Support alpha in Standard Surface to glTF PBR translation (AcademySoftwareFoundation#2591)

closes AcademySoftwareFoundation#2579

This converts opacity to alpha as implemented in standard_surface.mtlx, i.e with the limitation of monochromatic opacity.

Author: bowald

libraries/bxdf/translation/standard_surface_to_gltf_pbr.mtlx

@@ -22,6 +22,7 @@
     <input name="emission_color" type="color3" value="1, 1, 1" />
     <input name="normal" type="vector3" defaultgeomprop="Nworld" />
     <input name="tangent" type="vector3" defaultgeomprop="Tworld" />
+    <input name="opacity" type="color3" value="1, 1, 1" />
 
     <output name="base_color_out" type="color3" />
     <output name="metallic_out" type="float" />
@@ -40,6 +41,8 @@
     <output name="emissive_strength_out" type="float" />
     <output name="normal_out" type="vector3" />
     <output name="tangent_out" type="vector3" />
+    <output name="alpha_out" type="float" />
+    <output name="alpha_mode_out" type="integer" />
   </nodedef>
 
   <nodegraph name="NG_standard_surface_to_gltf_pbr" nodedef="ND_standard_surface_to_gltf_pbr">
@@ -213,6 +216,23 @@
       <input name="in" type="vector3" interfacename="tangent" />
     </dot>
 
+    <!-- Opacity -->
+    <luminance name="opacity_luminance" type="color3">
+      <input name="in" type="color3" interfacename="opacity" />
+    </luminance>
+
+    <extract name="opacity_luminance_float" type="float">
+      <input name="in" type="color3" nodename="opacity_luminance" />
+      <input name="index" type="integer" value="0" />
+    </extract>
+
+    <ifequal name="alpha_mode_from_opacity_luminance" type="integer">
+      <input name="value1" type="float" nodename="opacity_luminance_float" />
+      <input name="value2" type="float" value="1.0" />
+      <input name="in1" type="integer" value="0" />
+      <input name="in2" type="integer" value="2" />
+    </ifequal>
+
     <output name="base_color_out" type="color3" nodename="base_color" />
     <output name="metallic_out" type="float" nodename="metallic" />
     <output name="roughness_out" type="float" nodename="roughness" />
@@ -230,6 +250,8 @@
     <output name="emissive_strength_out" type="float" nodename="emissive_strength" />
     <output name="normal_out" type="vector3" nodename="normal" />
     <output name="tangent_out" type="vector3" nodename="tangent" />
+    <output name="alpha_out" type="float" nodename="opacity_luminance_float" />
+    <output name="alpha_mode_out" type="integer" nodename="alpha_mode_from_opacity_luminance" />
 
   </nodegraph>
 </materialx>