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For example if I call Unity 5's render pipeline "PBR" (Physically-Based Rendering) and also call Unreal Engine 4's render pipeline "PBR," does it mean that the same model with the same PB textures will be viewed identically in very similar or equal scene setups on both engines?

I just wondering if someone says "PBR," does it mean only one, single, right way to do "PBR," or do implementations vary from one to the other?

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Physically-Based Rendering (PBR) is really a modern tag people use to refer to the trend of making the rendering system "correct" with regard to the real-world physics of light interacting with surfaces, rather than an exact definition of a full rendering system. In other words, I would say PBR is more a definition of the aspiration of the system than what it strictly does.

See here for a good summary of the topic.

Therefore no two systems are guaranteed to try to get to the goal in the same way.

Different systems may model light more or less accurately in the simulation for example - i.e. one might have better global illumination system or better shadow mapping etc. So while the system might model light-surface interaction more accurately, and thus be described as a PBR system, the incident lighting on the surface in the first place might be a bit different giving different results in each system.

So, if given a single flat surface in a scene with one light it's reasonable to expect two PBR systems to give basically the same results. But when it comes to rendering a whole scene with lots going on, there are many variables in play that will probably lead to more obvious variations in the resultant image.

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The idea of physically based rendering is to reach a closer approximation of how light behaves in the real world and use this model to generate a 2D impression of a 3D scene. This is basically what we are doing since the invention of 3D graphics, just with more simplified models of optics. So yes, PBR is mostly a buzzword.

A perfect approximation of real-world physics of light would require to simulate every single photon accurately (and don't forget about the wave/particle duality of photons which is required to accurately simulate some optical phenomenons you encounter in real-life). That's the only way to create a truly photorealistic impression of any scene possible in real-life. But doing this in real-time would greatly exceed the abilities of todays computers by several orders of magnitude. That means you will always need to take some shortcuts and simplifications to get a reasonable compromise between image quality and rendering performance. How to make this compromise is up to each rendering pipeline.

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