Global Illumination. Frank Dellaert Some slides by Jim Rehg, Philip Dutre

Transcription

1 Global Illumination Frank Dellaert Some slides by Jim Rehg, Philip Dutre

2 Color and Radiometry What is color?

3 What is Color? A perceptual attribute of objects and scenes constructed by the visual system A quantity related to the wavelength of light in the visible spectrum A box of Crayola crayons A significant industry with conferences, standards bodies, etc. A challenge There are no second-rate brains in color vision Edwin Land

4 Why is Color Important? In animal vision food vs nonfood identify predators and prey Check health, fitness, etc. of other individuals. In computer vision Skin finder Segment an image

5 Reflectance Model c (!) = s(!) e(!)

6 Illumination Spectra Blue skylight Tungsten bulb

7 Reflectance Spectra

8 Macbeth ColorChecker

9 Hue-Saturation

10 Human Color Perception What is the retinal basis for color perception in humans?

11 Human Photoreceptors Fovea Periphery

12 Human Cone Sensitivities Spectral sensitivity of L, M, S cones in human eye

13 Color Names for Cartoon Spectra

14 Additive Color Mixing

15 Subtractive Color Mixing

16 Light Sources

17 Lambertian Surface The intensity of a pixel I(u,v) is: I(u,v) ^s ^n a I(u,v) = [a(u,v) ˆ n (u,v)] [s 0 ˆ s ] = b(u,v) s a(u,v) is the albedo of the surface projecting to (u,v). n(u,v) is the direction of the surface normal. s 0 is the light source intensity. s is the direction to the light source. Slide courtesy of David Kriegman

18 Orthographic Projection Simplification for light sources that are sufficiently far away from an object. All incoming light rays are parallel. Thus, while b vectors vary over the surface, s vector is constant. s b s s b b Pixels : b 1 T s, b 2 T s, b 3 T s,... " Bs

19 Area sources Examples: diffuser boxes, white walls. Radiosity: integrate over hemisphere section change variables and add up over the source Slide courtesy of David Forsyth

20 Ambient Illumination Add a constant to the radiosity at every point in the scene to account for brighter shadows than predicted by point source model Advantages: simple, easily managed Disadvantages: poor approximation

21 Local Shading Models

22 What about Shadows? Two kinds: occluder Attached Cast (We won t deal with these)

23 Area Source Shadows Penumbra Umbra Why don t we see more shadows indoors?

24 Shading models Local shading model Surface has radiosity due only to sources visible at each point Advantages: often easy to manipulate, expressions easy supports quite simple theories of how shape information can be extracted from shading Global shading model surface radiosity is due to radiance reflected from other surfaces as well as from surfaces Advantages: usually very accurate Disadvantage: extremely difficult to infer anything from shading values

25 Curious Experimental Fact Prepare two rooms, one with white walls and white objects, one with black walls and black objects Illuminate the black room with bright light, the white room with dim light People can tell which is which (due to Gilchrist) Why?

26 White Room A view of a white room, under dim light. Below, we see a cross-section of the image intensity corresponding to the line drawn on the image. Figure from Mutual Illumination, by D.A. Forsyth and A.P. Zisserman, Proc. CVPR, 1989, copyright 1989 IEEE

27 Black Room A view of a black room, under bright light. Below, we see a cross-section of the image intensity corresponding to the line drawn on the image. Figure from Mutual Illumination, by D.A. Forsyth and A.P. Zisserman, Proc. CVPR, 1989, copyright 1989 IEEE

28 What s going on here? local shading model is a poor description of physical processes that give rise to images This is a major nuisance

29 Global Shading Models Image by Henrik jensen

30 Illumination-Equation

31 Discretize: Linear System

32 Solving the Linear System B i = E i + " i # j F ij B j As a matrix equation: B = E + "FB, where " = diag(# i ) Solve following system: (I " #F)B = E Slight problem: F can have a trillion entries

33 Artifacts