Hey, I know this may be a common issue but I can’t for the life of me figure out what search terms to use, I have this weird light ghosting effect in lumen, I have a couple YouTube videos of my issue linked so you can check it out, but essentially I noticed when I was trying to add muzzle flashes to my scene that the underneath of wherever was being shot lit up in a weird way. It can be subtle at times because sometimes the muzzle flash lines up in a way that makes it look realistic, but the examples I have linked should makes it pretty clear that when the walls are lit up it has nothing to do with the light emitted into the scene, and everything to do with just a weird light burn in. I have tried most things like warm up frames, different engine version, different lighting setups, different anti aliasing, disabling AA, using MQR, changing all the settings in the light’s themselves. I just couldn’t figure it out and I’m using the forum as a last result, I hope I’m just an idiot and there is a super simple solution, any and all help would be super super appreciated, thank you for you time in advanced.
Unfortunately, that’s a problem pretty much fundamental to lumen. Because it’s real-time GI and reflections, it has to agressively reuse information spatially and temporally in order to get a clean result. One of the assumptions it runs on is that GI cannot vary too much from one frame to another.
What that means is that pretty much any rapidly-changing light flicker will break lumen, especially emissive surfaces. There are some caveats for that with punctual light sources like point or spot lights, but bottom line is that fast-moving emissives don’t work well.
You do have the option to force lumen to push through the temporal history faster, but that will either massively increase your noise, or you’ll have to increase the SPPs you’re tracing in order to supress said noise. You can kind of think of it like this:
These are the general tradeoffs that anyone using lumen can make, although the CVars do give case-specific optimizations. This is a very imperfect way of thinking about a complex system, but in general you can optimize for any two and lose at the third. You could have lighting that is very smooth and runs well, but it’ll probably ghost pretty brutally.
You could have super responsive lighting that’s less noisy, but you’ll need beefy hardware to get those SPPs, and it may not scale down too well.
And a general rule for emissive lighting with lumen: to cut down on noise, make the surfaces pretty dim, and pretty big. All emissive surfaces will be inherently noisy due to how the engine integrates them, but the bigger ones will be easier to compute, and the dimmer they are, the harder it is for the noise to be visible.
