Pulfrich Illusion Flashcards
Crossed & Uncrossed Disparity
The brain will interpret Objects with uncrossed disparity as further away in depth and Objects with crossed disparity as nearer in depth
Simple Harmonic Motion
Pendulum stationary until gravity overcomes ball’s inertia (the resistance of any physical object to a change in its state of motion)
Accelerates (speeds up) until the bottom
Reaches peak speed at the bottom
Gravity slows down (decelerates) the pendulum
Pendulum momentarily stationary at other side
Dimming Filter / Lens
Signal transduction from retina to binocular regions of the brain is faster for brighter (more luminous) images.
The filters you put over your eyes dim the image that falls on your retina.
Putting the filter on only one eye means that signals from that eye to the visual cortex are very slightly delayed.
For your information (not quizzed) – ideally we’d use a neutral density filter which reduces luminance from all wavelengths of light and all orientations equally.
Both directions combined.Left eye filtered.This slide is for revision at home
During left-to-right swing.
Acceleration of the pendulum combined with the filter dimming light to the left eye causes increasing uncrossed disparity as the ball moves from the left apex toward the bottom.
Thus the ball appears to get further away.
Deceleration on the upswing causes uncrossed disparity to decrease until the ball reaches the right apex.
Thus the ball appears to get closer.
Combined, this creates a backward arc.
During the right-to-left swing:
During the downswing crossed disparity increases as the ball accelerates toward the bottom.
Thus the ball appears to get closer.
During the upswing, crossed disparity decreases as the ball decelerates.
Thus the ball appears to get further away.
Combined, this creates a forward arc.
All together, it creates an illusion of motion in a clockwise ellipse.
Both directions combined.Right eye filtered.This slide is for revision at home.
During left-to-right swing.
Acceleration of the pendulum combined with the filter dimming light to the right eye causes increasing crossed disparity as the ball moves from the left apex toward the bottom.
Thus the ball appears to get closer.
Deceleration on the upswing causes crossed disparity to decrease until the ball reaches the right apex.
Thus the ball appears to get further away.
Combined, this creates a forward arc.
During the right-to-left swing:
During the downswing uncrossed disparity increases as the ball accelerates toward the bottom.
Thus the ball appears to get further away.
During the upswing, uncrossed disparity decreases as the ball decelerates.
Thus the ball appears to get closer.
Combined, this creates a backward arc.
Together, it creates an illusion of motion in an anticlockwise ellipse
What if you don’t have stereo vision?
10% of general population – deficient stereo vision.
Can’t see Magic Eye pictures
Can’t see the Pulfrich Illusion
But these people are still able to appreciate depth to some extent and don’t crash into things all the time. How?
Monocular depth cues
Parts of the visual scene that allow you to appreciate things in depth with only one eye.
Mono- = ‘one’
-ocular = ‘of the eye’
Can you think of any?
Occlusion
When things cover up a part of something else, we know that object must be closer than the one its occluding.
Retinal size
More distant objects project a smaller image to the retina.
Our brain knows that the ruler isn’t actually changing size, thus the changing retinal size is due to a change in distance of the ruler.
Linear perspective
Parallel lines appear to converge with distance
Texture Gradient
It’s a gradual change in the appearance of objects.
Close objects- Rough and more distinct
Far objects- Smooth and less distinct
Shape from Shading
Shading can give rise to depth perception.
Light typically comes from above – the protruding bits should get light and things under them should be shaded
Light Scatter
Distant objects appear hazier & bluer
Motion parallax
Perspective changes due to head movements similar to getting input from multiple eyes.
Sometimes you can see an animal waggle its head to get depth info via motion parallax.
As we move, objects that are closer to us move further across our visual field, compared to objects further away from us.
Monocular Cues Summary
Linear perspective
The way parallel lines converge.
Occlusion
Objects further away blocked ‘occluded’ by objects in front.
Retinal size
Nearer objects cover more of the retina (looks bigger).
Texture gradient
Similar to retinal size (texture elements decrease in size).
Light scatter
Further objects seem more hazy & blue (due to dust/moisture).
Motion parallax
Perspectives change due to head movements.
Compare monocular perspectives.
Shape from Shading
Shading can cue depth perception based on assumptions about direction of light.
