Part 4: DIRECT LIGHTING SYSTEMS

Terminology Note: Shadow Maps is the method (project setting). Cascade Shadow Maps (CSM) is a technique within that method, specifically used for Directional Lights. Local lights (Point, Spot, Rect) use traditional shadow maps without cascades.

It's useful to think in three layers:

1. What data does the shadow system use? (depth map, virtualized depth, distance field, ray hits)
2. How does it produce softness? (filtering/approximation vs true area-light sampling)
3. What breaks first? (aliasing, shimmering, temporal instability, resolution limits, noise)

1) What data is being queried?

• Shadow Maps / CSM: a depth texture from the light's point of view.
• VSM: still depth-based, but virtualized (tiled/cached) so it can allocate much more effective resolution.
• Distance Fields: a signed distance representation of meshes (precomputed) used to estimate occlusion.
• Hardware RT: ray queries against scene geometry (more physically grounded, more expensive).

2) How does "softness" happen?

• Shadow Maps / CSM: softness is typically a filtering trick applied to a fundamentally binary "in shadow / not in shadow" test.
• VSM: softness is generally an approximation driven by light size and distance (often looks great, but it isn't true area sampling).
• Distance Fields: softness comes from the distance information itself, which can produce pleasing large-scale soft shadows.
• Hardware RT: softness comes from sampling the light's area (closest to real penumbra behavior).

3) What problems should you expect?

• Shadow Maps / CSM: limited resolution and cascade boundaries can show up as aliasing, shimmering, and "popping."
• VSM: can show temporal instability (cache/page updates) during fast motion.
• Distance Fields: can show resolution artifacts depending on mesh DF quality and scale.
• Hardware RT: can introduce noise (and cost) depending on sample count and motion.

• Default for most projects: VSM (good quality, broad hardware support).
• Huge worlds / long-distance soft shadows: consider Distance Field Shadows (especially for far-range softness).
• Hero shots / accurate area-light penumbra: consider Hardware Ray Traced Shadows (budget permitting).
• Legacy pipelines / compatibility / existing tuning: Shadow Maps (CSM) still matters.

• Section 8 — Shadow Maps Method (Legacy): the classic workflow you'll still encounter, plus why it produces certain artifacts.
• Section 9 — Virtual Shadow Maps: how the tiled/virtual approach changes quality, performance, and motion behavior.
• Section 10 — Distance Field Shadows: where DF shines, what "resolution limits" actually look like, and the key constraints.
• Section 11 — Hardware Ray Traced Shadows: when RT shadows are worth it, and how to reason about cost vs accuracy.
• Section 12 — MegaLights: when lots of dynamic shadowed lights are needed, and how MegaLights changes the cost model.

Next, we'll start with Shadow Maps (Legacy) so you can recognize (and troubleshoot) the common failure modes that the modern systems were designed to improve.

Cascade Shadow Maps split the camera's view frustum into multiple distance bands ("cascades"), each with its own shadow map:

• Near camera = highest resolution shadows
• Far from camera = lower resolution shadows

This technique addresses the resolution problem unique to Directional Lights — they illuminate the entire scene, so a single shadow map would be stretched too thin.

• Shadow Maps dynamic shadows are relevant for Stationary and Moveable lights.
• Static lights normally rely on baked shadows (Lightmass). If you're using Force No Precomputed Lighting, you won't get baked shadows anyway.

• You're maintaining a legacy project or pipeline that already relies on Shadow Maps tuning
• You're intentionally avoiding VSM for compatibility or simplicity reasons

In project settings:

Edit → Project Settings → Engine → Rendering → Shadows → Shadow Map Method

• Shadow Maps = Traditional shadow mapping (CSM for Directional, standard for local lights)
• Virtual Shadow Maps = Modern VSM workflow

(TODO: Add screenshot of Shadow Map Method setting.)

When using the Shadow Maps method, CSM tuning for Directional Lights happens in the light's Details panel:

• Dynamic Shadow Distance (MovableLight / StationaryLight): how far CSM shadows render
• Num Dynamic Shadow Cascades: how many cascades (more = higher quality, higher cost)
• Cascade Distribution Exponent: pushes more resolution near camera (higher = tighter near camera)
• Cascade Transition Fraction: blends between cascades to reduce visible "bands"

Tip: If you see cascade "popping" while moving the camera, increase the transition fraction and/or adjust the distribution exponent.

All lights using the Shadow Maps method produce hard shadows by default:

This is a fundamental limitation of depth-based shadow mapping. The shadow map stores "is this pixel in shadow?" as a binary yes/no, resulting in sharp edges.

To get soft shadows in Shadow Maps mode, you have two options:

Option 1: PCSS Filtering (affects all lights)

r.Shadow.FilterMethod 1

This enables Percentage-Closer Soft Shadows (PCSS), which dynamically adjusts shadow softness based on the distance between occluder and receiver.

