Part 4: AMBIENT LIGHTING

Direct lighting comes from a specific source in a specific direction — a lamp, the sun, a spotlight. It creates defined highlights and casts shadows.

Ambient lighting is the fill that comes from everywhere — the sky, bounced light from walls, the general "atmosphere" of a scene. It:

• Fills shadows so they're not pure black
• Provides base illumination from all directions
• Sets the overall mood and color temperature of a scene

In Unreal, ambient lighting comes from three primary sources:

1. Sky Light — captures environment and provides diffuse fill
2. Sky Atmosphere — simulates realistic atmospheric scattering (outdoor)
3. HDRI — uses a 360° image for controlled studio-like lighting

TODO: Add Sky Light setup image

A Sky Light captures the environment around it and provides two key contributions to your scene:

1. Ambient Lighting — Soft, directionless fill light that simulates light from the sky/atmosphere
2. Reflections — Environment reflections on shiny/metallic surfaces

What Sky Light does:

• Fills shadows with ambient color (so they're not pure black)
• Provides base illumination from all directions
• Supplies environment data for reflections on surfaces
• Simulates the overall "atmosphere" of a scene

Key difference from other lights: Sky Light doesn't come from a specific direction — it surrounds your scene and contributes from all angles equally.

See Part 7: Reflections for more on how reflections work.

Adding a Sky Light:

1. Place Actors → Lights → Sky Light
2. By default, it captures the scene around it

Source Type options:

For most workflows, you'll use Sky Light indirectly through the HDRI Backdrop plugin, which includes its own Sky Light component.

HDRI (High Dynamic Range Image) is a 360° panoramic image that contains a full range of light values — from dark shadows to bright light sources.

HDRIs provide three things:

1. Background — 360° environment surrounding your scene
2. Ambient lighting — soft fill light from the environment (via Sky Light)
3. Reflections — realistic environment reflections on shiny surfaces (via Sky Light)

Unreal provides the HDRI Backdrop plugin for easy HDRI setup.

Enabling the plugin:

1. Edit → Plugins
2. Search "HDRI Backdrop"
3. Enable and restart if prompted

Adding HDRI Backdrop to your scene:

1. Place Actors panel → Lights → HDRI Backdrop
2. Drag into viewport

Size is the key parameter: If your HDRI isn't affecting the scene, increase Size significantly. If your reflections show up as black, the Size is probably too small.

The HDRI Backdrop Blueprint contains:

• Sky Light component — provides the ambient lighting and reflection environment
• Geometry component — the dome mesh displaying the HDRI as your background
• Automatic cubemap handling

Important: Delete any other Sky Lights in your scene — HDRI Backdrop has its own. Multiple Sky Lights will cause conflicts and unexpected results.

Select the HDRI Backdrop → expand the Skylight component in the Details panel to access Sky Light settings.

Lower Hemisphere is Solid Color:

• When enabled (default): Below the horizon is a flat color
• Prevents ground from "leaking" unrealistic light upward
• Can cause a visible black bar at the horizon if your HDRI doesn't extend below

If you see a black bar at the horizon:

• Your HDRI may not have below-horizon data
• Try a different HDRI with full sphere coverage
• Or disable "Lower Hemisphere is Solid Color" (may cause light leaking)

HDRI contributes to your scene through its internal Sky Light component:

What HDRI provides:

• ✓ Soft, directionless ambient fill light
• ✓ Environment reflections on shiny/metallic surfaces
• ✓ 360° background for your scene

What HDRI does NOT provide:

• ✗ Directional shadows — the "sun" in an HDRI won't cast shadows
• ✗ Sharp, focused lighting — it's all diffuse ambient

For directional lighting and shadows, you still need actual light actors (Directional, Rect, etc.).

See Part 7: Reflections for how to control reflection quality and methods.

1. ☐ Enable HDRI Backdrop plugin
2. ☐ Place HDRI Backdrop actor
3. ☐ Assign your HDRI cubemap
4. ☐ Set Size to 1000+ (adjust to scene scale)
5. ☐ Set Projection Center appropriately (0, 0, 3000 typical)
6. ☐ Delete any other Sky Lights
7. ☐ Adjust Intensity and Lighting Distance Factor as needed

When importing .hdr files:

1. Drag .hdr file into Content Browser
2. Open the texture in Texture Editor
3. Set Mip Gen Settings: NoMipmaps
4. Set Maximum Texture Size: Match your HDRI resolution (e.g., 4096, 8192)

Without these settings, your HDRI may appear low-resolution or muddy.

Built into Unreal:

• Engine Content — Enable "Show Engine Content" in Content Browser settings
• Engine Plugin Content — Some plugins include HDRIs (enable "Show Plugin Content")

Free online resources:

• Poly Haven (polyhaven.com/hdris) — high quality, CC0 license
• HDRI Haven — merged into Poly Haven
• sIBL Archive — various environments

Sky Atmosphere is Unreal's physically-based atmospheric scattering system. It simulates how sunlight scatters through Earth's atmosphere, creating realistic sky colors and ambient lighting.

Why it matters for ambient lighting: The color and intensity of your sky directly affects what your Sky Light captures. Sky Atmosphere + Directional Light + Sky Light creates a cohesive outdoor lighting baseline.

The Core Setup (Outdoor Realism):

1. Directional Light — represents the sun, set as "Atmosphere Sun Light"
2. Sky Atmosphere — generates realistic sky color based on sun position
3. Sky Light — captures the sky and provides ambient fill

Adding Sky Atmosphere:

1. Place Actors → Visual Effects → Sky Atmosphere
2. Select your Directional Light → Details → Light → enable Atmosphere Sun Light
3. Add a Sky Light (or let HDRI Backdrop handle it)

Key Settings:

Tip: For most outdoor scenes, the defaults work well. Rotate your Directional Light to change time of day — Sky Atmosphere responds automatically.

Sky Atmosphere vs HDRI — When to Use Which:

HDRI Pros:

• Consistent, art-directed look
• Works well for product shots and controlled environments
• Provides detailed environment reflections from real photographs

Sky Atmosphere Pros:

• Dynamic time of day (rotate sun, sky updates)
• Physically accurate scattering
• No HDRI file management
• Better for large outdoor scenes

Note: Height Fog and Volumetric Clouds are covered in Part 8: Atmosphere — these are separate from ambient lighting and have their own performance considerations.

Key Points:

• Sky Light captures the environment and provides ambient fill + reflections
• HDRI gives consistent, art-directed lighting — great for product shots
• Sky Atmosphere reacts to sun position — great for dynamic outdoor scenes
• Only have ONE Sky Light in your scene (HDRI Backdrop includes one)
• If reflections are black, increase HDRI Backdrop Size