In this lesson, you will learn how to shift Lumen from its default software-based tracing to Hardware Ray Tracing (HWRT) to achieve pixel-perfect reflections and sharper, physically accurate shadows.
The Shift to Hardware Ray Tracing
While Lumen’s default software ray tracing relies on Mesh Distance Fields—which are essentially low-resolution approximations of your scene—Hardware Ray Tracing utilizes your GPU’s dedicated RT cores. This allows the engine to intersect rays directly against the high-fidelity Nanite geometry or the raw mesh triangles, rather than a simplified representation.
When you enable this, you aren't just boosting resolution; you are solving "leaking" issues often seen in complex, thin-geometry scenes where Distance Fields lack the precision to capture fine-grained occlusion.
Enabling the Integration
To integrate this, you must first ensure your project is configured for hardware acceleration. Navigate to Project Settings > Rendering and under the Hardware Ray Tracing section, check Support Hardware Ray Tracing.
Once enabled, you need to tell Lumen to prefer this method. In your Post Process Volume settings, under Lumen > Reflections and Lumen > Global Illumination, change the Lumen Scene Quality or specific tracing modes to "Hardware Ray Tracing."
You will notice an immediate change in the behavior of your reflections:
// Within the Project Settings/Rendering/Lumen category:
// Ensure "Software Ray Tracing" is disabled or "Hardware Ray Tracing" is toggled
r.Lumen.HardwareRayTracing = 1
Why It Matters
When using HWRT with Lumen, your reflections are no longer limited by the resolution of the Lumen Scene (the simplified card-based representation). HWRT allows for:
- Precise Occlusion: Shadows cast by small or thin objects that would otherwise be ignored by distance fields.
- High-Fidelity Reflections: Reflections that correctly capture the geometric complexity of your Nanite meshes, even at oblique angles.
Keep in mind that while this offers the highest visual fidelity, it significantly increases the GPU cost. It is best used for high-end cinematic targets or "Ultra" graphics presets where your hardware budget allows for the overhead of direct triangle intersection.
Key Takeaway: Integrating Hardware Ray Tracing allows Lumen to bypass the limitations of Mesh Distance Fields by performing direct ray-triangle intersections, delivering the highest possible fidelity for complex shadows and reflections at the cost of higher GPU overhead.