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Just a question about English, why is it "physically based", as in "physically based rendering", instead of "physics based"? To me it sounds like "physically" s

Just a question about English, why is it "physically based", as in "physically based rendering", instead of "physics based"? To me it sounds like "physically" should be followed by some other word than "based", like "physically correct". But yet it's used this way.

fsloth 6y ago
Well, the rendering is not really "physics based". This would imply some sort of first-principles formulation, and you see those only in some renderers. Rendering algorithms are more like an artistic bag of tricks. The "physically based" implies the tricks are approximating the visual acuity of realistic scenes with physics inspired approximations and the like.
- q3k 6y ago
  PBR algorithms tend to at least start from the rendering equation and simplify/approximate from that. They attempt to maintain some semblance of accuracy by eg. employing the conservation of energy of emitted vs. received light of every point of the scene; or expressing matte texture by the statistical behaviour microfacets of a rough surface; or using actual radiometry/photometry units to express lighting elements of the scene; making the scene also be expressed in concrete real-world units, ... There's still a lot of cheating and smoke and mirrors, but at least all these operate from the idea that the math should check out. This is in contrast to more oldschool approaches that were fully dependent on tweaking things until they looked right - ie., taking blinn-phong in a scene with some 'ambient lighting' and operating in fully imaginary units, modified by an artist on scene-by-scene basis.
regularfry 6y ago
Because while it's inspired by physics, it's usually a limited model that's not 100% physically accurate. "Physically based rendering" doesn't have to include photon mapping, for instance, which means caustics won't be visible (without some other way of cheating them into place, anyway).
jayd16 6y ago
You're right, you would say tomato based soup and not tomato-y based soup. ...but english is not that rigorous and names stick.

fsloth 6y ago

Well, the rendering is not really "physics based". This would imply some sort of first-principles formulation, and you see those only in some renderers. Rendering algorithms are more like an artistic bag of tricks. The "physically based" implies the tricks are approximating the visual acuity of realistic scenes with physics inspired approximations and the like.

q3k 6y ago
PBR algorithms tend to at least start from the rendering equation and simplify/approximate from that. They attempt to maintain some semblance of accuracy by eg. employing the conservation of energy of emitted vs. received light of every point of the scene; or expressing matte texture by the statistical behaviour microfacets of a rough surface; or using actual radiometry/photometry units to express lighting elements of the scene; making the scene also be expressed in concrete real-world units, ... There's still a lot of cheating and smoke and mirrors, but at least all these operate from the idea that the math should check out. This is in contrast to more oldschool approaches that were fully dependent on tweaking things until they looked right - ie., taking blinn-phong in a scene with some 'ambient lighting' and operating in fully imaginary units, modified by an artist on scene-by-scene basis.

regularfry 6y ago

Because while it's inspired by physics, it's usually a limited model that's not 100% physically accurate. "Physically based rendering" doesn't have to include photon mapping, for instance, which means caustics won't be visible (without some other way of cheating them into place, anyway).

jayd16 6y ago

You're right, you would say tomato based soup and not tomato-y based soup. ...but english is not that rigorous and names stick.