What you'll learn:
- Exponential Height Fog and its key settings
- Volumetric fog for light shafts and god rays
- Practical density values for common scenarios
- Performance cost and when to use volumetrics
Section 18: Height Fog & Volumetrics (10 min)
Exponential Height Fog creates atmospheric haze that increases with distance and decreases with altitude. It's essential for grounding outdoor scenes and adding depth.
Adding Height Fog:
- Place Actors → Visual Effects → Exponential Height Fog
- Position doesn't matter much — it affects the whole scene
| Setting | What It Does | Range |
|---|---|---|
| Fog Density | Overall fog thickness | 0.0 – 1.0 (0.02 default) |
| Fog Height Falloff | How quickly fog thins at altitude | 0.0 – 2.0 (0.2 default) |
| Fog Max Opacity | Maximum fog visibility | 0.0 – 1.0 (1.0 = can go fully opaque) |
| Start Distance | Distance before fog begins | 0+ (0 = starts at camera) |
| Fog Inscattering Color | Color of the fog itself | RGB color picker |
| Directional Inscattering Color | Color toward the sun | RGB color picker |
Fog Density is your primary control. Start low (0.01–0.05) and increase gradually.
Fog Height Falloff controls vertical distribution:
- Low values (0.1–0.2) = fog extends high, affects more of the scene
- High values (0.5–1.0) = fog hugs the ground, clears at altitude
| Scenario | Fog Density | Height Falloff | Notes |
|---|---|---|---|
| Clear day | 0.005 – 0.01 | 0.2 | Subtle atmospheric perspective |
| Light haze | 0.02 – 0.04 | 0.2 | Visible but not obscuring |
| Morning mist | 0.05 – 0.1 | 0.5 | Ground-hugging, clears above |
| Heavy fog | 0.15 – 0.3 | 0.3 | Significantly reduces visibility |
| Interior dust | 0.01 – 0.02 | 0.1 | Very subtle, adds atmosphere |
| Dense atmosphere | 0.3+ | 0.2 | Alien worlds, underwater feel |
Tip: Start with "Light haze" values (0.02 density, 0.2 falloff) and adjust from there. It's easier to increase than to dial back from too much fog.
Volumetric Fog makes fog interact with lights, creating:
- Light shafts / god rays
- Visible light cones from spotlights
- Atmospheric light scattering
Without volumetrics, fog is just a flat color overlay. With volumetrics, lights actually illuminate the fog particles.
Enabling Volumetric Fog:
- Select your Height Fog actor
- Details panel → Volumetric Fog: ✓ Enabled
Per-Light Control: Each light has a Volumetric Scattering Intensity setting:
- 0 = This light doesn't affect fog (invisible in fog)
- 1 = Normal contribution
- >1 = Exaggerated effect (artistic choice)
| Setting | What It Does | Recommended |
|---|---|---|
| Scattering Distribution | Directionality of light scatter | 0.2 – 0.9 |
| Albedo | Fog color when lit | White or slight tint |
| Extinction Scale | How quickly light fades in fog | 1.0 default |
| View Distance | How far volumetrics render | 6000 – 10000 |
Scattering Distribution is key for look:
- 0 = Uniform scatter (light spreads evenly)
- 0.9 = Strong forward scatter (bright halo around sun, dramatic god rays)
- 0.5–0.7 = Balanced, natural look
Volumetric fog has a real GPU cost. Budget accordingly:
| Configuration | Approximate Cost | Notes |
|---|---|---|
| Height Fog only (no volumetrics) | ~0.5 ms | Very cheap |
| Volumetric Fog (default settings) | ~2–4 ms | Moderate |
| Volumetric Fog + many lights | ~4–8 ms | Can add up |
| High View Distance + density | ~6–10 ms | Expensive |
Optimization tips:
- Reduce View Distance if you don't need distant fog
- Limit how many lights have Volumetric Scattering Intensity > 0
- Use Directional Light for sun god rays (one light, big impact)
- Spot/Point lights in fog are expensive — use sparingly
Outdoor Day Scene:
Fog Density: 0.02
Height Falloff: 0.2
Volumetric Fog: Enabled
Scattering Distribution: 0.7
Directional Light Volumetric Intensity: 1.0
Moody Interior:
Fog Density: 0.01
Height Falloff: 0.1
Volumetric Fog: Enabled
Scattering Distribution: 0.3
Spot Lights Volumetric Intensity: 0.5
Dense Forest:
Fog Density: 0.08
Height Falloff: 0.4
Volumetric Fog: Enabled
Scattering Distribution: 0.8
Directional Light Volumetric Intensity: 1.5
Product Visualization (Clean):
Fog Density: 0.0
Volumetric Fog: Disabled
For product shots, usually no fog — keep it clean.
| Use Case | Recommendation |
|---|---|
| Outdoor environments with sun | ✓ Enable — god rays add realism |
| Moody interiors with visible light beams | ✓ Enable — adds atmosphere |
| Performance-critical gameplay | ⚠ Test carefully — can be expensive |
| Product visualization | ✗ Disable — keep focus on product |
| Stylized/clean art style | ✗ Usually disable |
| VR applications | ⚠ Expensive — use sparingly |
| Issue | Cause | Fix |
|---|---|---|
| Fog looks flat/boring | Volumetrics disabled | Enable Volumetric Fog |
| No god rays visible | Light's Volumetric Intensity = 0 | Increase to 1.0+ |
| Fog too uniform | Height Falloff too low | Increase to 0.3–0.5 |
| Performance tanking | Too many volumetric lights | Reduce lights with Volumetric Intensity > 0 |
| Fog clipping at distance | View Distance too low | Increase View Distance |
| Fog color looks wrong | Inscattering Color incorrect | Adjust to match sky/light color |
Key Points:
- Fog Density 0.02 is a good starting point for most scenes
- Height Falloff controls ground vs sky distribution
- Volumetric Fog enables light interaction (~2-4ms GPU cost)
- Scattering Distribution 0.7 gives natural god rays
- Disable volumetrics for product shots and performance-critical scenes