CGT 511 Perception. Facts. Facts. Facts. When perceiving visual information

Transcription

1 CGT 511 Perception Bedřich Beneš, Ph.D. Purdue University Department of Computer Graphics Facts When perceiving visual information light is the most important factor light is mostly reflected or scattered from objects light can be thought in two ways (light is dualistic) photons particles waves Facts Human beings perceive 80% of information with eyes Humans are sensitive to patterns, rather than the intensity Visual acuity is the precision of detection of patterns Facts Visual perception is sensitive to edges Slow gradient change is difficult to see Abrupt change is easily seen 1

2 Weber s Law It defines just noticeable difference. The ratio between the magnitude of the change and the magnitude of the stimulus is constant. The ratio of the increment threshold to the background intensity is a constant. Δ Weber s Law Example: Let s say you cannot distinguish between 100 g and 105 g. Let s say increase the magnitude to 200g. What will be the just noticeable difference? Weber s Law The will be: k= 0.05 Setting 200 Δ Δ You will not be able to distinguish between 200 and 210 g. Fechner s Law Generalizes Weber s law so it allows description of the strength of the sensory experience log Where: is the sensory experience is a scaling constant specific to stimulus, and is the intensity of the stimulus 2

3 Steven s Law Many researches have argued the logarithmic relationship is not always valid. Steven s Law defines strength of a stimulus Where is the sensory experience is a scaling constant specific to stimulus, and is the intensity of the stimulus, 1 is a stimulus specific exponent Light on a boundary boundary of two environments changes the light direction this is called refraction light is bend towards the normal vector to the surface Light on a boundary Light on a boundary Light on a boundary limit proces stacking prisms and going to the limit process gives lens 3

4 Camera Human Eye Wikipedia Human Eye cornea protects against damage iris colored, controlled by muscles retina layer of photo sensitive cells rods grayscale vision (very sensitive) cones color perception (0.1 sensitivity of rods) foeva approx. 1 O 5.2 O at the center of the visual axis the highest density of photosensitive cells blind spot Human Eye Rods and cones contain photopigments that react to light Rods are most sensitive at 500 nm (nanometers) are sensitive at low light levels scotopic vision (vision in the dark) Can respond to a single photon! 4

5 Human Eye Cones different cones have different sensitivity to light there are three types of cones: Long, Medium, and Short (sometimes called red, green, and blue) are sensitive at high light levels photopic vision Human Eye Color Problems Eye Color Sensitivity Animal sensitive to # of wavelengths dog 1 primates 2 3 Do dogs need humans 3 a color TV? pigeon 4 hummingbird 18 Color Color is subjective. Everybody sees something else. (Chine s paintings example) 5

6 Critical Flicker Frequency (CFF) response of the eye is not immediate critical flicker frequency CFF above this frequency flashes fuse into one image Humans CFF ~ 60Hz Bees CFF ~ 300 Hz Dynamic Range We can operate over intensities This is also called dynamic range Source Luminance (cd/m 3 ) Moonless overcast night Moonless clear night 0.03 Twilight 3 Overcast day 300 Day with sunlit clouds Adaptation Adaptation is the process of adjusting different levels of intensity of light Adaptation is not immediate Dark adaptation is the slowest 40 minutes are required to complete dark adaptation (!) (astronomers know it ) Light adaptation is much quicker Adaptation Auerbach & Wald, 1954, Gregory

7 Dynamic Range Dynamic Range Photopic vision (cones only) Scotopic vision (rods only) Mesopic vision (both) Luminance / ] White paper in: Starlight Moonlight Indoor Lighting Rod vision Cone vision Sunlight Scotopic vision Mesopic vision Photopic vision Dynamic Range Colors human eye can see 168 mil. Colors at one moment we can recognize colors Field of View Each eye can see approx 160 o horizontal and 135 o vertical There is a 120 o overlap so both eyes can see approx 200 o Eye must scan the area! 7

8 Visual Acuity visual acuity or spatial resolution is the ability to see fine scale patterns. High acuity area is around 1 o 5.2 o in the direction of the main axis Put the arm in front of you and it is approximately the size of your thumb Visual Acuity Lindsay and Norman Visual Perception is Inaccurate Visual Perception is Inaccurate 8

9 Visual Perception is Inaccurate Visual Perception is Inaccurate Summary Fundamental laws Refraction lens and camera human eye retina, iris, rods and cones, where and when? CFF, adaptation Field of View Readings Peter Shirley et al., Fundamentals of Computer Graphics, 2 nd edition, pages William Thompson et al. Visual Perception from a Computer Graphics Perspective 9