1	Graphics File Format and Data Compression Techniques (take 2) Week 6
2	Business Attendance (please sign in) Lecture notes now online
3	Homework 5 What did you think of the HW?
4	HW5 Questions a2b & b2a encode/decode Bitstream hamm/unhamm Noise
5	HW5 Hamming (7,4) Venn Diagram: Data: 1,2,3,4 Parity 5,6,7 Hamming (8,4) Extra overall parity bit Provides 2 bit err detection
6	HW5 Interoperability? Selected at random Any student’s a2b, hamm, noise, unhamm, b2a work together
7	HW5 Noise Fn. Anybody plot a histogram of the noise fn? Anybody experiment with various noise values? At what value did: Corruption begin? Signal completely lost?
8	Sensor distribution in Retina
9	Completion Phenomena Effect that ‘fills in’ missing visual data Makes our visual field appear Complete (no noticeable blind spot) Consistently High resolution Color
10	Optics Cornea and lens provide +60 to +80Dopters of optical power Focal length: 16 – 12.5mm Typical eye 24mm from cornea to retina Requiring 42D power
11	‘Extra’ focusing power Allows Focus from infinity to near Provides compensation for imperfect shape Eye too long (lens normal) Myopic (nearsighted) Eye too short (lens normal) Hyperopic (farsighted)
12	Chromatic Aberration Prisms break light into constituent colors due to refraction Lenses work by refraction and introduce chromatic aberrations ie a white dot focuses as a circular rainbow
13	Art parallels physics Artists consider red an ‘advancing’ color Correspondingly the eye needs to focus a little closer for red
14	Radiation Light is: Electromagnetic radiation Of frequencies our eyes are sensitive
15	EM Radiation ~fortin@harvard.edu
16	EM wavelengths ~fortin@harvard.edu
17	Blackbody Radiation Blackbody Perfect absorber and emitter of radiation Objects radiate EM based on their temperature Humans are ≈ 98.6F or 37C or 300K Humans radiate strongly at 10micron
18	Kelvin Temperature scale °K = °C + 273 0° K = absolute zero No motion No emission
19	Plank’s Curve Black Body Radiation By temperature Consider electric range…
20	Plank’s curve Heat an electric range: Warm: IR radiation (300K) Hot: Glows Dull Red (1000K) 1000K Plank’s curve barely intersects visible sptrm Very Hot: Glows Orange Even Hotter: Glows Yellow Hottest of all: Glows White (7000K) Every burn Magnesium? Peak at Violet, tail radiates all visible colors
21	Wein’s Law of Blackbody Radiation λ_max ≈ 1/t λ_max Peak emission wavelength in μm Temperature in Kelvin
22	Stefan Boltzman Law Total Energy Radiated = σ t⁴ σ Stefan Bolzman constant 5.67 x 10^-8 Watts m^-2 K^-4 Energy is proportional to 4^th power temp Double temperature 16x energy
23	Astronomical Applications Determine temperature of star By analyzing its radiation Our Sun Emissions peak in visible spectrum Much IR, little UV 5800K Surface temperature Bright Star Up to 60,000K Interstellar Gas 60K Mainly emits radio waves
