1	Announcements Project status reports on Wednesday prepare 5 minute ppt presentation should contain: problem statement (1 slide) description of approach (1 slide) some images (1 slide) current status + plans (1 slide)
2	Light Readings Szeliski, 2.2, 2.3.2
3	Light Readings Szeliski, 2.2, 2.3.2
4	Properties of light Today What is light? How do we measure it? How does light propagate? How does light interact with matter?
5	What is light?
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7	The light field Known as the plenoptic function If you know R, you can predict how the scene would appear from any viewpoint. How?
8	Stanford light field gantry
9	More info on light fields If you’re interested to read more: The plenoptic function Original reference: E. Adelson and J. Bergen, "The Plenoptic Function and the Elements of Early Vision," in M. Landy and J. A. Movshon, (eds) Computational Models of Visual Processing, MIT Press 1991. L. McMillan and G. Bishop, “Plenoptic Modeling: An Image-Based Rendering System”, Proc. SIGGRAPH, 1995, pp. 39-46. The light field M. Levoy and P. Hanrahan, “Light Field Rendering”, Proc SIGGRAPH 96, pp. 31-42. S. J. Gortler, R. Grzeszczuk, R. Szeliski, and M. F. Cohen, "The lumigraph," in Proc. SIGGRAPH, 1996, pp. 43-54.
10	What is light?
11	The visible light spectrum We “see” electromagnetic radiation in a range of wavelengths
12	Light spectrum The appearance of light depends on its power spectrum How much power (or energy) at each wavelength
13	The human visual system Color perception Light hits the retina, which contains photosensitive cells
14	Density of rods and cones Rods and cones are non-uniformly distributed on the retina Rods responsible for intensity, cones responsible for color Fovea - Small region (1 or 2°) at the center of the visual field containing the highest density of cones (and no rods). Less visual acuity in the periphery—many rods wired to the same neuron
15	Demonstrations of visual acuity
16	Demonstrations of visual acuity
17	Brightness contrast and constancy The apparent brightness depends on the surrounding region brightness contrast: a constant colored region seem lighter or darker depending on the surround: http://www.sandlotscience.com/Contrast/Checker_Board_2.htm brightness constancy: a surface looks the same under widely varying lighting conditions.
18	Light response is nonlinear Our visual system has a large dynamic range We can resolve both light and dark things at the same time One mechanism for achieving this is that we sense light intensity on a logarithmic scale an exponential intensity ramp will be seen as a linear ramp Another mechanism is adaptation rods and cones adapt to be more sensitive in low light, less sensitive in bright light.
19	Visual dynamic range
20	After images Tired photoreceptors Send out negative response after a strong stimulus
21	Color perception Three types of cones Each is sensitive in a different region of the spectrum but regions overlap Short (S) corresponds to blue Medium (M) corresponds to green Long (L) corresponds to red Different sensitivities: we are more sensitive to green than red varies from person to person (and with age) Colorblindness—deficiency in at least one type of cone
22	Color perception Rods and cones act as filters on the spectrum To get the output of a filter, multiply its response curve by the spectrum, integrate over all wavelengths Each cone yields one number Q: How can we represent an entire spectrum with 3 numbers?
23	Perception summary The mapping from radiance to perceived color is quite complex! We throw away most of the data We apply a logarithm Brightness affected by pupil size Brightness contrast and constancy effects Afterimages The same is true for cameras But we have tools to correct for these effects See Rick’s lecture notes on Computational Photography and HDR
24	Light transport
25	Light sources Basic types point source directional source a point source that is infinitely far away area source a union of point sources More generally a light field can describe any distribution of light sources
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28	The interaction of light and matter What happens when a light ray hits a point on an object? Some of the light gets absorbed converted to other forms of energy (e.g., heat) Some gets transmitted through the object possibly bent, through “refraction” Some gets reflected as we saw before, it could be reflected in multiple directions at once Let’s consider the case of reflection in detail In the most general case, a single incoming ray could be reflected in all directions. How can we describe the amount of light reflected in each direction?
29	The BRDF The Bidirectional Reflection Distribution Function Given an incoming ray and outgoing ray what proportion of the incoming light is reflected along outgoing ray?
30	Constraints on the BRDF Energy conservation Quantity of outgoing light ≤ quantity of incident light integral of BRDF ≤ 1 Helmholtz reciprocity reversing the path of light produces the same reflectance
31	Diffuse reflection Diffuse reflection Dull, matte surfaces like chalk or latex paint Microfacets scatter incoming light randomly Effect is that light is reflected equally in all directions
32	Diffuse reflection
33	Specular reflection
34	Specular reflection
35	Phong illumination model Phong approximation of surface reflectance Assume reflectance is modeled by three components Diffuse term Specular term Ambient term (to compensate for inter-reflected light)
36	BRDF models Phenomenological Phong [75] Ward [92] Lafortune et al. [97] Ashikhmin et al. [00] Physical Cook-Torrance [81] Dichromatic [Shafer 85] He et al. [91] Here we’re listing only some well-known examples
37	Measuring the BRDF Gonioreflectometer Device for capturing the BRDF by moving a camera + light source Need careful control of illumination, environment
38	BRDF databases MERL (Matusik et al.): 100 isotropic, 4 nonisotropic, dense CURET (Columbia-Utrect): 60 samples, more sparsely sampled, but also bidirectional texure functions (BTF)