GLTFExporter: Added KHR_materials_transmission and KHR_materials_volume support (#22214)

* GLTFExporter: Added KHR_materials_transmission and KHR_materials_volume support.

* GLTFExporter: Clean up.

examples/jsm/exporters/GLTFExporter.js

@@ -46,6 +46,18 @@ class GLTFExporter {
 
 		} );
 
+		this.register( function ( writer ) {
+
+			return new GLTFMaterialsTransmissionExtension( writer );
+
+		} );
+
+		this.register( function ( writer ) {
+
+			return new GLTFMaterialsVolumeExtension( writer );
+
+		} );
+
 	}
 
 	register( callback ) {
@@ -2215,6 +2227,93 @@ class GLTFMaterialsPBRSpecularGlossiness {
 
 }
 
+/**
+ * Transmission Materials Extension
+ *
+ * Specification: https://github.com/KhronosGroup/glTF/tree/master/extensions/2.0/Khronos/KHR_materials_transmission
+ */
+class GLTFMaterialsTransmissionExtension {
+
+	constructor( writer ) {
+
+		this.writer = writer;
+		this.name = 'KHR_materials_transmission';
+
+	}
+
+	writeMaterial( material, materialDef ) {
+
+		if ( ! material.isMeshPhysicalMaterial || material.transmission === 0 ) return;
+
+		const writer = this.writer;
+		const extensionsUsed = writer.extensionsUsed;
+
+		const extensionDef = {};
+
+		extensionDef.transmissionFactor = material.transmission;
+
+		if ( material.transmissionMap ) {
+
+			const transmissionMapDef = { index: writer.processTexture( material.transmissionMap ) };
+			writer.applyTextureTransform( transmissionMapDef, material.transmissionMap );
+			extensionDef.transmissionTexture = transmissionMapDef;
+
+		}
+
+		materialDef.extensions = materialDef.extensions || {};
+		materialDef.extensions[ this.name ] = extensionDef;
+
+		extensionsUsed[ this.name ] = true;
+
+	}
+
+}
+
+/**
+ * Materials Volume Extension
+ *
+ * Specification: https://github.com/KhronosGroup/glTF/tree/master/extensions/2.0/Khronos/KHR_materials_volume
+ */
+class GLTFMaterialsVolumeExtension {
+
+	constructor( writer ) {
+
+		this.writer = writer;
+		this.name = 'KHR_materials_volume';
+
+	}
+
+	writeMaterial( material, materialDef ) {
+
+		if ( ! material.isMeshPhysicalMaterial || material.thickness === 0 ) return;
+
+		const writer = this.writer;
+		const extensionsUsed = writer.extensionsUsed;
+
+		const extensionDef = {};
+
+		extensionDef.thicknessFactor = material.thickness;
+
+		if ( material.thicknessMap ) {
+
+			const thicknessMapDef = { index: writer.processTexture( material.thicknessMap ) };
+			writer.applyTextureTransform( thicknessMapDef, material.thicknessMap );
+			extensionDef.thicknessTexture = thicknessMapDef;
+
+		}
+
+		extensionDef.attenuationDistance = material.attenuationDistance;
+		extensionDef.attenuationColor = material.attenuationTint.toArray();
+
+		materialDef.extensions = materialDef.extensions || {};
+		materialDef.extensions[ this.name ] = extensionDef;
+
+		extensionsUsed[ this.name ] = true;
+
+	}
+
+}
+
 /**
  * Static utility functions
  */