Binocular cues to depth PSY 310 Greg Francis. Lecture 21. Depth perception

Transcription

1 Binocular cues to depth PSY 310 Greg Francis Lecture 21 How to find the hidden word. Depth perception You can see depth in static images with just one eye (monocular) Pictorial cues However, motion and binocular cues generally play an important role in depth perception Motion parallax Stereopsis 1

2 Motion and depth Consider a point far away Motion and depth When it moves in the world, the point also moves on the retina 2

3 Motion and depth Consider a point that is closer Motion and depth Have it move the same amount in the world Notice how much it moves on the retina 3

4 Motion and depth Compare the movements for near and far dots On the retina In the world Far It s a variation of the retinal size with distance property Near Motion and depth If you move, then the projection of light from nearby objects moves more quickly on the retina than for far objects Motionparallax.gif 4

5 Prof. Greg Francis Motion and depth Motion parallax is a property of physics Motion and projection on to a surface The retina A camera Motion parallax Many organisms use motion parallax to identify depth of objects Rabbits, prairie dogs, insects, humans, robots 5

6 Two views We noted last time that with a single view there is no way to identify the depth of a point Most of the monocular cues work with objects, not points Motion parallax can identify the depth of a point because it includes several different views For motion to exist, there must be more than one view Either you move Or the object moves We can get depth of a point from different kinds of views Two eyes Binocular vision Binocular vision Suppose you look at a scene with two shapes at different depths You focus (converge your eyes) on object B 6

7 Binocular vision Suppose you look at a scene with two shapes at different depths You focus (converge your eyes) on object B B is in the same place on the retina for the left eye and the right eye Binocular vision Suppose you look at a scene with two shapes at different depths You focus (converge your eyes) on object B A is in a different place on the retina for the left eye and the right eye 7

8 Binocular vision The difference in the position of A across the two eyes is called disparity Measured in terms of visual angle Two views Similarly, if you look at the tree, the policeman is in different places for the foveae of the left and right eye 8

9 Two views Even though the eyes are not far apart, the views can be quite different Two views Even though the eyes are not far apart, the views can be quite different Make them alternate to give a good impression of depth. Motion parallax. TwoViews.gif 9

10 Horopter Differences between the view of the two eyes can be used to identify depth What about points that fall on the same relative position of the two retinas? Horopter Suppose you stare at point D The point D falls on the central part of the fovea of each eye 10

11 Horopter Suppose you stare at point D The point D falls on the central part of the fovea of each eye Then point C will also fall on the same relative position Horopter Suppose you stare at point D The point D falls on the central part of the fovea of each eye Then point B will also fall on the same relative position The angle between D and B is the same for both eyes 11

12 Horopter The horopter is the set of points that fall on the same relative positions of the two eyes No disparity for these points Consider a point W The angles in the two eyes are different W Horopter Easier to see if we get rid of the background W D 12

13 Horopter Easier to see if we get rid of the background Easier to see if we re-position the lines W W Horopter Consider a point behind D The projection of W is on opposite sides of the projection of D Both on the nasal (nose) side of the retina Uncrossed disparity 13

14 Horopter Consider a point in front of D The projection of W is on opposite sides of the projection of D Both on the temporal (temples) side of the retina Crossed disparity W Lots of depth cues How do we know the two views of the eye are used to compute a depth percept? What about all the monocular cues? Does disparity do anything? Need two views that have no monocular cues Random dot stereogram 14

15 Random dot stereogram Random dots contain no monocular cues to depth Random dot stereogram Dots in the middle are in the same relative position 15

16 Random dot stereogram Dots in the middle are in the same relative position Random dot stereogram Dots in the surround are shifted 16

17 Random dot stereogram Dots in the surround are shifted Random dot stereogram The points in the different eyes introduce disparity You have to look at the images with a special device (stereoscope) Or learn how to cross or uncross your eyes to get the images to converge together And you see depth! 17

18 Single Image Stereograms The disparate points can be hidden in a single image We had an image here during lecture, but it was Causing problems for people to print out the notes. Single Image Stereograms Relax your eyes to look far away and get the white boxes to merge together We had an image here during lecture, but it was Causing problems for people to print out the notes. 18

19 Single Image Stereograms It takes practice Some people are stereo-blind Stereopsis It pretty much all works the way you would expect from the geometry And there are neurons in visual cortex that respond to different amounts of disparity Disparity is always relative to where you are looking The point of fixation has no disparity How do you know the depth of this point? 19

20 Depth perception Motion parallax Binocular cues Two views Disparity Stereopsis Conclusions Lots of other issues Motion and stereopsis together Next time Size perception How big is something? Related to depth perception Size constancy Moon illusion 20