Sky, clouds and volumetrics

Volumetric clouds are clouds rendered as 3D volumes of density that the engine marches rays through, rather than painted on a skybox or drawn as flat cards. Together with a physically based sky atmosphere, they let the whole sky change with the sun, cast shadows on the ground and, in some games, be flown through. Epic describes its system as "a physically-based cloud rendering system that uses a material-driven approach" (Unreal Engine Documentation: Volumetric Cloud Component).

The atmosphere itself (Rayleigh and Mie scattering, aerial perspective, sky lookup tables) is covered on fog and atmosphere. This page is about the clouds, and about how sky systems fit into time of day lighting.

#How volumetric clouds work

The engine defines a layer of sky between two altitudes. For each pixel, it marches a ray through that layer, samples the cloud material's density at steps along the way, and estimates how much sunlight reaches each sample. In Unreal the density comes from a volume material, which is what makes the system "material-driven" and lets artists build their own cloud types and times of day (Unreal Engine Documentation: Volumetric Cloud Component).

Because this is expensive, engines trace at reduced resolution. Unreal's default mode traces at quarter resolution, reconstructs at half and upsamples to full; a higher-quality mode does not support clouds intersecting opaque meshes (Unreal Engine Documentation: Volumetric Cloud Component). For games, Epic recommends "only use a single octave of light multiple scattering" (Unreal Engine Documentation: Volumetric Cloud Component).

#Clouds and scene lighting

• Cloud shadows. Enabling Cast Cloud Shadows on Unreal's sun lets the clouds "shadow scene elements" and produce light shafts (Unreal Engine Documentation: Volumetric Cloud Component).
• Sky light. With a dynamic sky, the ambient light must update too; Unreal's Sky Light uses Real Time Capture for this (Unreal Engine Documentation: Volumetric Cloud Component).
• Shared models. Ghost of Tsushima used one "Physically based sky model, used for sky, clouds, haze, and fog particles", with atmospheric lookup tables resampled each frame for the current sun and moon angles (SIGGRAPH 2021: Real-Time Samurai Cinema: Lighting…). One model keeps sky, clouds and fog consistent.
• Ray tracing. Unreal's clouds are not ray traced; they appear in reflections only through the sky light capture (Unreal Engine Documentation: Volumetric Cloud Component).

#Platforms

Unreal lists volumetric clouds as unsupported on mobile, supported on PlayStation 4 and Xbox One with shadow maps recommended for performance, and supported on current consoles and PC; its Sky Atmosphere does run on mobile with a sky dome mesh (Unreal Engine Documentation: Volumetric Cloud Component).

Related: fog and atmosphere, real-time lighting, exposure and post-processing, light and shadow.

#Further reading and watching