I’m thinking about the same question. So seems that Lumen replaced the global AO totally？ Because Lumen is a software raytracing strategy, if you want a global AO you need to use ppv and turn on the raytracing AO. And if you dont use raytracing, there won’t be an AO buffer for you to visualize, and that’s why it’s absolutely white I guess. But I’m curious what kind of AO strategy they use for lumen? They support material AO, so maybe it’s SSAO?
I think I figure it out. There is no additional pass for AO in software raytracing system of Lumen, but the influence of AO is included in Lumen lighting. So if you want a global AO image you’d better use a PPV and turn on the ray tracing AO. Hope that’s helpful!
Hey guys. Just thought of adding this note.
First off - Ambient Occlusion is something that software does to fake the soft shadowing when meshes interact with one another. The Engine can do this shadowing by itself without having to do AO pass - but, you see, it is very resource intensive to do this soft shadowing with just lighting, you end up dropping performance if you do this. So that’s why the concept of doing an AO pass exists. Screen space and Raytracing renderer uses AO to do this shadowing effect (as a bonus, artists get to art direct this pass too - like controlling falloff radius, intensity and colors. You see the concept here? You are not actually modifying your lights for these effects. This is all just a pass generated by engine being composited on top).
If you take a pathtracer renderer like Arnold or even UE’s pathtracer, it does this self shadowing without having to generate an AO pass. The downside to this is that you lose artistic control of AO because the AO pass does not exist in the first place (that’s what I think at least). Lumen however uses a technique called 'Virtual Shadow mapping" to achieve this look. You can visualise using the view modes on your viewport too.
But for more stylised work, I think Epic should add an ability to give AO pass and be able to composite on top of final image in Engine itself. For now, the option is to take the AO pass from SS or raytracing GI and composite in some editing software like AE, photoshop or Nuke.
EDIT: User “cschrubini” has linked a video in the below comments showing how to enable SSAO with Lumen.
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Hi,
I have similar problem. When I’m creating default Unreal (5.1) project (this one with race) - I have set Lumen and I can change for e.g colour of ambient occlusion - it works!
In my personal project I can see AO but i can’t change this colour for e.g.
Aaaand what is weir - it wokrs oly with scalability settings set on Low or Medium 
With High, Epic, Cinematic - nothing hapend
Any ideas?
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Because that’s something like SSAO, not Limen, which is only enabled when Hogh, Epic or Cinematic, not being allowed to separate AO, which seems to be integrated into the Limen GI.
Ue5-lightning .pdf (18.5 MB)
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Thanks for sharing!
This method worked fine.
This does not seem to work when using a post process material with a lumen scene.
It’s fine and all that Lumen changes how AO works, but the AO pass is still vital for masking specific effects in a post process material volume.
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AFAIK you can’t, unless you disable the integrated AO inside Lumen (Short Range AO), which is much more precise, so the tradeoff usually won’t worth.
Thanks Alberto
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This is the answer, disable short range ao:
r.Lumen.ScreenProbeGather.ShortRangeAO 0
r.Lumen.DiffuseIndirect.SSAO 1
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Ok guys let’s see…
AO Is an Outdated Paradigm
Ambient Occlusion is a relic of an era when computational constraints dictated visual approximations over physical accuracy. It consumes resources by loading additional textures or calculating shadows where are not needed to produce results that contradict the physics of light.
Let’s consider two examples: a narrow crack in a surface and the entrance of a dark cave.
The Crack
When you look at a narrow crack from the outside, it might appear dark to your eyes. This doesn’t mean there’s no light inside the crack—it simply means that the photons inside are not bouncing back toward your eyes effectively. In reality, the crack is full of photons scattering in every direction, and with the right conditions (e.g., a direct light source shining into it), you would be able to see the interior.
However, AO maps don’t simulate this behavior. Instead, they artificially darken the crack based solely on geometry, independent of whether a light source is present or not. This is a direct contradiction to how light behaves in reality.
The Cave
Similarly, when you look into a dark cave from the outside, you might perceive the interior as pitch black. But in reality, the cave is not empty of photons. The photons from the environment enter the cave and scatter across its surfaces, though they might lose energy and fail to reach your eyes effectively. If you were to walk into the cave with a flashlight, you’d see the walls illuminated because the light interacts with the interior and reflects back to your eyes.
AO fails here as well. It “bakes in” darkness inside the cave regardless of whether a light source could realistically illuminate it. This approach ignores the dynamic behavior of light entirely, making it a poor representation of reality.
In short, AO creates “fake darkness”
that is computationally expensive and unnecessary in the context of physically accurate rendering. The transition to ray and path tracing, lumen … should eliminate these issues by accurately simulating light behavior, making AO an outdated and inefficient approach.
AO should—and will—be phased out in favor of more authentic methods.
while it is agreeable for photorealistic results and high end rendering i would argue that one would love to have artistic control and the ability to have it nonetheless for artistic control or, for specific use case in which Lumen is not achieving the desired result, i am encountering the issue right now and very happy that i can turn back on SSAO as showed above.