Depth Cues, Visual Illusions and Perceptual Constancies
Your eyes receive a flat image, yet you see a stable 3D world. Learn the depth cues that build it, the illusions that trick it, and the constancies that keep it steady.
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From flat image to 3D world 👁️
Each eye receives a flat, two-dimensional image, yet you perceive **depth**, a stable world, and occasionally something that is not really there. This module maps the toolkit: the **depth cues** that build 3D from 2D, the **visual illusions** that trick perception, and the **perceptual constancies** that keep the world looking steady even as the image on your retina keeps changing. (The two big theories that explain WHY are in a separate module.)
The words you need 🗝️
Five terms organise the whole topic.
Depth from a single eye 📐
Most depth cues are **monocular** - they work with just one eye: - **Superimposition**: a nearer object partly hides one further away. - **Relative size**: of two similar objects, the smaller image looks further away. - **Linear perspective**: parallel lines appear to converge as they recede. - **Texture gradient**: detail looks finer and more crowded with distance. - **Height in the plane**: objects higher up in the view seem more distant. The one **binocular** cue on the spec is **stereopsis**, which needs both eyes.
Match each depth cue to how it works 🔗
- Superimposition
- Relative size
- Linear perspective
- Texture gradient
- A nearer object partly blocks one behind it
- The smaller of two similar images is judged further away
- Parallel lines seem to meet as they go into the distance
- Surface detail becomes finer and denser with distance
Which need only one eye? 👓
Select the TWO cues that are monocular (need only one eye).
- Height in the plane
- Linear perspective
- Stereopsis
- Using the difference between the two eyes' images
The two-eyed cue 👀
What is stereopsis, the binocular depth cue?
- The brain combines the slightly different image from each eye to judge depth
- Judging depth from the detail seen by one eye alone
- Assuming smaller objects are always closer
- Using colour to work out distance
When perception bends, and when it holds 🔄
Two topic ideas are opposites. In a visual illusion, perception goes WRONG; in a visual constancy, perception stays RIGHT even though the retinal image changes.
Match each illusion type to its meaning 🎭
- Fiction
- Ambiguous figure
- Distortion
- Visual constancy
- Perceiving a shape or edge that is not physically there
- One figure that can be seen in two different ways
- Misjudging the true size, length or angle of something
- Seeing an object as unchanged despite a changing retinal image
The shrinking friend 📏
A friend walks away from you. The image of them on your retina gets smaller, yet they still look their normal size to you. Which idea explains this?
- Size constancy
- A distortion illusion
- A fiction
- Stereopsis
What shapes a perceptual set 🧠
A perceptual set is a readiness to perceive things a certain way. It can be shaped by _____, such as being hungry; by _____, when you see what you expect to see; by _____, such as fear making something look threatening; and by _____, the shared assumptions of your society.
How size constancy works 🪜
An interactive activity.
Name what is happening 🧭
An interactive activity.
Your turn: explain depth 👁️
An interactive activity.
The grade-9 habit 🌟
Keep the three ideas cleanly apart. **Depth cues** BUILD 3D from a flat image - mostly monocular (superimposition, relative size, linear perspective, texture gradient, height in the plane) plus the binocular **stereopsis**. **Visual illusions** are where perception goes WRONG (fictions, ambiguous figures, distortions); **constancies** are where it stays RIGHT despite a changing retinal image (size, shape, colour); and **perceptual set** (motivation, expectation, emotion, culture) tilts what you perceive. These are the very facts Gibson's and Gregory's theories argue over, so revise them alongside that module.