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RTXGI Attic sample ambient light problem

in Attic RTXGI sample scene, even after all the lights have been turned off, there’s still this light to evenly light the entire scene, turn DDGI on or off doesn’t affect this global lighting, why the scene is lit in such a way?

Possible the Skylight? It’s illuminating even sealed off interiors on several occasions.

thanks, that seems to be the case in Attic, but I have problem to get DDGI working in any other scenes, like ArchViz interior (which itself showcases the ray tracing), I’m running RTXGI branch 4.26.2, RTXGI version is 1.1.30, r.GlobalIllumination.ExperimentalPlugin is set to 1, DDGI just doesn’t have any effects on a single directional light, if I add another point light, DDGI seems to kick in all right but with just one directional light, it doesn’t show up at all, I compared the result to UE Ray tracing GI (Brute force or final gathering) as well as path tracing, you can clearly tell that DDGI is just not working at all, wondering did I miss anything?

to demonstrate, here are two screenshot show the problem, one is with DDGI, you can see there are no light bounces at all, the other screenshot is with UE ray tracing GI set to brute force, the scene is properly lit by a single directional light but as you can see the DDGI probes are all black which means they dont receive any lights at all.