Part 5: INDIRECT LIGHTING

To see how indirect lighting (bounce light) works, we need reference materials that demonstrate the effect:

BP_LHT_RefLightingIndirect Blueprint: Create a Blueprint with spheres showing HSV Value effects on bounce:

1. White (V = 0.85) — High bounce, brightens nearby surfaces
2. Mid Grey (V = 0.5) — Medium bounce
3. Dark Grey (V = 0.18) — Low bounce
4. Black (V = 0.04) — Minimal bounce, absorbs most light

Add a white wall or floor nearby to see how each sphere's color bleeds onto adjacent surfaces.

Direct lighting: Light travels from source → surface → camera
Indirect lighting: Light bounces off surfaces before reaching camera

Indirect light is why:

• Shadows aren't pure black (light bounces into them)
• Rooms feel "filled" even with one window
• Colors bleed between nearby surfaces

Base Color (also called Albedo) represents reflectance — what percentage of light bounces back. This is the Value in HSV (Hue, Saturation, Value).

Here's why this matters for indirect lighting:

The Value (brightness) of your materials determines how much light bounces.

This is why realistic material values matter:

• Too-dark materials = dead, flat shadows (no bounce)
• Too-bright materials = washed out, no contrast (infinite bounce)
• Correct values (0.04 - 0.85) = natural light falloff

Practical example:

• A room with dark walls (0.1 Value) will have very little bounce light — shadows will be harsh
• The same room with light walls (0.7 Value) will have soft, filled shadows from all the bounced light

Lumen is Unreal Engine 5's Global Illumination (GI) system.

What Lumen Does:

• Calculates light bouncing between surfaces
• Provides ambient fill in shadows
• Makes emissive materials "light" the scene
• Updates dynamically as lights/objects move

Lumen uses different ray tracing methods to gather indirect lighting information:

Note: Distance Fields work when Hardware RT is off — they're Lumen's fallback for indirect lighting on non-RTX hardware.

Screen traces use the rendered image to approximate indirect lighting. This is fast but has limitations:

Screen Tracing Source — What data screen traces use:

Project Settings → Rendering → Lumen → Screen Tracing Source

Post Process Controls:

Important: Screen traces are view-dependent — if you look away from a surface, it can't contribute to GI via screen traces. Hardware RT and Distance Fields provide more stable results.

For highest accuracy, enable Hit Lighting mode:

Project Settings → Rendering → Lumen → Ray Lighting Mode → Hit Lighting

This uses hardware ray tracing to evaluate lighting at each ray hit rather than using the surface cache.

Note: Hit Lighting is more expensive but produces more accurate results, especially for complex lighting scenarios.

1. Create an enclosed room with a window opening
2. Add a single light outside the window — convert this setup into a Blueprint for reuse. This gives you a portable indirect lighting test rig you can drop into any level.
3. Lumen OFF: Interior is black except direct light path
4. Lumen ON: Light bounces inside, fills the room

Quick toggle via Project Settings:

Project Settings > Engine > Rendering > Global Illumination:
    Dynamic Global Illumination Method: Lumen / None

Each light has an Indirect Lighting Intensity property:

• 1.0 = Normal bounce contribution
• 0.0 = This light doesn't contribute to bounces

• 1.0 = Exaggerated bounce (artistic choice)

See Part 6: Exposure for Lumen quality settings and Post Process overrides.

An emissive material appears to glow — it outputs light values greater than 1.0.

Creating an Emissive Material:

1. Create new material
2. Add Emissive Color input
3. Multiply base color by intensity value

Emissive Color = Light Color × Intensity
Example: Orange × 100 = Bright orange glow

By itself: NO.
Emissive materials appear bright but don't cast light on other objects.

With Lumen: YES.
Lumen can make emissive surfaces contribute to indirect lighting.

Demonstration:

1. Hide all lights
2. Place emissive material in scene
3. Lumen OFF: Only the material glows, surroundings are black
4. Lumen ON: Emissive casts colored light on nearby surfaces

Emissive lighting via Lumen has the same issues as other indirect lighting:

• Noisy
• Inconsistent frame-to-frame
• Not reliable when consistency matters

Why Emissive Can Be Noisy:

• Mesh size and emissive intensity — Small meshes with high intensity are harder to sample accurately
• Related to Screen Traces — Screen-space limitations affect emissive contribution
• Sample count — Default sampling may not be enough for complex emissive setups

Solutions:

• Reduce intensity of emissive and increase exposure instead
• Use larger emissive surfaces rather than small bright ones
• Add support lights (actual light actors) near emissive surfaces for consistent results
• Increase Final Gather Quality (performance cost)

Don't rely on emissive as primary lighting when you need predictable, repeatable results.

Lumen uses temporal accumulation — it builds quality over multiple frames. Noise appears when:

• Camera or objects move quickly
• Lighting changes rapidly
• Sample counts are too low

These console variables control Lumen's spatial filtering and denoising:

Temporal Stability Settings:

Trade-off: Reducing update speed makes lighting more stable but slower to respond to changes. For cinematics, slower speeds reduce noise; for gameplay, faster speeds keep lighting responsive.

1. Start with default settings — Often good enough
2. If noisy: Increase SpatialFilterNumPasses to 5-6
3. If still noisy: Increase SpatialFilterHalfKernelSize to 2
4. For cinematics: Reduce update speeds in Post Process settings
5. For problem emissives: Add support lights or reduce emissive intensity