Depth cues or how your brain and eyes make 3 dimensions out of 2

1. Depth cues or how your brain and eyes make 3 dimensions out of 2 Navigate this presentation by left clicking with the mouse or by pressing the page down key

2. As the human retina is basically just a flat screen on the inside of the eye it can only receive information in 2 dimensions these being height and width. Human experience however tells us that the world is 3 dimensional. We experience both distance and depth in our personal environment. So where does this 3rd dimension come from? Put simply our perceptual processes create it by interpreting information from two types of depth cues. These are binocular and monocular depth cues. To perceive binocular depth cues you must have two eyes, whilst only one eye is needed for the perception of monocular cues.

3. Binocular cues Monocular cues • learned • innate • biological • environmental Although both types of cue are presented separately in this resource, in reality they all work together

4. There are two major binocular depth cues these are convergence and retinal disparity Convergence This acts like a rangefinder to tell us how far away an object is If an object is distant our eyes look straight ahead however as the object gets nearer our eyes swivel inwards (converge) to keep the object in focus The brain measures the amount of convergence to provide us with a rough estimate of distance

5. Retinal disparity Human eyes are spaced apart, separated by the nose. The average distance of this spacing is 7cms. This means that the brain receives two slightly different pictures of the same scene

6. Look at the picture below, it is the equivalent of visual input to the brain from both eyes. If you look carefully you will notice differences. This is due to the distance between the 2 eyes viewing the scene from a slightly different angle.

7. This is exactly how Victorian stereoscopes worked, two similar images of the same scene are presented one to each eye. The brain merges both images and the differences create an impression of depth The brain takes both images puts them together and uses the slight differences between them to create a perception of depth.

8. There are many monocular cues, these include • Superimposition • Relative size • Height in the horizontal plane • Linear perspective • Texture gradient • Motion parallax

9. We assume the tree is in front of the house because it obscures our view of it. It is superimposed on the front of the house, it overlaps it. Likewise we know the tutor is nearer to us than the blackboard because she obscures (overlaps) a portion of it Superimposition or overlap

10. We don’t interpret this picture as one large tank followed by smaller vehicles. We know all the tanks are the same size thus the smaller ones must be further away Relative size When we view 2 objects we know to be the same size and one appears smaller than the other we assume that the smallest is further away

11. In this scene the furthest objects, the trees, are also the highest. We know the boats are nearer to us than the trees because they are lower down The same effect works in real life as well as paintings Height in the horizontal plane

12. Parallel lines converge in the distance so the further away the lines are from the observer the closer together they will appear This fact was discovered and used by renaissance artists to great effect Linear perspective

13. artistic examples include this

14. and this

15. Linear perspective also occurs in the real world

16. Texture gradient Here the grain or texture appears to become finer as distance increases

17. The boulders in this Martian landscape demonstrate this, notice how they get smaller in the distance

18. Motion parallax When an observer passes through a scene it moves through the observers field of vision at different speeds

19. Distant objects appear to pass by slowly Near objects appear to pass by more quickly

20. Now we can try our own pictures