glTF Layer Order: Clearcoat, Iridescence, Sheen
When combining clearcoat, iridescence, and sheen on a single glTF surface, the material system evaluates layers in a strict bottom-to-top hierarchy to maintain physical plausibility and energy conservation. Iridescence acts directly on the base substrate to modify its specular reflectance, sheen sits above the base substrate to simulate surface microfibers, and clearcoat acts as the topmost protective layer covering everything else.
1.
The Substrate with Iridescence
(KHR_materials_iridescence)
The bottom of the evaluation stack is the standard PBR metallic-roughness base layer. When iridescence is present, it does not act as an independent outer coating; instead, it modifies the specular reflectance (Fresnel response) of the base substrate itself.
- The thin-film interference equations calculate phase shifts and wavelength-dependent reflections.
- These interference colors replace or blend with the standard dielectric or conductor specular response of the base surface.
- Diffuse contributions from the base substrate are tinted or attenuated accordingly based on the energy transmitted through the thin film.
2. The
Intermediate Sheen Layer (KHR_materials_sheen)
The sheen layer simulates soft, micro-fuzz reflections typically found on velvet or fabric. In the glTF layering stack, sheen sits directly on top of the iridescent base substrate, but beneath any clearcoat.
- Sheen adds an independent specular sheen lobe using the Charlie or similar microfacet distribution.
- To conserve energy, the sheen layer absorbs incoming and outgoing light, scaling down (attenuating) the underlying iridescent base layer's specular and diffuse contributions based on the sheen's albedo and roughness.
3. The Top
Clearcoat Layer (KHR_materials_clearcoat)
The clearcoat extension represents a smooth, transparent resin or lacquer applied over the entire surface. It occupies the highest level of the layer stack.
- The clearcoat specular reflection is computed first from the perspective of an external light source, using its own distinct roughness and normal maps.
- Because clearcoat reflects a portion of the incoming light at the outermost boundary, it attenuates all transmission to and reflection from the layers beneath it.
- Both the sheen layer and the iridescent base layer are darkened in
proportion to the clearcoat's Fresnel reflectance factor
(
1 - F_clearcoat), ensuring energy conservation across the composite material.
Composite Evaluation Summary
In terms of rendering composition and shading calculations:
- Base Layer: Compute base diffuse and specular terms, factoring in thin-film interference from Iridescence.
- Apply Sheen: Add the Sheen specular lobe on top, attenuating the iridescent base layer according to sheen reflectance.
- Apply Clearcoat: Add the Clearcoat specular lobe on the outer surface, attenuating the combined result of the sheen and iridescent base layers beneath it.