V-Ray Tip: V-Ray Light Cache: Fast, Stable Secondary GI Tuning

Light Cache is still the fastest way to build secondary GI in V-Ray. Dialing in its sample settings lets you balance stability, detail, and speed with precision.

Foundations

• Scene scale first: Sample Size is in world units. If your scene scale is off, Light Cache (LC) will either miss detail (too large) or get noisy and slow (too small).
• Use Brute Force for Primary GI and Light Cache for Secondary GI as a baseline; it’s predictable and easy to tune.

Key Light Cache controls

• Subdivs/Samples: Increase to reduce blotches in indirect lighting. Interiors with deep bounces and small apertures typically need more samples than open exteriors.
• Sample Size (world units): Controls GI detail captured by LC. Smaller values resolve tighter features (corners, trim, thin walls) but cost time/memory. Larger values run faster but can smear detail.
• Retrace Threshold: Retraces near-contact areas with accurate rays to fix light leaks, overly bright corners, and thin-geometry artifacts. Higher values improve contact quality at a small cost.
• Prefilter / Filter: Prefiltering smooths the cache before rendering and is helpful for animation flicker control. Filtering blends samples at render time for extra stability.
• Use Light Cache for Glossy Rays: Can speed interiors with lots of glossy surfaces, but may soften highlights and micro-detail. Keep off for approvals; toggle on when you need throughput.
• Mode & Reuse: For camera fly-throughs, build one cache with “Use camera path / Fly-through” and reuse it across frames. Save to file for render farms and consistency.

Practical starting points

• Typical interiors
  • Subdivs/Samples: medium–high
  • Sample Size: roughly 1–3% of a room’s characteristic dimension
  • Retrace Threshold: 0.8–1.0 for robust contact shadows
  • Prefilter: 10–20 for animation, 0–10 for stills
• Typical exteriors
  • Subdivs/Samples: low–medium
  • Sample Size: larger than interiors (features are broader)
  • Retrace Threshold: 0.5–1.0 depending on façade detail
  • Prefilter: low; exteriors flicker less by nature

Fast tuning workflow

• Block-in lighting and materials; render regions in Progressive mode.
• Start with a moderate Sample Size. If corners look washed or GI misses alcoves, shrink it. If render time and memory spike, increase it slightly.
• Raise Subdivs until blotches disappear at your target noise threshold.
• Enable Retrace to clean up contact areas; push up only as needed.
• For animations, add Prefilter, then cache with camera path and reuse.

Troubleshooting

• Splotchy GI: Increase Subdivs; consider a smaller Sample Size; check for overly glossy materials receiving LC.
• Flicker frame-to-frame: Use camera path/fly-through cache, enable Prefilter, and avoid tiny Sample Size for large scenes.
• Light leaks/over-bright corners: Raise Retrace Threshold; verify watertight geometry and correct normals.
• Softened glossy detail: Disable “use LC for glossy rays” on critical shots or only for select materials.

Production notes

• Lock, save, and version your LC files for repeatable results on farms.
• When in doubt, prioritize stable GI first, then optimize for speed.

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