Expected skylight functionality?

Hey everyone,
So I’m in the middle of putting the lighting and rendering tech through its paces, and Skylight has been giving me non-intuitive results. I have two questions:

First being that if I turn off “Use Ambient Occlusion” in the World Settings - should the skylight turn off as well? Example here:

I’ve literally only clicked the AO box, nothing else.

Second is that - I’m using the standard Grace Cathedral as my hdr map for the skylight and have shadows enabled…however I don’t really see anything in the way of “shadows” If you’re familiar with Grace, I should be getting very complex (or at the very least - SOME) shadowing from this light probe. Is skylight not intended to be used as an IBL-type solution? Am I missing a setting somewhere to get shadowing? This is a production quality light build with zero indirect bounces (looks pretty much the same with 3). I guess I’m not quite sure if this is the expected look and…if it is…what the intended functionality is over a regular ambient cubemap.


Hi Chris,

From looking at your scene outliner you should add a lightmass importance volume to encompass your scene since you are adjusting the lightmass settings from the World Properties tab. You may want to place a directional light in the scene as well if you’re wanting shadows for your assets.

Also, are you rebuilding lighting between checking and unchecking the box?

Here is some documentation for the Skylight that may help you out as well.

Thank you!


Hey Tim!
Thanks for responding - and for pointing out the importance volume - I totally forgot. Unfortunately, I get the same look and same interaction with AO (where the light disappears) even with the importance volume added to the scene. I am rebuilding the lighting between changes - I guess that was unclear from my initial post. I’m disabling AO, rebuilding the lighting, and seemingly the light breaks.

I had read that documentation, and it was unclear what the intended functionality for Skylight was (whether it’s more of an ambient light with some color direction but shadows essentially being ambient occlusion - or something that is a true Image Based Lighting solution). Based on your response - is the only way to get shadows casting for static meshes via replicating my image with directional/spotlights? Is there any plan on deck for adding shadowing support for HDR maps/light probes?
Appreciate your time!

You’re running into an unintended side effect. With NumBounces set to 0, Lightmass will only compute the Final Gather if AO is enabled. But the Final Gather is needed to compute sky light shadows. Set your NumBounces to the default and the issue will go away. This is a bug, thanks for mentioning. The sky light shadowing is supposed to be decoupled completely from AO.

The Skylight is intended to be an image based lighting solution for infinitely far away lighting with local shadowing. A few approximations have been made for performance though, specifically the shadowing and the lighting terms are computed separately and multiplied together later. This means that you won’t get changing colors in shadows based on which part of the background light source is shadowed.

Sky occlusion for stationary sky lights (when it is working properly) looks like this

The Ambient Cubemap is intended to be just a directional ambient term, without local shadowing. It’s really only useful for games with fully dynamic lighting where you want to prevent the lighting from ever going black.

Thanks for the response Daniel. Exactly what I was looking for.

It would be great to have the ability to do the shadow calculation at the same time as the lighting term. I know the lighting build times would be…significant…for a noise-free solution but coming from a world where I used wait 60 hours for a single frame - I’d be willing to make the sacrifice.

In any case, thanks for the information. Loving the engine so far.