01_visual_perception_OM1.pdf

Visual Perception Original Notes by Dr. Minglun Gong Edition by Dr. Oscar Meruvia-Pastor ©Copyright Outline Why learn about visual perception? Light Visible spectrum Human eyes Rods & cones Intensity perception Color perception...

Visual Perception Original Notes by Dr. Minglun Gong Edition by Dr. Oscar Meruvia-Pastor ©Copyright Outline Why learn about visual perception? Light Visible spectrum Human eyes Rods & cones Intensity perception Color perception Gong & Meruvia © 2 Why Learn About Visual Perception? If we: Understand light Which light properties are important to our eyes? Understand how human eyes perceive light How intensity, color, contrast, and tone are perceived Understand the limitations of our visual system We can design software & systems that work properly and produce good results Gong & Meruvia © 3 What is Light? Light: Electromagnetic radiation that can, within a certain range, produce a visual sensation Electromagnetic energy: Can be described as a wave propagation of electric & magnetic fields Low frequency waves → high frequency waves Radio waves, Microwaves Infrared, Visible light, Ultraviolet X-Rays, Gamma rays Gong & Meruvia © 4 Electromagnetic (EM) Spectrum Gong & Meruvia © 5 Visible Spectrum Visible light is electromagnetic energy within the 400~700nm wavelength Violet: 380 nm Blue: 450 - 490 Green: 490 - 560 Yellow: 560 – 590 Orange: 590 - 640 Red: 640 – 730 nm Gong & Meruvia © 6 Representing Light Key parameters of light are intensity & color: Intensity depends on the light’s amount of photons or wave amplitude (describes brightness). Color depends on the light source’s spectral distribution The visual effects of light can be described concisely by a triple-value vector (or triplet): Hue, Saturation and Lightness Mapping from spectral distributions to colors is many-to-one. Thus, different spectral energy distributions may look similar to humans Gong & Meruvia © 7 Hue, Saturation, & Lightness Representation Assume light spectra can be modeled by a normal distribution Mean → hue Variance → inverse of saturation Area → lightness/brightness Gong & Meruvia © 8 Spectrum of Light Sources Different types of light sources can be precisely described by their spectrum Superheated atoms show a distinctive atomic spectrum Light spectrum specifies the energy distribution at different wavelengths Total energy can be calculated by the integral of the function. Gong & Meruvia © 9 Human Eyes and the Human Visual System (HVS) Ciliary body controls the focus of the lens Iris controls the opening of the pupil 2mm (f/8.3) in bright 8mm (f/2.1) in dark Retina senses light 200° viewing angle 1/60° resolution Resolve 286dpi at 1ft viewing distance See 2.7M colors Gong & Meruvia © 10 Photoreceptors 2 kinds of photoreceptors are found in the human retina: Rod-shaped cells (90M): Responsible for vision at low light levels Cone-shaped cells (4.5M): Active at higher light levels than rods Responsible for color perception Rods Cones Gong & Meruvia © 11  Distribution of Photoreceptors The center of retina is called the fovea Has no rods but a very high density of cones Most stars would be seen off from the center at night Optical nerves (1M) exit the eye from optic disk No rods or cones Also called blind spot Gong & Meruvia © 12 Distribution of Cones 3 types of cones: Short-, middle-, & long- wavelength sensitive cones Referred as S, M, L cones Cones distribution: L (red): 64% M (green) 32% S (blue) 2% No blue cones in fovea Gong & Meruvia © 13 Color Perception Only the cones are responsible for color perception Peak sensitivities of S, M, & L cones: Located at 430nm, 560nm, & 610nm wavelengths Roughly equivalent to blue (436nm), green (546nm), & red (700nm) Gong & Meruvia © 14 Intensity Perception Static contrast ratio: Around 100:1 About 6.5 stops (6-7 bits of gray) Dynamic contrast ratio: Up to 1,000,000:1 About 20 stops From 𝟏𝟎−𝟔 to 𝟏𝟎𝟓 Lux (lumens/m2) Need up to 30 mins to adapt from sunlight to full darkness Gong & Meruvia © 15 Afterimage Cone cells can adapt to over-stimulation and lose sensitivity Over-stimulation causes afterimage to appear after the exposure to the original color image has ceased Strong colors create afterimage Red → cyan Blue → yellow Green → magenta Gong & Meruvia © 16 Mach Bands Eyes are sensitive to intensity differences, not absolute values When center rod receives light, it inhibits the response of nearby rods Enhances the contrast between light & dark Intensity objects Also creates the Mach band visual effect Gong & Meruvia © 17 Chromostereopsis Eye lens diffract lights of different colors differently Causes depth effect due exclusively to color Can be used to create 3D effect ____ appears closer ____ appears further