1. Visual Perception
How We See Things

2. Visual Perception It is generally agreed that we have five senses
Vision
Hearing
Touch
Taste
Smell
Of our five senses, the one with the highest bandwidth and most often used by computers to communicate with humans is vision
Which sense gets second billing? Which are sometimes used? Which are rarely used.

3. Human Visual System
Remarkably tolerant of bad data and yet amazingly sensitive
Tolerant
Sketches, original (distortion, noise), occlusions
Sensitive
Visible anomaly, adaptable to varying brightness, remarkable acuity
Vision perceives reality, it is not a measure of reality
E.g.
Brightness is the perceived characteristic of luminance
Color is the perceived characteristic of spectral frequencies
Images producing the same (or near same) percept are perceived as “realistic”
Goal of computer graphics is to produce desired effect in a computationally efficient manner

4. Eye Anatomy Focal System Imaging System Network Pupil and Lens
Retina with its rods and cones
Cones: color perception. Cones are especially dense around the fovea
Rods: brightness perception. Rods outnumber cones about 20:1
Network
Optic Nerve and Visual Cortex

5. The Color of Things
Color is the perceived appearance of an object’s emitted or reflected light
Visible light which gives color to an object appears from violet at 400nm to red at 700nm.
Below 400nm in decreasing wavelengths are ultraviolet, x-rays, and gamma rays. Above 700nm in increasing wavelengths are infrared, microwave, and radio waves.
Whether emitted or reflected is immaterial to the eyes response, for example, the moon only reflects light, but is commonly perceived as an emitter
Light waves in the visible band are perceived (as color) by the retina’s cones
Cones are of three (possibly four) types: blue, “red” and “green”
The chemical response of the cones to excitation by light causes the perception we see as color
Among the retina’s cells, there is interplay (lateral inhibition) which causes optical illusions such as Mach bands.
Also optical illusions e.g., after-images, unexplainable conditions (1, 2) exist

6. Color Models HLS: Hue – Lightness (or Luminence) – Saturation
We tend to refer to color naturally with this model, e.g., a blue car or a pink dress is referring to its hue.
The “color wheel” is characteristic of hue (0 – Red, 80 – Green, 160 – Blue)
The perceived color we ascribe to an object is called color constancy
E.g., a red coffee mug in a twilight lit room is likely no longer truly appearing red, but we would still describe it as such
RGB: Red, Green, Blue
This is the less natural, but more “native” way of specifying color.
Often in a computer graphics system, color is designated as 24-bit (or RGB) or 32-bit (or RGBA)
E.g., 0xff_00_00_00 would be fully-saturated red with no transparency
Experimenting w/Color (link)
Paint
Interpolation of Color

7. Additive and Subtractive Color
Mixtures of spectral frequencies are “additive”
E.g., full red, green, and blue produce white.
Mixtures of dyes or paints is “subtractive”
E.g. a complete mixture of cyan (blue and green), magenta (blue and red), and yellow (red and green) is black.
Color is a perception which is modified by its context (e.g., 1, 2, and 3)

8. Characteristics of Portraying Dynamic Effects
Frame Rate for Visual Persistence
Typically need 25 frames per second (i.e., refresh of 25Hz) to avoid most flickering. Effect completely ceases at 80Hz
Common Technologies
US Fiilm 24Hz, European Film 25Hz
US TV 60Hz (interlaced), European TV 50Hz (interlaced)
Typical LCD computer monitor 65Hz (non-interlaced)
Stereo LCD 120 Hz (for both left- and right-eye image)
Update Rate for Smooth Motion
The accepted minimum for smooth motion is 10Hz
More is necessary for small or fast-moving obejcts
Poor frame rates cause strobing
Unifying Frame Rate and Update Rates
Use front and back display buffers for displaying frame
Swap buffers at lesser or frame or update rate

9. Visual Cues for Distance and Motion
Distance Cues
Size
Occlusion
Linear Perspective
Texture or Surface Detail
Shadow
Stereopsis
Motion Parallax
Accomodation and Convergence
Atmosphere
Motion Cues
Position Change
Motion Blur