Note: PCSS adds a performance cost and may introduce aliasing artifacts in some scenarios.

Option 2: Increase Source Radius (local lights only)

For Point, Spot, and Rect lights, increasing the Source Radius property (or Source Width/Height for Rect Lights) will produce softer shadows — but only when combined with PCSS or other filtering.

• Hard shadow edges: All lights produce hard shadows by default
• Cascade lines / popping: (Directional only) transitions between cascades become visible during movement
• Aliasing / stair-step edges: limited resolution, especially in mid/far cascades
• Shadow "swimming": (Directional only) subtle instability as the cascades re-fit around the camera

VSM uses massive 16K×16K shadow textures split into tiles, with different structures per light type:

1. Scene is divided into pages/tiles
2. Only visible tiles get rendered
3. Tiles near camera get higher resolution
4. Distant tiles get lower resolution
5. Results are cached and reused across frames

Note: Local lights provide significant resolution increase vs traditional shadow maps, but very large local lights can run out of virtual resolution. Use Directional Lights for large-scale shadows.

VSM is the right default for most projects:

• Works with all light mobilities (Static, Stationary, Moveable)
• Works on any GPU — no special hardware required
• Excellent shadow detail and resolution
• Handles many lights efficiently
• Good performance with static or slow-moving content

VSM behaves differently depending on distance and mesh type:

Far Distance:

• Nanite vs Non-Nanite meshes may have different shadow behavior at long distances
• VSM clipmaps handle this automatically, but you may see quality differences

Close Distance:

• Generally excellent detail due to high virtual resolution
• The clipmap system allocates more pages near the camera

VSM builds quality over multiple frames through temporal accumulation. When the camera or objects move quickly:

• Flickering at shadow edges
• Shimmering artifacts
• A "crawling" look during camera motion

This isn't a bug — it's the cache updating. Usually subtle, but noticeable on fast-moving shots.

VSM calculates soft shadows mathematically based on Source Radius/Angle and distance. The softness looks good, but it's an approximation — all lights produce the same generic soft edge regardless of their actual shape.

Penumbra noise is controlled by ray count. At Epic scalability, both local and Directional Lights use 8 rays by default.

• Fewer rays = visible noise in penumbra
• Setting to 0 = disables SMRT, reverts to single-sample hard shadows

Most VSM controls live under these namespaces:

• r.Shadow.Virtual.* — general VSM settings (resolution, caching, page management)
• r.Shadow.Virtual.SMRT.* — soft shadow ray tracing settings (ray counts, samples)

Tip: Use r.Shadow.Virtual. in the console and press Tab to see all available options.

To visualize how VSM is working, use the viewport debug modes:

Show → Virtual Shadow Map → [Visualization Mode]

VSM Debug Visualization

Tip: VSM is already enabled by default. You don't need to change any settings unless you want to use a different shadow system.

A distance field stores, for every point in space around a mesh, how far away the nearest surface is. This pre-computed data lets the engine quickly determine shadow coverage without tracing individual rays.

Distance fields are generated at mesh import time, but the system must be enabled in project settings.

Enable Mesh Distance Fields

Required for Distance Field Shadows.

Edit → Project Settings → Engine → Rendering → Software Ray Tracing

☑ Generate Mesh Distance Fields

After enabling, reimport meshes or restart the editor for distance fields to generate.

Per-Light Enable:

Distance Field Shadows Setting

• Select the light actor
• Details panel → Distance Field Shadows: Enabled

Distance Field Settings

This is an important distinction for Directional Lights:

Why the difference? CSM and Distance Fields are separate systems that can complement each other. VSM is a unified system that replaces both — it handles near and far shadows internally, so Distance Fields become an alternative rather than a supplement.

Good for:

• Large open worlds with distant shadows
• Soft area shadows from Directional Lights
• Stylized projects where approximate softness is acceptable

Limitations:

• Resolution depends on distance field quality
• Thin geometry may not cast accurate shadows
• Memory overhead from distance field data
• Less accurate than hardware ray tracing for close-up area light shadows

Note: With CSM workflow, Distance Field Shadows and CSM can work together — DF for distant/soft shadows, CSM for nearby detail. With VSM, you choose one or the other.

To visualize mesh distance fields in the viewport:

Show → Visualize → Mesh DistanceFields

Mesh Distance Fields Visualization

If meshes appear pink/magenta, distance fields are not generated for them — check that Generate Mesh Distance Fields is enabled and reimport the meshes.

With hardware ray tracing, Unreal shoots rays from surfaces toward the light source:

• Point lights: One ray toward a single point — simple shadow
• Area lights (Rect Light, lights with Source Radius): Multiple rays toward different points across the light's surface

The result: shadow shapes that actually reflect the light's geometry. A Rect Light casts a shadow with a rectangular penumbra. A light with Source Radius creates round soft edges. This matches real-world photography.

