Week+Three


 * __LIGHTNESS, BRIGHTNESS, CONTRAST__ **

Ecological optics/perception - Perception is in service of action = for evolutionary advantage - Simultaneous brightness contrast = gray on dark bg looks lighter than gray on light bg - Mach banding = effect in matching colors/shades together (ie polygon vs flat rendering) o At pint where uniform area meets a luminance ramp, bright band is perceived o Simulated by Gaussian principles - Hermann grid illusion = black spots appear at intersection of bright lines o Black dots appear because of how ganglion cells in the back of the retina interpret light o There is more inhibition at points between 2 squares - Chevreul illusion (ie pixel arrays) o With a sequence of gray bands, bands appear darker at one edge than the other o Shaded edges to make each color stand out instead of flowing as a fading color

- Neurons detect differences o Receptors act as transducers o Receptors transmit signals about relative amount of energy (ie light)  Light differs from one receptor to another o Primary visual processing areas are in the visual cortex at the back of the brain - Receptive fields o Visual area over which cell responds to light (ie psp custom filter)

Contrast effects and artifacts in cg - Shading of facets o Uniform = 1 value for polygon – high peak o Gouraud = value for edges – some peak o Phong = surface normal interpolated between edges (no mach banding) – smooth

Edge enhancement - Lateral inhibition o Edge detection process = signals positions and contrasts of edges - Cornsweet effect o 2 same brightness can look diff by having an edge that shades off 2 sides gradually o Brain does perceptual interpolation = central region appear lighter than outer region

Luminance = amount of light (energy) coming from a region of space (measurement = energy/unit area) Brightness = perceived amount of light coming from a glowing source Lightness = perceived reflectance of a surface (ie paint shade)

Stevens power law = perceived sensation is proportional to stimulus intensity

Perception of surface lightness - Depends on adaptation and contrast - Direction of illumination and surface orientation - ‘reference white’ = lightest obj in scene to determine gray values of other obj - Specular ratio (ie black and white)