Lecture Five Human Visual System

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Lecture FiveHuman Visual System
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The Human Eye
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HVS Function

• Part of the nervous system
• Light enters through the pupil (2mm-9mm)
• The lens focuses the light onto the retina
• Retina cells are of two types cones and rods
• Huge dynamic range of around 80dB
• 30,000 candela/m2 (white A4 in bright sunlight)
• 0.03 cd/m2 (same page in moonlight)

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Gamma Correction

• We do not perceive intensity on a linear scale
• Requirement for gamma correction in CRTs
• Intensity k . (Voltage)?
• k and ? are constants for a particular CRT
• Leads to a logarithmic intensity scale
• I1 k . I0
• I2 k2 . I0 ...
• In kn . I0

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Contrast Sensitivity of the Eye
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Contrast Sensitivity of the Eye

• The perception of fine detail is dependent on
  luminance level
• The eye is good at identifying sharp boundaries
• Detail can be missed if the changes are gradual
• The eye prefers richness in colour than
  resolution
• consider PlayStation or Nintendo
• The eye can only resolve about 40 grey levels

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Implications for Multimedia

• Preserve edge information
• Consider brightness for fine detail
• Consider number of colour shades in use
• Consider when to use inverse video
• Carefully choose contrasting colours
• Useful properties for image video coding...