Visual Perception How We See Things

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

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

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

The Color of Things Color is the perceived appearance of an object’s emitted or reflected light o Visible light which gives color to an object appears from violet at 400nm to red at 700nm. o Below 400nm in decreasing wavelengths are ultraviolet, x-rays, and gamma rays. Above 700nm in increasing wavelengths are infrared, microwave, and radio waves. o 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 o Cones are of three (possibly four) types: blue, “red” and “green” o The chemical response of the cones to excitation by light causes the perception we see as color o Among the retina’s cells, there is interplay (lateral inhibition) which causes optical illusions such as Mach bands.Mach bands o Also optical illusions e.g., after-images, unexplainable conditions (1, 2) existafter-images12

Color Models HLS: Hue – Lightness (or Luminence) – Saturation o We tend to refer to color naturally with this model, e.g., a blue car or a pink dress is referring to its hue. o The “color wheel” is characteristic of hue (0 – Red, 80 – Green, 160 – Blue) o 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 o This is the less natural, but more “native” way of specifying color. o 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

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

Characteristics of Portraying Dynamic Effects Frame Rate for Visual Persistence o Typically need 25 frames per second (i.e., refresh of 25Hz) to avoid most flickering. Effect completely ceases at 80Hz o 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 o The accepted minimum for smooth motion is 10Hz More is necessary for small or fast-moving obejcts Poor frame rates cause strobingstrobing Unifying Frame Rate and Update Rates o Use front and back display buffers for displaying frame o Swap buffers at lesser or frame or update rate