V-Ray Tip: VRayMtlWrapper Shadow-Catcher and Holdout Workflow

Use VRayMtlWrapper to turn any surface into a controllable matte/shadow catcher for precise live-action compositing and clean AOVs—without disrupting lighting in the rest of the scene.

What VRayMtlWrapper enables:

• Matte surface: Hide geometry from camera while preserving shadows, reflections, and contact interactions for CG integration.
• Alpha control: Drive how the object contributes to the alpha (including negative contribution for holdouts).
• Independent toggles: Generate/receive GI, receive/cast shadows, and visibility in reflections/refractions—individually.
• Flexible passes: Cleaner shadow and holdout passes with fewer comp hacks.

Quick setup: shadow catcher on a ground plane

• Assign VRayMtlWrapper to the ground.
• Enable “Matte surface.”
• Alpha contribution: -1.0 (fully removes the ground from RGB while preserving shadow detail in alpha).
• Receive shadows: On. Affect alpha: On (so shadows imprint in the alpha channel).
• GI: Set Generate GI = 0 and Receive GI = 0 to avoid the invisible ground influencing GI; enable only if you need secondary bounce realism on the CG.
• Shadow brightness: Use 1.0 for physically faithful shadows. Raise slightly (e.g., 1.1–1.2) to match plate lift or reduce overly deep shadows.
• Reflections/refractions: On if you want CG to pick up plate/environment reflections from the matte surface; Off for a pure shadow pass.
• Preview with your plate in the VFB background to judge contact and density. Composite the EXR over the plate (straight/unpremultiplied preferred).

Holdout and object occlusion

• Use Matte surface On + Alpha contribution -1.0 to create a holdout object that occludes CG behind it.
• Keep “Visible in reflections/refractions” On if the holdout should appear in glossy/transparent objects; Off if not.
• For plate-matched edges, add a small VRayDirt pass for subtle contact darkening around intersections.

Alpha and compositing checks

• Render 16/32-bit EXR with separate shadow/mask elements for grading headroom.
• Use Premultiplied alpha in delivery only if required by the pipeline; otherwise keep Straight for maximum flexibility.
• If using Light Selects or LightMix, verify that matte contributions remain consistent across relit states.

Performance and quality tips

• Disable GI on large matte surfaces to avoid unnecessary rays and color bleed.
• Use Adaptive Lights for scenes with many emitters to stabilize shadow noise on the matte.
• Clamp fireflies early (VFB “Clamp output” or material roughness tuning) to keep shadow passes clean.
• Cryptomatte plus Wrapper mattes gives robust object/ID isolation for last‑minute comp adjustments.

Common pitfalls to avoid

• Double-multiplying the plate: keep the plate out of the beauty render; composite it in post.
• Mismatched color space: ensure the plate and CG share the same display transform (e.g., ACEScg to display) before judging shadow density.
• Incorrect alpha: if the ground shows in RGB, confirm Alpha contribution = -1 and Matte surface is enabled.

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