Arnold : Prefer ai:volume over ai:surface for volume objects

Fixes #5830.

Changes.md

@@ -23,13 +23,15 @@ Improvements
   - Added Arnold-specific extension parameters.
   - Added parameter tooltips.
 - LightEditor : Added columns for Arnold-specific parameters on USD lights.
+- Arnold : If it exists, an `ai:volume` attribute is preferred over an `ai:surface` attribute when resolving shaders for volumes.
 
 Fixes
 -----
 
 - Viewer : Fixed Cycles shader balls.
 - TweakPlug : Fixed incorrect results and potential crashes in list modes.
 - USD : Fixed `Unsupported value type "StringData" for parameter "input"` warning when converting `UsdTransform2d` shaders with no `in` connections to Arnold.
+- Arnold : Fixed rendering of `ai:volume` shaders loaded from USD (#5830).
 
 API
 ---

python/IECoreArnoldTest/RendererTest.py

@@ -4314,6 +4314,58 @@ def testArnoldOSLWithCode( self ) :
 
 		del renderer
 
+	def testVolumeShaderAttribute( self ) :
+
+		import IECoreVDB
+
+		renderer = GafferScene.Private.IECoreScenePreview.Renderer.create(
+			"Arnold",
+			GafferScene.Private.IECoreScenePreview.Renderer.RenderType.Batch,
+			str( self.temporaryDirectory() / "test.ass" )
+		)
+		universe = ctypes.cast( renderer.command( "ai:queryUniverse", {} ), ctypes.POINTER( arnold.AtUniverse ) )
+
+		for withVolume in ( True, False ) :
+			for withSurface in ( True, False ) :
+				for step in ( 0.0, 0.1 ) :
+					with self.subTest( withVolume = withVolume, withSurface = withSurface, step = step ) :
+
+						attributes = IECore.CompoundObject( { "ai:shape:step_size" : IECore.FloatData( step ) } )
+
+						if withVolume :
+							attributes["ai:volume"] = IECoreScene.ShaderNetwork(
+								shaders = { "shader" : IECoreScene.Shader( "standard_volume" ) },
+								output = ( "shader", "out" ),
+							)
+
+						if withSurface :
+							attributes["ai:surface"] = IECoreScene.ShaderNetwork(
+								shaders = { "shader" : IECoreScene.Shader( "standard_surface" ) },
+								output = ( "shader", "out" ),
+							)
+
+						attributes = renderer.attributes( attributes )
+
+						meshName = f"mesh{withVolume}{withSurface}{step}"
+						renderer.object( meshName, IECoreScene.MeshPrimitive.createSphere( 1 ), attributes )
+
+						vdbName = f"vdb{withVolume}{withSurface}{step}"
+						renderer.object( vdbName, IECoreVDB.VDBObject(), attributes )
+
+						meshShader = arnold.AiNodeGetPtr( arnold.AiNodeLookUpByName( universe, meshName ), "shader" )
+						self.assertEqual(
+							arnold.AiNodeEntryGetName( arnold.AiNodeGetNodeEntry( meshShader ) ),
+							"standard_volume" if ( withVolume and step ) else "standard_surface" if withSurface else "utility"
+						)
+
+						vdbShader = arnold.AiNodeGetPtr( arnold.AiNodeLookUpByName( universe, vdbName ), "shader" )
+						self.assertEqual(
+							arnold.AiNodeEntryGetName( arnold.AiNodeGetNodeEntry( vdbShader ) ),
+							"standard_volume" if withVolume else "standard_surface" if withSurface else "utility"
+						)
+
+		del renderer
+
 	def testInternedStringAttributes( self ) :
 
 		r = GafferScene.Private.IECoreScenePreview.Renderer.create(

src/IECoreArnold/Renderer.cpp

@@ -922,6 +922,7 @@ namespace
 bool isConvertedProcedural( const AtNode *node );
 
 IECore::InternedString g_surfaceShaderAttributeName( "surface" );
+IECore::InternedString g_volumeShaderAttributeName( "volume" );
 IECore::InternedString g_lightShaderAttributeName( "light" );
 IECore::InternedString g_doubleSidedAttributeName( "doubleSided" );
 IECore::InternedString g_setsAttributeName( "sets" );
@@ -951,6 +952,7 @@ IECore::InternedString g_volumeVisibilityAutoBumpAttributeName( "ai:autobump_vis
 IECore::InternedString g_subsurfaceVisibilityAutoBumpAttributeName( "ai:autobump_visibility:subsurface" );
 
