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examBoard: AQA
examType: GCSE
lessonTitle: Occlusion
Occlusion is one of the most powerful visual cues our brain uses to make sense of the world around us. It happens when one object partially blocks our view of another object. Even though we can't see the whole of the blocked object, our brain fills in the gaps and understands that it continues behind the blocking object.
Key Definitions:
When one object blocks another from view, we automatically understand that the blocked object is further away. This happens instantly and without conscious thought. Occlusion is such a strong depth cue that it can override other visual cues when they conflict.
Look around you right now - you'll see occlusion everywhere! A cup in front of your book, a tree in front of a building, or your hand in front of your face. Each time, your brain instantly knows which object is closer based on which one is blocking the other.
Occlusion works because our visual system follows certain rules when interpreting what we see. These rules have developed through evolution to help us navigate our 3D world effectively.
When we see a partially hidden object, our brain doesn't just register what's visible - it actively fills in what's missing. This process is called "amodal completion" and it happens automatically. Our brain makes an educated guess about the full shape of the object based on what it can see and what it knows about objects in general.
We can recognise objects even when they're partially hidden. For example, you can still identify a cat even if it's partly behind a sofa.
Occlusion immediately tells us which object is in front and which is behind, creating a clear depth order in our perception.
By combining occlusion with other visual cues, our brain builds a complete 3D model of the environment around us.
Psychologist J.J. Gibson was one of the first to study occlusion systematically. In his experiments in the 1950s, he showed that even infants understand occlusion, suggesting it might be an innate ability rather than something we learn. Gibson proposed that occlusion is one of the most reliable depth cues because it works in all lighting conditions and at all distances.
Babies as young as 3-4 months can understand occlusion, showing surprise when objects move in ways that shouldn't be possible based on occlusion cues. This early development suggests that understanding occlusion is fundamental to how we perceive the world.
Researchers test babies' understanding of occlusion by showing them "possible" and "impossible" events. For example, if a toy train goes behind a screen and comes out the other side too quickly, babies will look longer at this impossible event, showing they understand something isn't right.
While basic occlusion understanding seems innate, our ability to use occlusion cues gets better with experience. Artists and designers often train to better understand how occlusion works to create more realistic images.
Occlusion doesn't work alone - it's part of a toolkit of visual cues our brain uses to understand the world. These cues work together to give us a complete picture of our environment.
Our visual system uses multiple cues to determine depth and distance. When these cues agree, our perception is strongest. When they conflict, some cues (like occlusion) tend to dominate others.
Objects that are partially occluded often appear smaller due to distance, reinforcing our depth perception.
Occlusion works with texture gradients (how texture details appear to get smaller with distance) to create a strong sense of depth.
Our two eyes see slightly different views of occluded objects, providing additional information about their position in space.
Visual constancies are how we perceive objects as staying the same despite changes in our sensory input. Occlusion plays a key role in maintaining these constancies.
Even when an object is partially occluded, we perceive its size as constant. This helps us recognise that a person walking behind a tree is still the same height, even though we can only see part of them.
We perceive objects as having the same shape even when partially occluded. Your brain knows a football is still round even if half of it is hidden behind a wall.
The Kanizsa Triangle is a famous visual illusion where we see a white triangle that doesn't actually exist. The illusion works because our brain interprets the black "pac-man" shapes as being occluded by a white triangle. This shows how powerful occlusion is - we literally see objects that aren't there because our brain expects occlusion to work in certain ways!
Understanding how occlusion works has important real-world applications in many fields:
Game designers use occlusion to create realistic 3D environments and to optimise performance by not rendering objects that are completely occluded.
Teaching computers to understand occlusion is crucial for technologies like self-driving cars that need to recognise partially hidden objects.
Artists use occlusion to create depth in 2D images, making paintings and drawings appear three-dimensional.
Several important studies have helped us understand how occlusion works in human perception:
These studies show how fundamental occlusion is to our visual system and how it develops from a very early age.
Elizabeth Spelke showed that 4-month-old infants understand basic principles of occlusion, suggesting this ability develops very early. Babies showed surprise when objects appeared to move through solid barriers, violating occlusion principles.
Stephen Palmer demonstrated how our brains complete occluded objects based on principles like symmetry and simplicity. His work showed we don't just see what's visible - we actively construct a complete mental image.
Occlusion is a fundamental visual cue that helps us navigate our 3D world. It works automatically and develops early in life, suggesting its importance to human perception. By understanding how occlusion works, we gain insight into how our brain processes visual information and constructs our experience of reality.
The next time you look around, pay attention to all the occluded objects in your environment - you'll start to notice how much your brain is filling in and how crucial this process is to your everyday visual experience!
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