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Indeed. The book[1] on the subject states that the point is that, unlike the rendering methods that came before, the math is based on physics and physical princ

Indeed. The book[1] on the subject states that the point is that, unlike the rendering methods that came before, the math is based on physics and physical principles. Of course one has to simplify and approximate, but the approximations should be grounded in physics and obey the underlying physical principles. For example, in the early days people created reflection models without considering energy reciprocity for example. Well that's a physical principle that's very well established in physics[2] so any physically based renderer should only have reflection models that obey that principle. [1]: https://www.pbrt.org/ https://www.pbrt.org/ [2]: https://en.wikipedia.org/wiki/Helmholtz_reciprocity https://en.wikipedia.org/wiki/Helmholtz_reciprocity

zokier 3y ago
In a sense all (non-stylized) rendering is physics-based in the sense that all that physics is a model, and indeed the field of physics has different models of differing accuracy/precision/applicability. So even pre-PBR could be called physical in the sense that it represents some sort model of light, the rest is more of question on degree of accuracy and applicability. What I'm aiming at is that the line drawn for PBR is drawn in sand and feels somewhat arbitrary.
- magicalhippo 3y ago
  It's not that arbitrary to me. The earlier work was sometimes physically-motivated, and sometimes even based on physics. For example using Lambertian reflection[1] for diffuse surfaces. But the key, in my view, to PBR is that the whole pipeline is based on physical principles and math, while the old renderers were not. So previous renderers would not bother properly normalizing the Lambertian coeffients for example, leading to violation of energy conservation. [1]: https://en.wikipedia.org/wiki/Lambertian_reflectance https://en.wikipedia.org/wiki/Lambertian_reflectance

zokier 3y ago

In a sense all (non-stylized) rendering is physics-based in the sense that all that physics is a model, and indeed the field of physics has different models of differing accuracy/precision/applicability. So even pre-PBR could be called physical in the sense that it represents some sort model of light, the rest is more of question on degree of accuracy and applicability. What I'm aiming at is that the line drawn for PBR is drawn in sand and feels somewhat arbitrary.

magicalhippo 3y ago
It's not that arbitrary to me. The earlier work was sometimes physically-motivated, and sometimes even based on physics. For example using Lambertian reflection[1] for diffuse surfaces. But the key, in my view, to PBR is that the whole pipeline is based on physical principles and math, while the old renderers were not. So previous renderers would not bother properly normalizing the Lambertian coeffients for example, leading to violation of energy conservation. [1]: https://en.wikipedia.org/wiki/Lambertian_reflectance https://en.wikipedia.org/wiki/Lambertian_reflectance