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← BSDF and BSSRDF - Interactive 3D Graphics

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Showing Revision 2 created 05/25/2016 by Udacity Robot.

  1. I simply made up the anisotropic function out of my head. However, there's
  2. considerable research about how to capture BRDFs from materials, and how to
  3. make functions to compactly represent them. BRDFs are just the start. There's
  4. also the BSDF, the Beet Sugar Development Foundation. We're more interested in
  5. the Bi-Directional Scattering Distribution Function. This type of function
  6. captures both how light reflects from and transmits through material. There's
  7. also the BSSRDFs which stands for Bidirectional Surface Scattering Reflectance
  8. Distribution Function. Say that one three times fast. This function is
  9. important for materials like marble and milk. For these materials in
  10. particular, the light enters one location on the surface, bounces around inside
  11. the material, and comes out somewhere nearby. One other extremely important
  12. material that has this sort of scattering is skin. Getting skin to look good
  13. for interactive rendering can be quite involved. But the results are more
  14. convincing than using some simple reflection model. See the additional course
  15. materials for more information. That said, the key factor here is scale. The
  16. effect of subsurface scattering lessens as the viewer's distance from the
  17. object increases. Close up, a photon might exit at a location that's a fair
  18. number of pixels away from where it entered the surface. From farther away,
  19. they may be no change in pixel location. In fact the diffuse component for all
  20. non-metallic materials comes from subsurface scattering. It's just that in many
  21. cases this scattering is over an imperceptably small distance. Metals
  22. themselves are essentially all specular. Let me say that again, because all
  23. this time we've been living a lie. Metallic objects have no lambertian diffuse
  24. term. Well, not a lie, I just like being dramatic. Really, diffuse is simply an
  25. approximation of which we should be aware. Using it's fine, even high-quality
  26. applications do so. It's quick to compute and looks plausible. In reality,
  27. metals can indeed be given a roughened surface to give them a glossier, diffuse
  28. look. So, a diffuse term is fine. However, on a an atomic level, metallic
  29. objects have a free floating soup of electrons on the surface which absorbs and
  30. reemits incoming photons. If your surface represents a shiny metal, you
  31. probably don't want a diffuse term. Insulators have a diffuse term because the
  32. photons undergo subsurface scattering. Most of the time the entry and exit
  33. points are so close together it doesn't matter. But the direction of exit
  34. certainly does. Materials such as that in an unglazed clay pot, concrete, and
  35. even the moon itself, are rough enough that the lambertian reflection model
  36. doesn't capture them fully. This again turns out to be a matter of scale,
  37. having to do with the relationship of surface roughness with subsurface
  38. scattering. Admittedly, trying to capture all of these effects leads to a lot
  39. of work and possibly inefficient shaders. These subsurface scattering
  40. renderings are from 3D Studio Max and rendered offline, not at interactive
  41. rates. The main thing is to realize we don't have to stick with illumination
  42. models from the 1970's because of inertia or ignorance. Using reflection models
  43. based on how the real world works has a number of advantages. First and
  44. foremost, if everything is properly modeled, your virtual world acts like the
  45. real world. Change lighting conditions, and you don't have to tweak material
  46. settings to look good. For design software, this assurance can mean that you
  47. can trust what you see on the screen to have some relationship to what you
  48. manufacture. Physically based rendering is also a great help to virtual world
  49. content creators, such as game and film makers. It's a time saver to have
  50. predictable illumination models, as the artist does not have to learn obscure
  51. sliders that have no real world counterparts. It's vastly reassuring, knowing
  52. that materials won't show some glitch from a certain angle, and knowing that
  53. lighting can be changed without destroying the sense of realism. Rather than
  54. limit creativity, a well-designed system makes for a more productive and
  55. unrestrictive environment.