@@ -393,6 +393,22 @@ Output the RGB and alpha channels of a color4 as separate outputs. # Proposals: PBR Nodes +### Zero-Weight BSDF and EDF Distribution Values + +The PBS library should define empty values for BSDF and EDF distributions. An empty BSDF represents no surface scattering contribution, and an empty EDF represents no emission contribution. + +This clarification is needed because the same authored graph can currently have target-dependent results. For example, consider a `layer(top=multiply(dielectric_bsdf, 0.0), base=oren_nayar_diffuse_bsdf)`. In GLSL, `dielectric_bsdf` computes a non-identity throughput, `multiply` clears the response but preserves that throughput, and `layer` still attenuates the diffuse base. In OSL, `multiply` is emitted as closure multiplication, so `0.0 * dielectric_bsdf` is the null closure and `layer` returns the diffuse base unchanged. + +Multiplying a BSDF or EDF by zero, should produce the empty distribution of the corresponding type. This rule applies only to distribution-function composition. + +For vertical layering, the empty BSDF should have zero reflectance and act as the identity top layer: + +```text +layer(empty, base) = base +layer(multiply(top, 0), base) = base +``` + +BSDF and EDF nodes with zero weights should produce empty distribution values, unless the node explicitly documents independent non-distribution effects.