24	Photographers? What color do tungsten bulbs glow? Why does halogen appear white?
25	(Visible) Light Usually defined range 380 to 780 nanometers 1nm=10E-9m
26	Other EM Radiation Radio > 1mm waves FM, TV: few meters or 100MHz AM: few tenths of kilometers or 1000KHz Infra red: wavelength longer than red 1000nm (1 micron) to 1mm Absorbed by most materials causing heat NOT HEAT BY ITSELF
27	UV EM between light and X-rays Near UV 200-300nm Far UV 91-200nm Extreme UV 10-91nm Radiated by objects (gases) > 1000K
28	XRays EM between 0.01 and 10nm Dental X-Ray 0.012nm short X-Ray Penetrate soft tissue Absorbed by minerals in bones Generated by gases at 1-100 million degrees Think stellar corona
29	Gamma Rays EM less than 0.01nm Gas near 1billion degrees
30	Color Perception Rhodopsin Photopigment found in rods Sensitive to light in bell shaped curve Most sensitive to 500nm light Three types of photopigments in cones S (420nm short wavelength) M (530nm medium wavelength) L (560nm long wavelength)
31	Photo-pigment Sensitivity
32	Photo-pigment Sensitivity Asymmetric More sudden high frequency cutoff Extremely sensitive (the ultimate) Chemical reaction triggered by a single photon!! Changes cell membrane’s electrical potential (charge)
33	Principle of Univariance Rod or Cone only signals arrival of light NOT the frequency of the light Likelyhood of photon being triggered based on: Spectral sensitivity of receptor Frequency of light Intensity of light
34	Univariance Example If a receptor is 33% sensitive to a given freq 1 in 3 photons (on average) will be absorbed/triggered
35	Color, intensity detection Single receptors can not determine color or intensity Color, intensity only determined over time and spatially related receptors Similar to use of dither in output Spatial resolution traded off for color/intensity
36	Retina Retina is a compromise between Dense packaging of sensors No S cones at all at center of Fovea Allows more dense packing of M,L cones Color perception
37	Temporal Smoothing Properties: Sensitive even single photons Chemical reaction lasts milliseconds Subsequent stimulation increases reaction Implications: Critical Flicker Frequency Below CFF we percieve flashing light Above CFF flashes fuse into steady light
38	Critical Flicker Frequency As high as 60Hz in Humans As high as 300Hz in Bees Influenced by Ambient light Size of image Duty cycle
39	Measuring Intensity Candelas per square meter Light generated by a hypothetical candle 0.00003 Moonless overcast night 0.003 Moonless clear night 0.03 Moonlit clear night 3 Twilight 30 Very Dark Day 300 Overcast Day 3000 Clear Day 30,000 Day with sunlit clouds From: Lighting Handbook
40	Photopic vs Scotopic vision At Photopic levels (daylight) Rods hyperpolarized Cones primary sensor At Scotopic levels (low light) Rods primary sensor Rods 10x as sensitive as cones Our color perception shifts with light level Called the Purkinje shift
41	Physical filter Retina is ‘inside out’ Light has to pass thru membrane and sensors themselves to be detected Lens yellows with age
42	Contrast Sensitivity: Weber function Smallest intensity difference noticeable (JND) Between contiguous regions ΔI/I < 0.02 are not noticeable
43	Contrast ISO contrast chart:
44	Sensitivity to sine waves From Glassner 1995
45	Sine wave sensitivity tests
46	(spatial) Frequency Perception Grows until age 20 Brain develops to handle hi-frequencies? Drops after 20 Due to decreased pupil size?
47	Color Perception Objects don’t possess color They merely absorb, reflect and occasionally re-radiate light Objects also emit EM radiation But as we have discussed, mostly invisible unless very hot
48	Colors Black objects Absorb perceptual light (and get hot as a result) Grey objects Absorb light evenly across all freqs White objects Reflect all freqs. of light Good roof color! Fluorescent objects Absorb ultraviolet light Re-radiate that energy as perceptual light
49	Perceptual oddities Many! Mach banding Contrast constancy Seem to arise from vision ‘assuming’ natural world Metamerism (2 colors only differ in some circumstances)
50	Mach Banding
51	Contrast constancy
52	Depth Perception Not going to discuss: Binocular vision Parallax Oculomotor depth Monocular depth cues
53	How to represent color? Scientifically Measure spectral energy of light at every position Perceptually inspired Use a color model based on human perception
54	Color Models RGB (Red, Green, Blue) Additive model Devices that emit color (ie computer monitor) Used extensively in the computer industry CMY(K) (Cyan, Magenta, Yellow, (Black)) Subtractive model Printed material (inks absorb light) HSB (Hue, Saturation, Brightness) Easiest for humans to navigate YIQ, YC_RC_B (luminance, 2 chroma) Used in NTSC video
55	Additive Color Ambient light is black Sum of colors is white
56	Additive Color Devices Consider Projection TV Ambient light Temporal Color Fusion DLP projection, sequential color CRT monitor Phosphor selection Shadow Mask LCD Spatial Resolution/MSFT’s ClearType
57	Subtractive Color Reflective page is white Sum of colors is black
58	CMYK Example
59	Visualizing Color Spaces
60	Color Gamut Relative coverage CMYK has the smallest coverage