Fog
FogVolumetric fog represents a participating medium: light is scattered toward the camera and attenuated as it travels through the air. Unlike a flat distance-fog blend, a volumetric grid can describe lighting that varies through space, including bright shafts and darker regions behind obstructions.
Spectral samples baked probe illumination to light that medium. This includes separate direct, emissive, bounced, and ambient contributions, so fog can reflect the same baked lighting field used elsewhere in the scene. The probes supply illumination; the fog's density and scattering properties determine how that illumination becomes visible.
Features > Fog controls this lighting and the volumetric grid. Local density, bounds, color, and camera fades belong to Spectral Fog Volume. Native height-fog properties remain on Unreal's Exponential Height Fog actor.
The lighting controls below are saved with the level and update live. The separate Fog Quality overrides, including Fog Filtering, are project-wide. Importing another level's settings copies its probe-fog lighting balance, not these quality overrides.
Supported fog sources
Probe lighting supports both Spectral Fog Volume for bounded regions and Exponential Height Fog for a height-based atmosphere. For Exponential Height Fog, enable Volumetric Fog on its component and provide baked probe coverage where the volumetric lighting is evaluated. A Spectral Fog Volume is not required just to light native volumetric height fog.
This integration affects the volumetric portion of Exponential Height Fog, not its separate non-volumetric height-fog approximation. For the engine-side distinction, see Epic's Volumetric Fog reference.
Probe fog lighting requires desktop SM5-or-higher deferred, non-stereo rendering and static lighting. Mobile shader platforms, forward shading, and stereo views are excluded. An existing native volumetric lightmap is preserved rather than replaced by the auxiliary probe bridge. See the platform matrix before choosing this feature for mobile or VR.
Use Probe Fog Lighting
Default: disabled.
Allows fog to use Spectral's baked probe lighting. The region needs valid probe coverage for that lighting to be available. Enabling the switch does not place fog, generate a probe grid, or bake illumination.
The lighting multipliers appear when this is enabled. Disabling it removes this probe-lighting contribution; it is not the fog volume's Enabled switch.
Fog Overall Intensity
Default: 1. Minimum: 0; no declared hard maximum.
Multiplies the combined probe-lighting contribution. 0 removes that contribution, 1 keeps its original scale, and larger values brighten it.
This changes illumination, not the amount of fog. Use the volume's Density if you want the air itself to be thicker or thinner.
Fog Direct
Default: 1. Minimum: 0.
Scales the unbounced baked contribution from point, spot, and rect lights. Use it to balance those local light sources against indirect and ambient illumination.
It reads the stored direct contribution. Changing this multiplier does not recapture a moved light or include a new light.
Fog Emissive
Default: 1. Minimum: 0.
Scales the separately stored direct emissive-lighting contribution. This is baked illumination from emissive surfaces, not the fog volume's own Emissive color.
A multiplier cannot create a missing data channel. If older data does not contain the separate emissive contribution, rebake before evaluating this control.
Fog Indirect (Bounce)
Default: 1. Minimum: 0.
Scales bounced lighting from the probes. Lower it to reduce broad indirect fill without removing the local-light or sky/sun channels. Higher values can brighten shadowed fog but do not improve the bake's accuracy.
Fog Ambient (Sky and Sun)
Default: 1. Minimum: 0.
Scales the unbounced sky and directional-light contribution. The sun belongs here rather than in the point/spot/rect Direct control.
If reducing Direct leaves a strong directional contribution, check this channel before assuming the settings failed.
Fog Probe Directionality
Default: 1. Minimum: 0; slider up to 2, with no declared hard maximum.
Controls how strongly the sampled probe lighting retains direction. 0 makes it uniform; 1 retains the baked directional character. Values above 1 strengthen directionality.
This is separate from the fog volume's Scattering Distribution, which controls how fog scatters light relative to the view. One changes the sampled lighting field; the other changes its scattering response.
Changing lighting versus changing its balance
These controls rebalance existing baked channels without a rebake. Changes to lights, emissive geometry, probe spacing, or coverage still need a bake. Save the level after tuning.
See Fog quality and materials for Fog Filtering, the seven grid and temporal controls, and the remove/reapply material nodes.