In the Unreal Engine’s LumenTranslucencyVolumeLighting.usf file, there is a function TranslucencyVolumeIntegrateCS. During its calculation, there is a variable called TranslucencyVolumeIntensityScale, which is given a fixed value of 4. This determines the brightness of the translucency during volume rendering. I want to know why this value is 4? Or is it some kind of magic value?
// line 318
float TranslucencyVolumeIntensityScale = 4;
Lighting.R.V *= TranslucencyVolumeIntensityScale;
Lighting.G.V *= TranslucencyVolumeIntensityScale;
Lighting.B.V *= TranslucencyVolumeIntensityScale;
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Hi there,
Thanks for reaching out about this. As far as I know, the literal “4” is an intentional, hard‑coded gain applied only to the Lumen translucency GI that’s consumed by lit translucent materials (not by Volumetric Fog). It’s there to bring the perceived brightness of volume‑integrated, low‑order SH lighting into line with the rest of the translucency lighting model and UE’s historical “Translucency Lighting Volume” look. In other words, it’s an empirical calibration to compensate for the way this volume GI is computed and consumed (limited ray directions, two‑band SH, and the subsequent material lighting model), so translucent materials don’t look too dim compared to direct lights and other GI paths.
So, in other words, the number 4 is a magic value in the sense that this is not a physically-based value per se, but rather a multiplier used to match the look of other legacy lighting systems. I hope that makes sense, but if there are still outstanding questions, please let me know.
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