 IECore::InternedString g_arnoldSurfaceShaderAttributeName( "ai:surface" );
+IECore::InternedString g_arnoldVolumeShaderAttributeName( "ai:volume" );
 IECore::InternedString g_arnoldLightShaderAttributeName( "ai:light" );
 IECore::InternedString g_arnoldFilterMapAttributeName( "ai:filtermap" );
 IECore::InternedString g_arnoldUVRemapAttributeName( "ai:uv_remap" );
@@ -1059,6 +1061,13 @@ class ArnoldAttributes : public IECoreScenePreview::Renderer::AttributesInterfac
 				m_surfaceShader = shaderCache->get( surfaceShaderAttribute, attributes );
 			}
 
+			const IECoreScene::ShaderNetwork *volumeShaderAttribute = attribute<IECoreScene::ShaderNetwork>( g_arnoldVolumeShaderAttributeName, attributes );
+			volumeShaderAttribute = volumeShaderAttribute ? volumeShaderAttribute : attribute<IECoreScene::ShaderNetwork>( g_volumeShaderAttributeName, attributes );
+			if( volumeShaderAttribute )
+			{
+				m_volumeShader = shaderCache->get( volumeShaderAttribute, attributes );
+			}
+
 			if( auto filterMapAttribute = attribute<IECoreScene::ShaderNetwork>( g_arnoldFilterMapAttributeName, attributes ) )
 			{
 				m_filterMap = shaderCache->get( filterMapAttribute, attributes );
@@ -1375,9 +1384,9 @@ class ArnoldAttributes : public IECoreScenePreview::Renderer::AttributesInterfac
 
 				AiNodeSetBool( node, g_matteArnoldString, m_shadingFlags & ArnoldAttributes::Matte );
 
-				if( m_surfaceShader && m_surfaceShader->root() )
+				if( AtNode *shader = preferredShader( geometry ) )
 				{
-					AiNodeSetPtr( node, g_shaderArnoldString, m_surfaceShader->root() );
+					AiNodeSetPtr( node, g_shaderArnoldString, shader );
 				}
 				else
 				{
@@ -1995,6 +2004,7 @@ class ArnoldAttributes : public IECoreScenePreview::Renderer::AttributesInterfac
 			h.append( m_sidedness );
 			h.append( m_shadingFlags );
 			hashOptional( m_surfaceShader.get(), h );
+			hashOptional( m_volumeShader.get(), h );
 			hashOptional( m_filterMap.get(), h );
 			hashOptional( m_uvRemap.get(), h );
 			hashOptional( m_lightShader.get(), h );
@@ -2018,10 +2028,33 @@ class ArnoldAttributes : public IECoreScenePreview::Renderer::AttributesInterfac
 			h.append( m_sssSetName.c_str() ? m_sssSetName.c_str() : "" );
 		}
 
+		AtNode *preferredShader( const AtNode *geometry ) const
+		{
+			if( m_volumeShader )
+			{
+				// Prefer the volume shader if we have one, and the geometry is either
+				// a volume or a shape being rendered as a volume (non-zero step size).
+				bool preferVolume = AiNodeIs( geometry, g_volumeArnoldString );
+				if( !preferVolume && AiNodeEntryLookUpParameter( AiNodeGetNodeEntry( geometry ), g_stepSizeArnoldString ) )
+				{
+					preferVolume = AiNodeGetFlt( geometry, g_stepSizeArnoldString ) > 0.0f;
+				}
+				if( preferVolume )
+				{
+					return m_volumeShader->root();
+				}
+			}
+
+			// Otherwise use the surface shader. We use this even for volume geometry,
+			// because Gaffer has always assigned volume shaders as `ai:surface`.
+			return m_surfaceShader ? m_surfaceShader->root() : nullptr;
+		}
+
 		unsigned char m_visibility;
 		unsigned char m_sidedness;
 		unsigned char m_shadingFlags;
 		ArnoldShaderPtr m_surfaceShader;
+		ArnoldShaderPtr m_volumeShader;
 		ArnoldShaderPtr m_filterMap;
 		ArnoldShaderPtr m_uvRemap;
 		IECoreScene::ConstShaderNetworkPtr m_lightShader;