From 01fc05fdc7415a357b0845ea27ad14a416fe2e20 Mon Sep 17 00:00:00 2001
From: Jonathan Stone <jstone@lucasfilm.com>
Date: Wed, 4 Jan 2023 12:23:33 -0800
Subject: [PATCH] Fix opacity edge case in UsdPreviewSurface (#1186)

This changelist fixes an edge case for the opacity computation in UsdPreviewSurface, making surfaces visible when their non-zero opacity is exactly equal to the opacity threshold.

This aligns the MaterialX graph for UsdPreviewSurface with its specification (https://graphics.pixar.com/usd/release/spec_usdpreviewsurface.html), which states:

"... when opacityThreshold is greater than zero, the opacity values less than the opacityThreshold will not be rendered, and the opacity values greater than or equal to the opacityThreshold will be fully visible."
---
 libraries/bxdf/usd_preview_surface.mtlx | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/libraries/bxdf/usd_preview_surface.mtlx b/libraries/bxdf/usd_preview_surface.mtlx
index 5b9ac546e1..916a7a746f 100644
--- a/libraries/bxdf/usd_preview_surface.mtlx
+++ b/libraries/bxdf/usd_preview_surface.mtlx
@@ -272,12 +272,12 @@
       <input name="low" type="float" value="0.00001" />
       <input name="high" type="float" value="1.0" />
     </clamp>
-    <ifgreater name="cutout_opacity" type="float">
+    <ifgreatereq name="cutout_opacity" type="float">
       <input name="value1" type="float" nodename="opacity_clamped" />
       <input name="value2" type="float" interfacename="opacityThreshold" />
       <input name="in1" type="float" value="1" />
       <input name="in2" type="float" value="0" />
-    </ifgreater>
+    </ifgreatereq>
     <surface name="surface_constructor" type="surfaceshader">
       <input name="bsdf" type="BSDF" nodename="coat_bsdf" />
       <input name="edf" type="EDF" nodename="emission_edf" />