V-Ray Tip: V-Ray Caustics Strategy: When to Render True Caustics versus Fake

Caustics are gorgeous but costly. Use them strategically, and fake them when possible to keep renders predictable and fast.

When to render true caustics

• Hero closeups of glass, gemstones, liquids, or polished metals where energy focus, parallax, and edge sparkle are story-critical.
• Product shots that must match photography (watches, perfume, optics) where refractive realism informs material credibility.
• Sequences with significant motion or light changes where baked or projected fakes won’t hold up.

How to keep real caustics under control

• Enable the Caustics solver only for the shots that demand it; disable caustics generation on non-essential lights to reduce noise.
• Drive caustics from a single small, bright source (or the sun). Broad fill lights contribute noise with minimal perceived benefit.
• Set reasonable trace depths: Reflection 4–6, Refraction 6–8. Higher depths can explode render time with marginal quality gains.
• Isolate output: render the Caustics element and composite additively over Beauty. Exclude it from denoising to preserve fine detail.
• Use conservative highlight clamping to tame fireflies, then confirm with VFB History/Compare to ensure dynamic range is intact.
• Validate real-world scale, normals, and IOR. Incorrect scales or non-solid glass create unstable and noisy caustics.
• For thin glass fakes, “Affect shadows” may help, but turn it off when you enable true caustics to avoid double-darkening.
• Consider Light Path Expressions to isolate higher-order specular paths for grading without touching the rest of the image.

Fast and believable fakes

• Project a caustic gobo: map a high-contrast caustics texture to a VRayLight or VRayDecal aimed at the receiver; animate UVs/rotation for water shimmer.
• Bake once, reuse often: render a high-quality caustics pass from a representative rig, bake to a texture, then reproject via Triplanar or Decal across shots.
• Use animated normal/displacement on water plus a subtle luminance modulation (VRayCompTex) to suggest sparkle without heavy specular paths.
• Keep fakes on their own Light Select/Render Element so Light Mix or comp can adjust intensity and color without re-rendering.
• HDRI interiors: rely on Adaptive Dome Light for base GI and add a projected caustic pattern only where the audience expects it (pool floors, tabletops).

Workflow tips

• Start with a Material Override to establish a baseline. Add true caustics last and measure time/quality deltas.
• Iterate with Region/Render Mask around specular edges and high-contrast contact patches where caustics concentrate.
• On GPU, monitor VRAM; refractive meshes and large HDRIs inflate memory. Use RTX acceleration where supported and keep textures optimized.
• Document per-light caustics toggles and intensities so farm renders remain consistent across shots and artists.

Rule of thumb: If physical accuracy won’t be noticed (wide shots, heavy motion blur/DOF), fake it. Save true caustics for moments where they carry the frame.

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