Unreal 5.5 Mega Lights strong noise only when active

[kharnholio1]

Hello everybody,

Although Mega Lights works wonders I do see a lot more noise in shadow areas when I activate it compared to when it is not active. This happens regardless of the amount of lights and is most notably in shadow areas. Attached you can find screenshots of a test scene.

Does anyone have an idea how to fix it to a pre mega lights level?

[glitchered]

it’s normal to have noise when using huge area lights. there’s a limitation how far you can spread those and keep it spatially managable. reminder… at some point it was called stochastic shadows. means… it’s randomly sampling or randomly shooting rays and gathers the shadow data and temporally accumulates it. you’d need more rays and temporal samples to get it smoother over time. at performance cost, tho.

for now… just limit the size of area lamps and keep it well textured. means… add some surface detail that makes up for the limitation and noise in the shadow.

[kharnholio1]

Although I do get your workaround, I wonder why this is not needed when turning Mega lights off, as then the surfaces look almost perfectly clean (see screenshot)

If anything I would have thought that mega lights would improve the noise situation and not make it worse.

I do have a bump in that material which helps to hide the noise a bit when being close like 10-50cm from the surface. As the noise is about 10x10px big hiding it only works at a distance where the bump has a similar size on screen and then it is still recognizable …

Is this behavior known by the devs?
As it only happens when MegaLights are active:
Will this be fixed in future updates?
Am I missing something else?

[ZacD]

There’s no noise without megalights because rasterized lighting uses cheap approximations for light and shadows that don’t produce noise, but also aren’t 100% realistic and don’t support things like soft shadows on area lights. Megalights is ray tracing the direct light and shadows, but only can do so many samples in realtime, so there’s noise.

[kharnholio1]

Thanks Zac for shedding some light on this topic.

Unfortunately the UE 5.5 Post Processing Volume does not seem to have a “Samples Per Pixel” setting for Global Illumination with Lumen available, which I suppose is the one causing the noise on the directly lit surfaces. Regardless I tried to increase the sample count for AO and Translucency without an effect.

As there is also a “Samples Per Pixel” setting on each light source available I tried increasing that from it´s default value of 1 to 2 / 4 / 8 / 16 / 32 / 64 to no avail. As there is only this single light source active and I restarted the project as well in the meantime I wonder what else could be done …

[ZacD]

The noise is not from GI, Megalights still uses lumen for GI, but Megalights only handles the direct lighting.

[kharnholio1]

Ones again thanks for sharing these insights
Is there any way to reduce that noise then? I do not see any sample count options besides the ones I posted screenshots of …

[ZacD]

Try playing with these console commands:

r.MegaLights.NumSamplesPerPixel 16
r.MegaLights.Volume.NumSamplesPerVoxel 4

[kharnholio1]

Holy smokes, thanks A LOT @ZacD
Although I lost about 10fps from 80 down to 70
r.MegaLights.NumSamplesPerPixel 16 did the trick