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What enabled physically-based rendering?

The earliest game to use PBR is Alien: Isolation, as far as I know.

How come it took until late 2014 for a game to come out that uses it? Is it the middleware of a particular company which has succeeded in making PBR the default shading tool?

In other words, what factor(s) caused such a drastic improvement in shading?

Remember Me had it before then–a custom addition to UE3 since UE4 wasn’t available at the time. I wouldn’t be surprised if something had it before then too.

Support for PBR is similar to how VFX started doing things physically based/linear workflow. The hardware is fast enough to support it and it makes things more standardized once people start using the same settings for things which makes the whole process faster and easier. Before, you would just make something and adjust until it looks good, but now you can start with real-world values so you don’t have to make guesses, and once you’ve done something you know how to do it again later.

It’s lot older than that.
http://advances.realtimerendering.com/s2010/index.html
Cry engine 3 presentation talk about. “physically-based lighting model” back in 2010.

http://advances.realtimerendering.com/s2011/index.html
“Physically-based lighting in Call of Duty: Black Ops”

Lighting/Shading is synonyms here.

Wasn’t it Pixar who developed PBR and first used it for Monsters University?

Short answer.
Nope.

Disney did publish some very thorough research a few years ago and presented a fairly unified model for all of the different pieces. It seems like around that time is when everybody started investigating it more carefully. It wasn’t necessarily new though.

This may be the one:
https://disney-animation.s3.amazonaws.com/library/s2012_pbs_disney_brdf_notes_v2.pdf

I think it’s kind of like Crytek’s wind shader method for trees. People started picking it up when there was a good amount of literature on the subject, and because it gives a nice result without too much effort. Now, like the wind method, it’s become somewhat of a standard thing.