UE4 4.24 SSGI + AO Implementation

[Poet_11 (William K)]

@Rawalanche

I know quite a few folks from the high end studios you named, and I can assure you most of them don’t use AO as a separate pass they put on top of the renders. AO still has it’s place in form of a map/shader to mask out concave areas for procedural weathering effects, but that’s about it. No competent CG artist in this decade spends additional time and resources to render an additional pass used just to introduce a bias to their renders.

AO in UE4 is present because by default, and also historically, UE4 did not have any realtime GI solution. You have just solid ambient light color which you need to combine with AO to get at least remotely believable result. But if you have an actual accurate GI, which SSGI is, adding more AO will just give you worse results. And there’s nothing artistic about that.

You can clearly see that on the GIF posted above. You can see that SSGI at default setting still creates a lot more pronounced and detailed concave light falloff than SSAO. There’s no need to introduce any more AO on top of that, and you can also see that SSGI with AO set at 1.5 just burns concave shadows, making the light falloff appear lot less realistic.

That doesn’t mean that requirement of “deeper shadows” for artistic purpose is invalid, but that should be done on color grading level, after the rendered result. Not by doing ******** with light transport

I actually work with and have worked in recent past with some of those companies mentioned. I can assure you we all render out passes in form of render elements including an AO pass were required to comp them in later during compositing. Sure you can use them in the shader as well and we do!
But we also choose to render out AO automatically during the main render pass which includes all the separate elements. These are for artistic control. Let us not argue on something I factually know is 100% true today :). I’ve been int he industry professionally for over 25 years now. This doesn’t mean AO is used everywhere it just means it is there when you need it and you will need it during comps.

As for UE4, it is a cheat software, because it is a game engine and serves its purpose as that, SSGI is not an accurate GI solution it is a hack of a hack one bounce color bleed! and a post process at that! it is light years behind anything close to a raytraced GI solution let alone unlimited bounces. Even Epic says use it in a very subtle manner. Which is why some people use another hack called post process AO which suffers form similar problems to make up for others shortcomings. used both together and you will risk getting muddy dirty images. Some care others don’t.

But I mentioned earlier in my post, I don’t recommend using post process AO since it doesn’t give good results, use Material AO for clean results. and work around dynamic contact shadows with other custom setups not requiring post process AO.