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Propose zero-weight BSDF layer semantics. #2971

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Propose zero-weight BSDF layer semantics.
tdavidovicNV May 31, 2026
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tdavidovicNV Jun 9, 2026
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Merge branch 'main' into dev/tdavidovic/zero_weight_proposal
tdavidovicNV Jun 10, 2026
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tdavidovicNV Jun 12, 2026
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30 changes: 23 additions & 7 deletions documents/Specification/MaterialX.Proposals.md
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Expand Up @@ -5,7 +5,7 @@ MaterialX Proposals v1.39

# MaterialX: Proposed Additions and Changes

**Proposals for Version 1.39**
**Proposals for Version 1.39**
September 15, 2024


Expand All @@ -19,17 +19,17 @@ As such, those forward-looking proposals have been moved from the formal Specifi

## Table of Contents

**[Introduction](#introduction)**
**[Introduction](#introduction)**

**[Proposals: General](#propose-general)**
**[Proposals: General](#propose-general)**

**[Proposals: Elements](#propose-elements)**
**[Proposals: Elements](#propose-elements)**

**[Proposals: Stdlib Nodes](#propose-stdlib-nodes)**
**[Proposals: Stdlib Nodes](#propose-stdlib-nodes)**

**[Proposals: PBR Nodes](#propose-pbr-nodes)**
**[Proposals: PBR Nodes](#propose-pbr-nodes)**

**[Proposals: NPR Nodes](#propose-npr-nodes)**
**[Proposals: NPR Nodes](#propose-npr-nodes)**


<p>&nbsp;<p><hr><p>
Expand Down Expand Up @@ -393,6 +393,22 @@ Output the RGB and alpha channels of a color4 as separate outputs.

# Proposals: PBR Nodes<a id="propose-pbr-nodes"></a>

### 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.


<p>&nbsp;<p><hr><p>
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