# Perception (9/22)

## Visual perception

Because data visualization harnesses our powerful visual perception capacity, it is important to understand the characteristics of human visual system.

### Optical illusion

One way to clearly see the biases and distortions in our perception is examining optical illusions. One of the most important illusions is about lightness and contrast. For instance, look at the image below. The square A looks much darker than B, even though their lightness is exactly same.

This happens because our visual system try its best to infer the innate properties (brightness, color, etc.) of the objects we see. For instance, we perceive the 'true' color of an object regardless of the lighting condition. Also, our visual system processes 'difference' much better than 'absolute value'. Therefore, perceived lightness highly depends on the lightness of the surrounding.

When two areas with different lightness meet, the lightness different at the interface dominates our perception. Below, the left side looks darker even though they are the same shade of gray.

It become clear if we block the interface.

The shaded diamonds illusion clearly demonstrates this effect.

### Psychophysics

Psychophysics is a discipline that studies the quantitative relationships between stimuli and perception. There are several 'laws' of Psychophysics. Weber-Fechner's law notes that the JND (just noticeable difference) between two stimuli is proportional to the size of the stimuli. For instance. Stevens' power law tells that the perceived strength ($$S$$) of a stimulus ($$I$$) scales as a power-law ($$S = kI^{\alpha}$$).

Although these laws are not as robust as the laws of Physics, it is important to know about them. For instance, it is a very important lesson that we perceive length much more accurately than area or volume.

### Pre-attentive processing

Pre-attentive processing is important in visualization because often the goal of visualization is immediate discovery of important patterns in the data. It is essential to use the visual encodings that enable pre-attentive processing to highlight differences.

## References

Books Laws Optical illusions Pre-attentive processing
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