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Real-Time Relighting

This presentation discusses real-time relighting techniques for achieving interactive rendering with reasonable quality. It covers relighting algorithms for distant environment lights, natural illumination, function approximation, spherical harmonics, wavelet transforms, and their applications in animation production.

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Real-Time Relighting

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  1. Real-Time Relighting Digital Image Synthesis Yung-Yu Chuang 1/10/2008 with slides by Ravi Ramamoorthi, Robin Green and Milos Hasan

  2. Realistic rendering • We have talked about photorealistic rendering for complex materials, complex geometry and complex lighting. They are realistic but slow.

  3. Real-time rendering • Its goal is to achieve interactive rendering with reasonable quality. It’s important in many applications such as games, visualization, computer-aided design, …

  4. Real-Time relighting • Lighting is the process of adjusting lights. It is an important but time-consuming step in animation production pipeline. • Relighting algorithms for two kinds of lights • Distant environment lights • Near-field lights for production

  5. Relighting algorithms for distant environment lights

  6. Natural illumination People perceive materials more easily under natural illumination than simplified illumination. Images courtesy Ron Dror and Ted Adelson

  7. Natural illumination Rendering with natural illumination is more expensive compared to using simplified illumination directional source natural illumination

  8. Reflection maps Blinn and Newell, 1976

  9. Environment maps Miller and Hoffman, 1984

  10. HDR lighting

  11. Examples of complex environment light

  12. Examples of complex environment light

  13. Direct lighting with complex illumination q p

  14. Function approximation • G(x): the function to approximate • B1(x), B2(x), … Bn(x): basis functions • We want • Storing a finite number of coefficients cigives an approximation of G(x)

  15. Function approximation • How to find coefficients ci? • Minimize an error measure • What error measure? • L2 error • Coefficients

  16. Function approximation • Basis Functions are pieces of signal that can be used to produce approximations to a function

  17. Function approximation • We can then use these coefficients to reconstruct an approximation to the original signal

  18. Function approximation • We can then use these coefficients to reconstruct an approximation to the original signal

  19. Orthogonal basis functions • Orthogonal Basis Functions • These are families of functions with special properties • Intuitively, it’s like functions don’t overlap each other’s footprint • A bit like the way a Fourier transform breaks a functions into component sine waves

  20. Integral of product

  21. Basis functions • Transform data to a space in which we can capture the essence of the data better • Spherical harmonics, similar to Fourier transform in spherical domain, is used in PRT.

  22. Real spherical harmonics • A system of signed, orthogonal functions over the sphere • Represented in spherical coordinates by the function where l is the band and m is the index within the band

  23. Real spherical harmonics

  24. Reading SH diagrams Thisdirection + – Not thisdirection

  25. Reading SH diagrams Thisdirection + – Not thisdirection

  26. The SH functions

  27. The SH functions

  28. Spherical harmonics

  29. Spherical harmonics m 0 l 1 2 -2 -1 0 1 2

  30. SH projection • First we define a strict order for SH functions • Project a spherical function into a vector ofSH coefficients

  31. SH reconstruction • To reconstruct the approximation to a function • We truncate the infinite series of SH functions to give a low frequency approximation

  32. Examples of reconstruction

  33. An example • Take a function comprised of two area light sources • SH project them into 4 bands = 16 coefficients

  34. Low frequency light source • We reconstruct the signal • Using only these coefficients to find a low frequency approximation to the original light source

  35. Harr wavelets • Scaling functions (Vj) • Wavelet functions (Wj) • The set of scaling functions and wavelet functions forms an orthogonal basis

  36. Harr wavelets

  37. Example for wavelet transform • Delta functions, f=(9,7,3,5) in V2

  38. Wavelet transform • V1,W1

  39. Example for wavelet transform • V0,W0 ,W1

  40. Example for wavelet transform

  41. Quadratic B–spline scaling and wavelets

  42. 2D Harr wavelets

  43. Example for 2D Harr wavelets

  44. Applications 19% 5% L2 1% 15% L2 3% 10% L2

  45. Relighting algorithms for animation production

  46. Relighting for production • Lighting is a time-consuming process. • Artists adjust lighting parameters and wait for a couple of hours or days to get feedback. • Local shading with complex scene and many lights • Interactive relighting • Interative visual eedback • Fixed scene and camera • Lower quality • Scalable with sene complexity and number of lights

  47. Deep framebuffer • Gershbein and Hanrahan, SIGGRAPH 2000

  48. Deep framebuffer

  49. Deep framebuffer

  50. LPICS • Pixar, SIGGRPH 2005. A practical realization for the deep framebuffer approach on GPUs LPICS 0.1s Final renderer 2,000s video

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