WEBVTT 00:00:00.130 --> 00:00:03.550 I simply made up the anisotropic function out of my head. However, there's 00:00:03.550 --> 00:00:06.494 considerable research about how to capture BRDFs from materials, and how to 00:00:06.494 --> 00:00:11.647 make functions to compactly represent them. BRDFs are just the start. There's 00:00:11.647 --> 00:00:15.872 also the BSDF, the Beet Sugar Development Foundation. We're more interested in 00:00:15.872 --> 00:00:19.408 the Bi-Directional Scattering Distribution Function. This type of function 00:00:19.408 --> 00:00:23.227 captures both how light reflects from and transmits through material. There's 00:00:23.227 --> 00:00:27.436 also the BSSRDFs which stands for Bidirectional Surface Scattering Reflectance 00:00:27.436 --> 00:00:32.188 Distribution Function. Say that one three times fast. This function is 00:00:32.188 --> 00:00:35.364 important for materials like marble and milk. For these materials in 00:00:35.364 --> 00:00:38.492 particular, the light enters one location on the surface, bounces around inside 00:00:38.492 --> 00:00:42.808 the material, and comes out somewhere nearby. One other extremely important 00:00:42.808 --> 00:00:46.633 material that has this sort of scattering is skin. Getting skin to look good 00:00:46.633 --> 00:00:50.130 for interactive rendering can be quite involved. But the results are more 00:00:50.130 --> 00:00:53.548 convincing than using some simple reflection model. See the additional course 00:00:53.548 --> 00:00:57.764 materials for more information. That said, the key factor here is scale. The 00:00:57.764 --> 00:01:00.788 effect of subsurface scattering lessens as the viewer's distance from the 00:01:00.788 --> 00:01:04.748 object increases. Close up, a photon might exit at a location that's a fair 00:01:04.748 --> 00:01:08.788 number of pixels away from where it entered the surface. From farther away, 00:01:08.788 --> 00:01:12.877 they may be no change in pixel location. In fact the diffuse component for all 00:01:12.877 --> 00:01:16.786 non-metallic materials comes from subsurface scattering. It's just that in many 00:01:16.786 --> 00:01:21.224 cases this scattering is over an imperceptably small distance. Metals 00:01:21.224 --> 00:01:24.842 themselves are essentially all specular. Let me say that again, because all 00:01:24.842 --> 00:01:29.420 this time we've been living a lie. Metallic objects have no lambertian diffuse 00:01:29.420 --> 00:01:33.898 term. Well, not a lie, I just like being dramatic. Really, diffuse is simply an 00:01:33.898 --> 00:01:37.670 approximation of which we should be aware. Using it's fine, even high-quality 00:01:37.670 --> 00:01:42.190 applications do so. It's quick to compute and looks plausible. In reality, 00:01:42.190 --> 00:01:45.094 metals can indeed be given a roughened surface to give them a glossier, diffuse 00:01:45.094 --> 00:01:49.984 look. So, a diffuse term is fine. However, on a an atomic level, metallic 00:01:49.984 --> 00:01:53.746 objects have a free floating soup of electrons on the surface which absorbs and 00:01:53.746 --> 00:01:57.998 reemits incoming photons. If your surface represents a shiny metal, you 00:01:57.998 --> 00:02:01.588 probably don't want a diffuse term. Insulators have a diffuse term because the 00:02:01.588 --> 00:02:05.060 photons undergo subsurface scattering. Most of the time the entry and exit 00:02:05.060 --> 00:02:08.973 points are so close together it doesn't matter. But the direction of exit 00:02:08.973 --> 00:02:12.435 certainly does. Materials such as that in an unglazed clay pot, concrete, and 00:02:12.435 --> 00:02:15.635 even the moon itself, are rough enough that the lambertian reflection model 00:02:15.635 --> 00:02:19.968 doesn't capture them fully. This again turns out to be a matter of scale, 00:02:19.968 --> 00:02:22.896 having to do with the relationship of surface roughness with subsurface 00:02:22.896 --> 00:02:27.646 scattering. Admittedly, trying to capture all of these effects leads to a lot 00:02:27.646 --> 00:02:31.625 of work and possibly inefficient shaders. These subsurface scattering 00:02:31.625 --> 00:02:34.838 renderings are from 3D Studio Max and rendered offline, not at interactive 00:02:34.838 --> 00:02:39.039 rates. The main thing is to realize we don't have to stick with illumination 00:02:39.039 --> 00:02:43.601 models from the 1970's because of inertia or ignorance. Using reflection models 00:02:43.601 --> 00:02:47.060 based on how the real world works has a number of advantages. First and 00:02:47.060 --> 00:02:50.030 foremost, if everything is properly modeled, your virtual world acts like the 00:02:50.030 --> 00:02:53.855 real world. Change lighting conditions, and you don't have to tweak material 00:02:53.855 --> 00:02:57.505 settings to look good. For design software, this assurance can mean that you 00:02:57.505 --> 00:03:00.085 can trust what you see on the screen to have some relationship to what you 00:03:00.085 --> 00:03:04.770 manufacture. Physically based rendering is also a great help to virtual world 00:03:04.770 --> 00:03:08.370 content creators, such as game and film makers. It's a time saver to have 00:03:08.370 --> 00:03:11.604 predictable illumination models, as the artist does not have to learn obscure 00:03:11.604 --> 00:03:16.118 sliders that have no real world counterparts. It's vastly reassuring, knowing 00:03:16.118 --> 00:03:19.062 that materials won't show some glitch from a certain angle, and knowing that 00:03:19.062 --> 00:03:23.410 lighting can be changed without destroying the sense of realism. Rather than 00:03:23.410 --> 00:03:26.324 limit creativity, a well-designed system makes for a more productive and 00:03:26.324 --> 00:03:28.190 unrestrictive environment.