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> I find the physically based rendering pretty disingenuous considering the huge gap still remaining between real-world and standard pbr > ... I accept that i

> I find the physically based rendering pretty disingenuous considering the huge gap still remaining between real-world and standard pbr > ... I accept that it is just a domain-specific term that shouldn't be taken too literally. Actually it is supposed to be taken literally. That's why it's physically based rendering and not physically correct rendering. The goal is to create a system based on real world physics to get as close as possible within the constraints of current computer hardware. I believe this article gets it wrong: > The primary goal of physically-based rendering (PBR) is to create a simulation that accurately reproduces the imaging process of electro-magnetic spectrum radiation incident to an observer. This simulation should be indistinguishable from reality for a similar observer. I don't think the stated goal of PBR is to be indistinguishable from reality for a similar observer. At least, I have never seen this stated anywhere. However, it's worth noting that PBR is a loaded term and includes both physically based lighting, and physically based shading (surface properties). If we are purely talking about lighting, that is, modelling the way light bounces in a physical way, it might be possible to get an exact match to physical reality. Light bounces in a relatively simple way. The problem is that surfaces are not simple. They absorb, reflect, diffract, scatter, and so on. So even if the equations modelling the reaction of light are 100% physically correct, it's an entirely different story to model a complex surface like skin in a physically correct way. This article is about light, so it's talking about physically based lighting. In which case the phrase this simulation should be indistinguishable from reality for a similar observer is more reasonable, but I think it should still be qualified for clarity.

magicalhippo 3y ago
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
kelsolaar 3y ago
OP author here! > I don't think the stated goal of PBR is to be indistinguishable from reality for a similar observer. At least, I have never seen this stated anywhere. I’m, admittedly, highly biased as I work for Wētā FX, a high-end visual effects vendor. We have been developing for over a decade a spectral renderer, i.e. Manuka [1], whose design principles and philosophy are aimed at recreating how a motion picture camera, its lenses, filters, and even colour science, image the real world so that we can integrate virtual cg elements into plates as if they were part of the original photography. This hopefully gives some context to your quote. Whether we are successful at this is a different topic but this is a core business offering we are hired for. - [1] https://www.wetafx.co.nz/research-and-tech/technology/manuka/ https://www.wetafx.co.nz/research-and-tech/technology/manuka...
- esperent 3y ago
  Hey, thanks for writing the article! My nitpicks aside, I learned a lot. I'm always interested in reading more about spectral rendering. > I’m, admittedly, highly biased as I work for Wētā FX, a high-end visual effects vendor That's fair. I can see it being a stated goal of PBR at Weta. However, I'm at the extreme other end of the rendering spectrum. I work with WebGL (I contribute a lot to three.js) and being indistinguishable from reality is most assuredly NOT a goal of a real time in browser rendering engine. I mean, we can dream. But it will be a long, long time before we could realize that dream so our time is better spent finding approximations that can work even on a midrange mobile device.

magicalhippo 3y ago

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

kelsolaar 3y ago

OP author here! > I don't think the stated goal of PBR is to be indistinguishable from reality for a similar observer. At least, I have never seen this stated anywhere. I’m, admittedly, highly biased as I work for Wētā FX, a high-end visual effects vendor. We have been developing for over a decade a spectral renderer, i.e. Manuka [1], whose design principles and philosophy are aimed at recreating how a motion picture camera, its lenses, filters, and even colour science, image the real world so that we can integrate virtual cg elements into plates as if they were part of the original photography. This hopefully gives some context to your quote. Whether we are successful at this is a different topic but this is a core business offering we are hired for. - [1] https://www.wetafx.co.nz/research-and-tech/technology/manuka/ https://www.wetafx.co.nz/research-and-tech/technology/manuka...

esperent 3y ago
Hey, thanks for writing the article! My nitpicks aside, I learned a lot. I'm always interested in reading more about spectral rendering. > I’m, admittedly, highly biased as I work for Wētā FX, a high-end visual effects vendor That's fair. I can see it being a stated goal of PBR at Weta. However, I'm at the extreme other end of the rendering spectrum. I work with WebGL (I contribute a lot to three.js) and being indistinguishable from reality is most assuredly NOT a goal of a real time in browser rendering engine. I mean, we can dream. But it will be a long, long time before we could realize that dream so our time is better spent finding approximations that can work even on a midrange mobile device.