Part 11: INDIRECT LIGHTING SYSTEMS

It's useful to think in three layers:

1. What data does the system use? (screen pixels, ray hits, distance fields)
2. How accurate is the result? (approximation vs physically traced)
3. What breaks first? (view-dependence, noise, resolution limits)

1) What data is being queried?

• Screen Tracing: the rendered image — fast but only knows what's on screen
• Hardware RT: ray queries against scene geometry — accurate but expensive
• Distance Fields: a signed distance representation of meshes — precomputed approximation

2) How accurate is the result?

• Screen Tracing: good approximation, but view-dependent — looking away breaks it
• Hardware RT: most physically accurate — rays trace actual geometry
• Distance Fields: approximate — resolution depends on mesh DF quality

3) What problems should you expect?

• Screen Tracing: artifacts when relevant geometry is off-screen or occluded
• Hardware RT: noise depending on sample count, higher GPU cost
• Distance Fields: resolution artifacts on thin geometry or complex shapes

You don't pick one — Lumen uses all of them together.

Lumen automatically blends these methods based on what's available and what works best for each situation:

1. Screen Tracing handles nearby, on-screen surfaces (fast)
2. Distance Fields handle off-screen and distant geometry (works everywhere)
3. Hardware RT (if available) provides higher accuracy where needed

What you control:

The practical takeaway: Most projects use the defaults. You adjust settings when you have specific problems — noise, view-dependent artifacts, or performance issues.

• Section 42 — Screen Tracing: the fast path, its limitations, and how to control it
• Section 43 — Hardware Ray Tracing for GI: when accuracy matters
• Section 44 — Lumen Quality & Noise Control: tuning for your use case

In Part 8: Ambient Occlusion, we added a Post Process Volume with Infinite Extent (Unbound) enabled. We'll use that same volume to control Lumen settings.

Many Lumen controls live in Post Process settings — including Screen Traces, quality settings, and update speeds.

Screen traces ray-march through screen-space depth data:

1. Start from a surface point
2. Step through screen pixels looking for intersections
3. Use hit pixels to estimate indirect lighting contribution

Pros:

• Very fast — uses existing render data
• No special hardware required
• Good for nearby surfaces

Cons:

• View-dependent — can only trace what's on screen
• Looking away from a bright surface removes its GI contribution
• Doesn't work for off-screen or occluded geometry

To toggle Screen Traces, select your Post Process Volume and navigate to:

Details Panel → Global Illumination → Lumen Global Illumination → Screen Traces

Check the checkbox to enable the override, then set to On or Off.

TODO: Screenshot - Post Process Volume showing Global Illumination > Lumen Global Illumination > Screen Traces path

Test 1: Toggle On/Off

With Lumen enabled in your GI demo room:

1. Toggle Screen Traces Off — watch how indirect lighting changes
2. Toggle Screen Traces On — the screen-space contribution returns

Test 2: View-Dependence

With Screen Traces On:

1. Position your camera facing a bright colored surface (like the blue rectangle from Part 6)
2. Notice the color bleeding onto nearby walls — screen traces are sampling the visible surface
3. Rotate the camera so the colored surface moves off-screen
4. Watch the color bounce disappear — screen traces can only sample what's currently rendered

This is why screen traces are called "view-dependent." If a surface isn't visible on screen, it can't contribute to GI through this method.

TODO: Screenshot - Color bounce visible when surface is on-screen

TODO: Screenshot - Color bounce disappears when surface is off-screen

Project Settings → Rendering → Lumen → Screen Tracing Source

Lumen has two modes for how it evaluates lighting at ray hit points:

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

Note: Hit Lighting is more expensive but produces more accurate results, especially for:

• Complex multi-bounce scenarios
• Small emissive surfaces
• Highly detailed geometry

Hardware ray tracing for GI requires:

• NVIDIA RTX 2000+ or AMD RX 6000+
• DirectX 12 with Shader Model 6
• Support Hardware Ray Tracing enabled in Project Settings

Without RT hardware, Lumen falls back to Distance Fields and Screen Tracing.

Use Hardware RT when:

• You need highest quality indirect lighting
• Emissive materials should accurately light the scene
• You're doing cinematics or virtual production

Stick with Surface Cache when:

• Performance is critical
• You need to support non-RTX hardware
• Default quality is sufficient for your art style

These settings live in your Post Process Volume under Global Illumination → Lumen Global Illumination:

Remember: Check the checkbox next to each setting to override it. Higher values = better quality but more expensive.

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

Emissive lighting via Lumen can be particularly noisy:

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.

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

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

Key Points:

• Lumen uses multiple tracing methods adaptively — Screen Tracing, Hardware RT, Distance Fields
• Screen Tracing is fast but view-dependent — looking away breaks GI contribution
• Hardware RT provides highest accuracy but requires RTX hardware
• Surface Cache is the default; Hit Lighting is more accurate but expensive
• Lumen has noise — use spatial filtering CVars and update speed settings to control it
• Emissive can be noisy — use support lights for consistent results

You now understand how indirect lighting is calculated. But there's one more element that contributes to the rendered image — reflections. In Part 12: Reflections, we'll explore what reflections are, how material properties control them, and set up reference materials. Then in Part 13: Reflection Systems, we'll cover the different methods (Screen Space, Lumen, Ray Traced) and when to use each.