Part 9: AMBIENT OCCLUSION SYSTEMS

AO settings are spread across three locations:

This is important: AO methods don't just exist independently — they have a priority system.

Settings Location: Post Process Volume

SSAO calculates occlusion using only the rendered depth buffer — what's visible on screen.

How it works:

1. For each pixel, sample nearby depth values
2. If nearby pixels are closer (occluding geometry), darken the pixel
3. Blur the result for smoother occlusion

Pros:

• Works on any geometry
• No precomputation required
• Fast and widely compatible

Cons:

• View-dependent — occlusion changes as camera moves
• Limited to screen-space information
• Can miss occlusion from off-screen geometry

Settings Location: Sky Light (must be Movable)

DFAO uses the Mesh Distance Fields we enabled in Part 2 and used for shadows in Part 4.

Requirements:

1. Project Settings: Generate Mesh Distance Fields must be enabled
2. Sky Light Mobility: Must be set to Movable (Static/Stationary Sky Lights cannot use DFAO)

Why Movable? DFAO requires dynamic world-space calculations. Static and Stationary Sky Lights use baked precomputed lighting and cannot access distance field data for dynamic AO.

How it works:

1. For each pixel, ray march through the distance field data
2. Distance fields provide occlusion information from all geometry (not just on-screen)
3. Result is view-independent and stable

Pros:

• View-independent — consistent regardless of camera angle
• Stable, dynamic, works in world space
• Handles large-scale occlusion well

Cons:

• Requires Movable Sky Light
• Resolution limited by distance field quality
• Doesn't capture small detail as well as SSAO

Threading the Distance Field story: We enabled Distance Fields in Part 2, used them for soft shadows in Part 4, and now use them again for DFAO. Later, in Part 11: Indirect Lighting Systems, Distance Fields will appear again for Lumen's software ray tracing.

Settings Location: Post Process Volume

RTAO performs accurate AO via hardware ray tracing, independent of screen space or distance fields.

Requirements:

1. Project Settings: Support Hardware Ray Tracing enabled
2. Hardware: RTX/DXR capable GPU + DirectX 12
3. Post Process Volume: Ray Traced Ambient Occlusion enabled

How it works:

1. For each pixel, trace rays into the scene
2. Rays test against actual geometry for occlusion
3. Result is physically accurate

Pros:

• Highest quality AO
• Accurate for all geometry sizes
• View-independent

Cons:

• Requires RT hardware
• Most expensive method
• May have noise at low sample counts

When Ray Traced AO is enabled in your Post Process Volume, it replaces both SSAO and DFAO. This is the highest quality option and fully overrides other methods.

When both DFAO and SSAO are active (no RTAO), they combine based on the Combine Mode setting on the Sky Light:

This allows DFAO to handle large-scale occlusion while SSAO adds fine contact detail.

With your Post Process Volume selected, navigate to:

Details Panel > Rendering Features > Ambient Occlusion

To disable SSAO explicitly:

• Set Intensity to 0, or
• Uncheck the Ambient Occlusion override entirely

If you have hardware ray tracing enabled, you'll find RTAO settings in the same section:

Details Panel > Rendering Features > Ambient Occlusion:
    Ray Traced Ambient Occlusion: ✓ On

Note: When RTAO is enabled, it completely replaces SSAO and DFAO. You don't need to disable them separately.

DFAO is controlled through your Movable Sky Light:

Sky Light > Details > Distance Field Ambient Occlusion

Combine Mode Explained

For most projects, Multiply produces richer ambient occlusion by letting both methods contribute.

If using RTAO, you can also enable ray traced shadows from the Sky Light:

Sky Light > Details > Light:
    Cast Ray Traced Shadow: ✓ On

This enhances AO and indirect lighting quality by casting ray traced shadows from the sky light. Only effective when RTAO is enabled.

Best Quality (with RT hardware)

Post Process Volume:
    Ray Traced Ambient Occlusion: ✓ On
    Samples Per Pixel: 2-4 (balance quality/performance)

Sky Light:
    Cast Ray Traced Shadow: ✓ On

RTAO overrides SSAO and DFAO automatically.

Good Quality (without RT hardware)

Project Settings:
    Generate Mesh Distance Fields: ✓ On
    Ambient Occlusion: ✓ On

Sky Light (Movable):
    Occlusion Combine Mode: Multiply
    Occlusion Contrast: 0.5

Post Process Volume:
    Intensity: 0.5
    Radius: 200

DFAO handles large-scale, SSAO adds fine detail, Multiply combines them.

Key Points:

• Three AO methods: SSAO (Post Process Volume), DFAO (Sky Light), RTAO (Post Process Volume)
• RTAO overrides both SSAO and DFAO when enabled — highest quality
• DFAO + SSAO combine based on Sky Light's Combine Mode (Minimum or Multiply)
• DFAO requires Movable Sky Light — Static/Stationary cannot use distance field AO
• Settings are spread across Project Settings, Post Process Volume, and Sky Light
• Use Min Occlusion on Sky Light to prevent areas from going pure black
• Distance Fields connect Part 2 (setup) → Part 4 (shadows) → Part 9 (AO) → Part 11 (Lumen software RT)

You now have ambient lighting (Parts 6-7), ambient occlusion fundamentals (Part 8), and ambient occlusion systems (Part 9) working together. But in the real world, light doesn't stop when it hits a surface — it bounces. In Part 10: Indirect Lighting, we'll explore Lumen and how light bounces create the subtle color bleeding and soft illumination that makes scenes feel real.