Color Processing (continued from previous day)
 

VI. Higher-Level Processing:  Color Constancy

• Why do we have color constancy?
• Explaining Color Constancy
• Memory color
• Chromatic adaptation
• Retinex theory
• Physiological mechanisms

Some definitions:

Memory Color

• A phenomenon in which an object's characteristic color influences our perception of its color.

Chromatic Adaptation

• A weakened response to a color stimulus due to previous exposure to other chromatic stimuli. For example, adaptation to a red light selectively bleaches your long-wavelength cone pigment, which decreases your sensitivity to red light and causes you to see reds and oranges as less saturated and bright, as compared to before adaptation.

Retinex Theory

• A theory of color constancy that is based on the comparison of the lightness readings of surfaces.
• According to this theory, a colored scene is analyzed into 3 lightness records.
• These lightness records give a measure of how much of the available red, green, and blue light a surface reflects.
• These lightness records can be thought of as equivalent to viewing the scene through three monochromatic (single-colored) filters, one at a time – one red, one green, one blue.
• For example, when a scene is viewed though a red filter, one sees an array of reds ranging from very light to very dark. The lightest areas of a scene viewed through a red filter reflect a lot of long-wavelength light and the darker areas reflect little of the long-wavelength light. So red areas in the scene will be the lightest when viewed through a red filter. In contrast, when a scene if viewed though a green filter, green areas in the scene will be the lightest.
• When comparing the output of the 3 lightness records, the lightest part of all three records will be perceived as white, since white reflects more of any wavelength than colored surfaces.