Use RT shadows when:

• You need accurate shadow shapes from area lights
• You're doing cinematics or virtual production
• Shadow quality is critical to your visual target
• You have RTX hardware and frame budget to spare

Stick with VSM/Distance Fields when:

• You need to support non-RTX hardware
• Performance is critical (real-time gameplay)
• Shadow shape accuracy isn't important to your art style

Tip: Many projects use both — VSM for gameplay, RT for cutscenes. You can switch per-light or globally.

Far Distance — Culling CVars:

At far distances, Unreal culls objects from the ray tracing scene (TLAS) to save performance. This can cause shadows to disappear on distant objects.

Solid-Angle Culling Explained:

The r.RayTracing.Culling.Angle CVar controls "cone" culling — it removes objects that appear too small in angle from the camera's perspective, especially outside the direct view.

• Base threshold is 5 degrees, and the CVar is a multiplier
• Default 1 = ~5° threshold
• 0.5 = ~2.5° (less aggressive, keeps more geometry)
• 0 = disable culling entirely (expensive)

Tip: If distant objects are missing RT shadows, try r.RayTracing.Culling 0 to test, then tune the radius/angle for your scene.

Close Distance — Near Clip:

For very close objects, RT can miss geometry due to camera near clip settings. If RT shadows disappear on close objects:

r.SetNearClipPlane 0.005

This allows RT to trace geometry much closer to the camera (useful for product visualization, VR, etc.).

Step 1: Enable Hardware Ray Tracing

Enable Hardware Ray Tracing

Master switch for all RT features. Requires editor restart.

Edit → Project Settings → Engine → Rendering → Hardware Ray Tracing

☑ Support Hardware Ray Tracing

Hardware Ray Tracing Settings

Step 2: Enable RT Shadows

Per-Light (Recommended) — Selective control:

• Select light actor
• Details panel → Cast Ray Traced Shadows: Enabled

Cast Ray Traced Shadows - Use Project Setting

Cast Ray Traced Shadows - Enabled

Global — All lights use RT:

Enable Ray Traced Shadows Globally

Enables RT shadows for all lights in the project at once.

Edit → Project Settings → Engine → Rendering → Ray Tracing

☑ Ray Traced Shadows

Ray Traced Shadows Global Setting

RT shadows use sampling — more rays = cleaner shadows but higher GPU cost.

Why noise happens: With limited rays, you're sampling a probability. 4 rays might give 2 blocked, 2 unblocked = 50% shadow. More samples averages this out for cleaner results.

Per-Light: Details panel → Ray Tracing → Samples Per Pixel

Global: Console variable r.RayTracing.Shadow.SamplesPerPixel [value]

Samples Per Pixel - 1 sample

1 Sample - Noisy Shadow

Samples Per Pixel - 10 samples

10 Samples - Clean Shadow

Traditional deferred lighting tends to have constant quality but cost scales with number of lights.

MegaLights flips that:

• More constant performance cost
• Quality can decrease when too many important lights hit the same pixels (denoiser has to work harder)

• Dense scenes with lots of fixtures (city nights, sci‑fi corridors, interiors with practicals)
• Lots of shadowed local lights where VSM/RT-per-light would be too expensive
• Textured area lights and volumetric fog lighting that would otherwise blow your budget

• Enable in Project Settings → Rendering → Direct Lighting → MegaLights
• MegaLights is designed around Hardware Ray Tracing support
• Per light, you can disable it with Allow MegaLights
• Per light, you can choose MegaLights Shadow Method:
  • Ray Tracing (default/recommended): no extra per-light shadow map cost, but depends on ray tracing scene quality
  • Virtual Shadow Maps: captures full Nanite detail, but adds per-light overhead and only approximates area shadows

• Avoid lights with huge attenuation bounds affecting the whole level
• Don't place lights inside geometry (wasted samples + can increase noise)
• Merge "clusters of tiny lights" into a few larger area lights where possible

r.MegaLights.Allow 0

Key Points:

• UE5 supports multiple shadow systems, including legacy CSM and modern VSM
• VSM works on any GPU with good quality; shows temporal artifacts on fast motion
• Distance Fields enable long-distance soft shadows (Stationary/Moveable only)
• Hardware RT produces accurate shadow shapes but requires RTX hardware
• MegaLights (UE5.5 experimental) enables hundreds of dynamic shadowed lights with constant performance
• Choose based on your hardware targets, performance budget, and visual requirements

You now know how to create direct lighting and choose the right shadow system. But how do you know if your lighting is too expensive? In Part 5: Performance & Debugging, we'll learn the tools to measure, diagnose, and optimize your lighting setup.