Visual Perception

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 • Pupil and Lens • Imaging System • 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

7. Visual Cues • Distance Cues • Size • Occlusion • Linear Perspective • Texture or Surface Detail • Shadow • Stereopsis • Motion Parallax • Accomodation and Convergence • Atmosphere • Motion Cues • PositionChange • Motion Blur

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