MegaLights

MegaLights is Production Ready in UE 5.8. It replaces several deferred direct-lighting/shadow paths for local lights and supports many dynamic shadowed area lights. Hardware ray tracing is recommended; each light can use ray-traced or VSM shadowing.

Directional Light support is opt-in with r.MegaLights.DirectionalLights 1 and disabled by default. Epic recommends Deferred Lighting with VSM for a strong sun because the MegaLights path has quality limitations; a dim, soft moonlight can be a better fit. Sky Light remains outside MegaLights.

Understand the budget

MegaLights traces a fixed sample budget per pixel toward important lights. Runtime cost is more stable than per-light traditional paths, but quality/noise worsens when too many important lights compete at one pixel. More lights are not free artistically even when the timing remains bounded.

Diagnose

1. Use MegaLights light finder, ray visualizer, and shadow-caster/mismatch views.
2. Compare complete GPU frame time, not only one MegaLights event; it replaces multiple passes.
3. Inspect off-screen and tiny bright lights, huge attenuation radii, overlapping volumes, and shadow methods.
4. Test motion/noise at target internal resolution and TSR quality.

Scale

Control	Effect
r.MegaLights.Allow 0	disable per tier/device profile for a fallback path
r.MegaLights.DownsampleMode	adjust sampling/tracing resolution; stronger downsampling saves work but can reduce quality
r.MegaLights.NumSamplesPerPixel	more samples improve stability/quality and increase GPU cost
Allow MegaLights per light	remove incompatible/problem lights from the path
Shadow Method per light	choose ray tracing or VSM with different quality/CPU/memory/GPU tradeoffs

Start from the Shadow Quality scalability group in BaseScalability.ini. High-quality translucency, volumetric fog, hair, textured area lights, and soft shadows have their own cost; scale only after the visualizer identifies them.