are smaller (relative size), a lamp post hides part of a car (superimposition), the cobbles get finer (texture gradient), and distant houses are higher up the image (height in the plane).
Worked example

In a photo, train tracks seem to meet at the horizon. Which depth cue is this, and is it monocular or binocular?

  1. Parallel lines appearing to converge in the distance is linear perspective.
  2. It works in a flat photo with one eye, so it is monocular.

Answer: Linear perspective, a monocular cue.

Key idea

Monocular depth cues (superimposition, relative size, linear perspective, texture gradient, height in the plane) need one eye and work in pictures. Stereopsis is a binocular cue: the brain combines the slightly different images from each eye.

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