table of contents

a look at how humans think and see

19) Visual Illusions: Perception by Comparison
by David Rudd Cycleback

(c) cycleback 2003, 2005 all rights reserved


















































































































































































































































































































































































































































































































































































































































































































































































Human perception of things is greatly influenced by nearby objects, qualities and other information. Both consciously and subconsciously we judge things through comparison. To measure fabric one compares the cloth to a yard stick. To judge the size of someone's hand, you might press your palms against hers. To judge someone's speed, you might race him or watch him race someone else.

In often less exacting comparisons, humans judge the length, height, angle, shape, color and distance by comparing one object to others in the scene. Looking at a family snapshot photo, you might guess the height of a stranger by comparing him to someone you know. You will guesstimate the distance to a house by comparing its size to the sizes of closer houses and trees. You will guesstimate an angle by comparing it to a level line ("Appears to be about 10-15% off from level."). Often these guesstimates are accurate within a reasonable degree. You might guess that stranger in the photo is 6 feet tall, as you know your aunt is 5"5." When you meet him, you may discover he's 5'11." Not perfect, but a darn good guess-especially as you were unable to clearly see what shoes they had been wearing.

A problem is that, while comparing to other objects is essential to making judgments, comparisons often lead to errors. Sometimes seemingly logical comparisons can produce answers that are completely wrong. These errors happen when assumptions about an object or about the overall scene is wrong.

What happens if you incorrectly remembered your aunt as 6 feet tall instead of 5'5," perhaps as the last time you met her was when you were five years old and substantially shorter? Your guess of the height of the man would be similarly off. You might wrongly guess he was 6'7." What happens if she was wearing flats in the photo, while he, shy about his height, was wearing lifts? What happens if due to illness the man couldn't make the family reunion and a cousin expertly photo-shopped images of him into some of the photos?

The following pictures show how your perception can be distorted by surrounding information.

The men are the same size. It is the skewed 'diminishing scale' lines that cause you to perceive the men as different sizes.



The above circle is perfectly round. It is the surrounding and overlapping lines that make you perceive it as lopsided.



Circles A and B are the same size. Without the surrounding grey circles, they would appear the same size.


Which cyclist is going fastest? Most will say the cyclist on the left is going the fastest and the one on the right the slowest. There are, however, many unanswered questions and ambiguities that make it impossible to know for certain who is going the fastest. For examples: Did they start at the same place? Did they start at the same time? Are they bicycling moving foreword or backward? Are they moving? (I've seen race cyclists stand still during a race). Even if it's a normal 1-2-3 Go! race, it's very much possible the guy on the right is going the fastest and the guy on the left is going the slowest at the moment the image was shot. Catching up, slowing down and switching positions are a normal parts of all races. main



(c) david rudd cycleback, all rights reserved