1. Illumination and Direct Reflection Kurt Akeley CS248 Lecture 12 1 November 2007 http://graphics.stanford.edu/courses/cs248-07/

2. CS248 Lecture 12Kurt Akeley, Fall 2007 Quantum electrodynamics We’re not going to talk about this

3. CS248 Lecture 12Kurt Akeley, Fall 2007 Our premise Goals: n Communicate, take advantage of human perception, and/or n Model reality Radiative heat-transfer approximation n Treat light as packets of energy (photons) n Model their transport as a flow Simplifications: n Ignore QED effects n Diffraction, interference, polarization, … n Assume geometric optics n Photons travel in straight lines n Intensities can be added

4. CS248 Lecture 12Kurt Akeley, Fall 2007 Point Light Source

5. CS248 Lecture 12Kurt Akeley, Fall 2007 Irradiance

6. CS248 Lecture 12Kurt Akeley, Fall 2007 Solid angles Area=1

7. CS248 Lecture 12Kurt Akeley, Fall 2007 Radiant Intensity (point source, uniform)

8. CS248 Lecture 12Kurt Akeley, Fall 2007 Radiant Intensity (point source, nonuniform)

9. CS248 Lecture 12Kurt Akeley, Fall 2007 Radiant Intensity (point source, nonuniform) r dA

10. CS248 Lecture 12Kurt Akeley, Fall 2007 Illumination (point source) Of an oriented unit area by a point light source n l r definition cancellation dot product def. Projected area factor

11. CS248 Lecture 12Kurt Akeley, Fall 2007 Reflected Light

12. CS248 Lecture 12Kurt Akeley, Fall 2007 Radiance (from oriented differential area)

13. CS248 Lecture 12Kurt Akeley, Fall 2007 Radiance is distance invariant Sample color value is determined by radiance n Distance doesn’t matter n Intuitively doubling the distance n Reduces the energy from a unit area by factor of 4 n Increases the area “covered” by the sample by a factor of 4 Multi-sample antialiasing filters radiance values Why does a fire feel warmer, but have the same radiance (apparent brightness), when you are closer to it?

14. CS248 Lecture 12Kurt Akeley, Fall 2007 Diffuse reflection Scatter proportion Function of θ r n Invariant to θ i n Goniometric diagram (Lambertian scatter)

15. CS248 Lecture 12Kurt Akeley, Fall 2007 Diffuse reflection cancellation prev. slide

16. CS248 Lecture 12Kurt Akeley, Fall 2007 Lambertian radiance n Lambertian scatter n Goniometric diagram (Lambertian scatter) Lambertian Radiance

17. CS248 Lecture 12Kurt Akeley, Fall 2007 The moon

18. CS248 Lecture 12Kurt Akeley, Fall 2007 Isotropic scatter (dusty surface) n Goniometric diagram (Lambertian scatter) Isotropic scatter Radiance n

19. CS248 Lecture 12Kurt Akeley, Fall 2007 Retroreflection (2-D) The moon is actually somewhat retroreflective

20. CS248 Lecture 12Kurt Akeley, Fall 2007 BRDF Relates n Incoming irradiance to n Outgoing radiance Degrees of freedom n 4 in general (anisotropic) n 3 in isotropic case n Add one for spectral Isotropic:

21. CS248 Lecture 12Kurt Akeley, Fall 2007 Anisotropic Texture filtering (2 lectures ago) Surface characteristics

22. CS248 Lecture 12Kurt Akeley, Fall 2007 BSSRDF

23. CS248 Lecture 12Kurt Akeley, Fall 2007 How can you implement BRDFs?

24. CS248 Lecture 12Kurt Akeley, Fall 2007 Texture mapping Paints images onto triangles Paints images onto points, lines, and other images Ties the vertex and pixel pipelines together n Rendered images can be used as textures n To modify the rendering of new images –That can be used as textures … Implements general functions of one, two, or three parameters n Specified as 1-D, 2-D, or 3-D tables (aka texture images) n With interpolated (aka filtered) lookup Drives the hardware architecture of GPUs n Multi-thread latency hiding n “shader” programmability Adds many capabilities to OpenGL n Volume rendering n Alternate color spaces n Shadows …

25. CS248 Lecture 12Kurt Akeley, Fall 2007 Shading vs. lighting Lighting n Light transport n Interaction of light with surfaces Shading n Interpolation of radiance values n Examples: n Smooth shading (aka Gouraud Shading) n Flat shading (aka constant shading) Shader n Program run per vertex/primitive/fragment n Really more of a “lighter” than a “shader”

26. CS248 Lecture 12Kurt Akeley, Fall 2007 Summary Diffuse lighting Radiance specified by n l n Cosine fall-off is due to irradiance, not scattering Many factors are ignored (often even the r 2 fall-off) Bidirectional reflectance distribution function (BRDF) n Ratio of reflected radiance to incident irradiance n Integrate over all incident light to get reflected radiance n 5 DOF including spectral information n 3 DOF for isotropic, non-spectral Texture mapping is a powerful, general-purpose mechanism n It’s not just painting pictures onto triangles!

27. CS248 Lecture 12Kurt Akeley, Fall 2007 Assignments Next lecture: Z-buffer Reading assignment for Tuesday’s class n FvD 15.1 through 15.5

28. CS248 Lecture 12Kurt Akeley, Fall 2007 End