Probe-lit fog

This tutorial adds fog to an already working probe-lit scene. You will first establish visible fog, then enable the baked contribution, isolate its lighting channels, and check the result while moving.

Finish Probe lighting first so you know the field has valid data. Fog density and illumination are different: probes supply light, not the fog itself.

1 Check the rendering path

Use a supported desktop SM5-or-higher deferred, non-stereo path with static lighting and shader hooks enabled. See platform compatibility before planning this feature for a target.

Mobile, forward shading, and stereo views do not gain the integration by checking Use Probe Fog Lighting. A registered native volumetric lightmap is preserved rather than replaced by the auxiliary probe bridge.

If the expected contribution is absent, investigate that existing lighting data and configuration. Do not delete build data as a first troubleshooting step.

2 Choose one fog source for the first test

Start with either a local volume or height fog so the cause of changes is easy to see. You can combine sources later.

Local Spectral Fog Volume

1. Add Spectral Fog Volume from Place Actors > Spectral.
2. Fit its box to a small, visible region in the test room.
3. Keep it enabled and start with a modest Density and a neutral Color.
4. Use Fade Distance to soften the inside edges of the box.
5. Keep the view range appropriate for the test area.

Inspect the region from outside and inside. Raising Density makes the medium thicker; it is not the correct first fix for missing lighting.

Exponential Height Fog

1. Place Unreal's Exponential Height Fog, or select the existing actor.
2. Enable Volumetric Fog on its component.
3. Set density and height falloff so the atmosphere is visible without obscuring the test scene.
4. Check that active scalability settings allow volumetric fog.
5. Choose a suitable volumetric view distance.

You do not need a Spectral Fog Volume to use the probe contribution with Exponential Height Fog. It affects the volumetric portion, not the separate non-volumetric height-fog approximation.

Checkpoint: the intended medium is visible. If it is not, check the fog actor, bounds, scalability, view range, and renderer before tuning probe brightness.

3 Confirm baked coverage

Show the probe field with Visualize Global Probes and check that it covers the space occupied by the fog. A probe box fitted only to a small corner cannot describe illumination throughout an entire atmosphere.

If coverage needs changing, adjust bounds or spacing, run Bake Lighting, and save. If the field already covers the region and its lighting is current, adding the fog actor alone does not require recapturing that lighting.

Turn off probe markers before comparing the rendered fog.

4 Enable the probe contribution

Open Features > Fog and enable Use Probe Fog Lighting. Start with Fog Overall Intensity: 1 and the individual lighting multipliers at 1.

Toggle the probe-lighting switch off and on from the same camera position. You are looking for the contribution from the baked field, not for all fog to disappear when the switch is off.

If there is no difference, check coverage and valid bake data, the supported rendering path, shader hooks, and any existing native volumetric lightmap. Large multipliers cannot create data that is missing.

5 Balance one lighting channel at a time

Temporarily reduce one multiplier to 0 to identify what it contributes, then restore it and tune from there.

Control	What to look for
Fog Direct	Unbounced baked light from Point, Spot, and Rect Lights.
Fog Emissive	Direct baked illumination from emissive surfaces.
Fog Indirect (Bounce)	Broad fill from light bounced around the scene.
Fog Ambient (Sky and Sun)	Unbounced sky and Directional Light contribution.
Fog Overall Intensity	The combined probe contribution after balancing its parts.

The sun belongs to Ambient (Sky and Sun), not the local-light Direct channel. The Fog Emissive multiplier is not the fog volume's own Emissive color.

Compare Fog Probe Directionality at 0 and 1 to see the difference between uniform and directional sampled lighting. Restore a suitable value before judging the volume's Scattering Distribution, which controls the medium's view-dependent scattering rather than the stored lighting field.

Too bright does not always mean too denseTip

If the fog has the right thickness but too much illumination, tune its lighting balance. If it hides too much of the scene, tune density. Change one at a time so you can see which problem you are solving.

6 Separate live light beams from baked fill

A live light's Volumetric Scattering Intensity controls its own volumetric contribution. Lower it if a flashlight beam is too strong without dimming its surface lighting.

Setting that multiplier to 0 does not remove the same light's old contribution from the baked probes. Check both sources if the fog remains bright.

7 Walk through the result

Move the camera slowly into and out of the local volume, then turn quickly and pass nearby objects. Check near fades, box boundaries, distant cutoff, and temporal trails.

• Visible cell boundaries: compare Trilinear and Bicubic in the Fog Filtering dropdown under Features > Fog > Fog Quality. It smooths the existing grid without a rebake. If detail is still missing, compare grid resolution and view distance.
• Trails behind moving lights: inspect temporal history and the light's scattering strength.
• Hard local edges: check the fog volume's inward Fade Distance.
• Lighting changes at an unrelated boundary: inspect probe coverage and overlaps.

Use Visualize Fog as a diagnostic, then return to the normal view. Under Fog Quality, make one small comparison at a time. Finer grids cost more, and these overrides affect the project rather than only the selected box.

Fog Filtering works with local Spectral Fog Volume and volumetric Exponential Height Fog. UE 5.6, UE 5.7, and UE 5.8 before 5.8.3 offer Trilinear and Spectral's custom Bicubic. With native filtering support from UE 5.8.3, the menu offers Trilinear, native Bicubic, native Tricubic, and Spectral Bicubic. Tricubic also filters through depth. None of these options adds grid cells or changes temporal history. Compare GPU cost as well as appearance before choosing a filter.

8 Save and verify

Save the level's fog and lighting balance. Save project configuration changes if you adjusted Fog Quality. Test the intended renderer and scalability in a packaged build.

Changing density or rebalancing existing probe channels is a live adjustment. Changing lights, emitting geometry, or the captured probe grid needs a rebake.

If a custom post-process effect should leave fog lighting unchanged, use the Remove Fog and Reapply Fog workflow at the documented material stage. It is separate from setting up the fog itself.