Chapter+Three+-Lightness,+Brightness,+Contrast,+Constancy

=Chapter Three -Lightness, Brightness, Contrast, Constancy=

- Receptive field of a cell = visual area over which a cell responds to light - Firing rate of cells is based on two gausians Simultaneous brightness and contrast (ie gray on a dark bg looks lighter than on a light bg)
 * On = response to light in the centre = white, bright
 * Off = response to light in the surround = black, decreasing light

Mach band = at the point where a uniform area meets a luminance ramp, a bright band is seen - Perception of illusory patterns (ie hexagon turns into a rounded shape) Chevreul Illusion = bands appear darker at one edge than at the other
 * smooth surfaces are often displayed using polygons (to speed up rendering)
 * enhances the boundaries at the edges of polygons

Contrast mechanisms - Uniform shading = surface color corresponds to light but exaggerated by chevreul (^^^ = ^V><) - Gouraud shading = edges between facets are reduced (X = <) - Phong shading = smooth lines (< = C)

Edge enhancement - Cornsweet effect = areas in centre of circle look lighter than the surrounding area

Color constancy = we experience colored surfaces not colored light Lightness constancy = overall reflection of a surface

Luminance = phys measurement of light coming from some region of space - Measured from the electromagnetic spectrum - Text and background contrast ratio = 3:1

Brightness = perceived amount of light coming from a source (vivid or saturated) - Perceived amount of light coming from self-luminous sources - Stevens power law = measures impact of simple sensations

Adaptation, contrast, lightness constancy - First age mechanism of light constancy = adaptation
 * Change sensitivity factor to overall level of illumination (ie blind from dark to light room)

- Second stage of level invariance = lateral inhibition

Perception of surface lightness - Reference white = lightest obj in the scene is used to determine gray values of other obj around - Ratio of non/specular reflection

Contrast crispening - Distorts gray values by enhancing it when close to the light part of the background