Extremely Long Lightbuild Times with 4.26

Development Rendering
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Hi Unreal Community and Epic Games,

I’ve run into some serious problems with Building the Lighting for my Game Level. I have a 500m x 500m open world map. The landscape is not that high poly, at 500,000 Tris for the landscape itself.
I have about 50,000 Grass Instances, each one being 200 tris, and about 2000 Trees each at 10,000 Tris. The Lightmap Resolution for all foliage instances is set to 8 in the foliage Lightmap Resolution overide section. I have a LightMass Importance Volume tightly fitting the landscape.

The Lighting has been building for the last 5 hours, and is only at 50%, which means I’m looking at approximately 10 hours to build the lighting for this level.

My PC Specs are:
I9 9900K CPU
32 GB Corsair Vengeance DDR4 Ram
ASUS RTX 2080 Strix, 8GB GPU.
ASUS Maximus 11 Code Motherboard.
Corsair Neutron 400GB PCI SSD.

Additionally, I have run into another serious problem when placing foliage instances. After I have built the lighting on previous builds of the Level, when I place any more than 100-200 foliage instances using the Foliage Paint Brush, I immediately get a huge hit to the Frame Rate, when looking at the newly painted Foliage. The frame rate drops from approx 140 FPS, with the Lighting built, to around 30-40 FPS when looking at the new Foliage with lighting not built on it. And the Frame Rate drop remains until I build the lighting again. This makes placing any significant quantity of Foliage a real hassle, as the Frame Rate starts to drop, and makes the viewport difficult to use. The only work around for this I have found is to switch into Unlit mode, and this resolves the issue of the laggy viewport, but means I have to work with the Viewport in Unlit, and this is less than ideal for artistic purposes.

I’ve been working with Unreal for years now (since 2014) and I’ve only run into these problems with Lightmap Building and Laggy viewport when placing foliage since Unreal 4.25 and 4.26. I’m sure, if my memory serves me correctly, that I’ve not had these issues in earlier versions of the Engine. For example, back in 2019, with Unreal 4.21 I made a few open world levels of similar size and with similar quantities of Foliage, but had perfectly reasonable Light Build times of approx 30 minutes, and did not encounter a laggy viewport when placing foliage.

Has something changed in 4.25 / 4.26 that is causing these extremely long Lightbuild times and the laggy viewport? My engine settings are at Default as far as I know.

Any advice on this issue would be greatly appreciated. Thanks! :slight_smile:

P.S. AS soon as I can im gonna try building the Lighting for this map in an earlier engine version and compare Lightbuild times. I will post the results as soon as I can.

2

So I’ve solved the issue for the purposes I need for my Game Level.

I was facing up to 10 hour build times, which was very inconvenient. Anyways I solved the issue by:

1: Reducing the LightMap Resolution on all my Static Meshes and Foliage Meshes.
2: Changing the Lightmass Settings in World Settings in the Game Level.

I increased the Static Lighting Level Scale from 1 to 8.
I decreased the Number of Indirect Lighting Bounces from 3 to 0.
I decreased the Number of Sky Lighting Bounces from 1 to 0.
And I increased the Volumetric Lightmass Detail Cell Size from 200 to 1000.

After these changes the Lighting for the Level built in about 15 minutes. Yay!

So yeah, that’s a solution to hugely reduce LightBuild times. Obviously, these changes are going to reduce the quality of the Lightmap Build on the environments. Fortunately, my game ia a Low Poly style so it doesn’t reduce the visual quality. For a more realistic game, it might impact visual quality.

But yeah, that’s a method for hugely reducing Light Build Times. Cheers!