three.js透光率实现原理归纳

.transmission : Float
透光率（或者说透光性），范围从0.0到1.0。默认值是0.0。
很薄的透明或者半透明的塑料、玻璃材质即便在几乎完全透明的情况下仍旧会保留反射的光线，透光性属性用于这种类型的材质。
当透光率不为0的时候, opacity透明度应设置为1.

会增加不少开销 额外渲染一次 单独渲染 作为transmissionSamplerMap传入
uniforms.transmissionSamplerMap.value = transmissionRenderTarget.texture;

function renderTransmissionPass( opaqueObjects, transmissiveObjects, scene, camera ) {

	const overrideMaterial = scene.isScene === true ? scene.overrideMaterial : null;

	if ( overrideMaterial !== null ) {

		return;

	}

	if ( currentRenderState.state.transmissionRenderTarget[ camera.id ] === undefined ) {

		currentRenderState.state.transmissionRenderTarget[ camera.id ] = new WebGLRenderTarget( 1, 1, {
			generateMipmaps: true,
			type: ( extensions.has( 'EXT_color_buffer_half_float' ) || extensions.has( 'EXT_color_buffer_float' ) ) ? HalfFloatType : UnsignedByteType,
			minFilter: LinearMipmapLinearFilter,
			samples: 4,
			stencilBuffer: stencil,
			resolveDepthBuffer: false,
			resolveStencilBuffer: false,
			colorSpace: ColorManagement.workingColorSpace,
		} );

		// debug

		/*
		const geometry = new PlaneGeometry();
		const material = new MeshBasicMaterial( { map: _transmissionRenderTarget.texture } );

		const mesh = new Mesh( geometry, material );
		scene.add( mesh );
		*/

	}

	const transmissionRenderTarget = currentRenderState.state.transmissionRenderTarget[ camera.id ];

	const activeViewport = camera.viewport || _currentViewport;
	transmissionRenderTarget.setSize( activeViewport.z, activeViewport.w );

	//

	const currentRenderTarget = _this.getRenderTarget();
	_this.setRenderTarget( transmissionRenderTarget );

	_this.getClearColor( _currentClearColor );
	_currentClearAlpha = _this.getClearAlpha();
	if ( _currentClearAlpha < 1 ) _this.setClearColor( 0xffffff, 0.5 );

	_this.clear();

	if ( _renderBackground ) background.render( scene );

	// Turn off the features which can affect the frag color for opaque objects pass.
	// Otherwise they are applied twice in opaque objects pass and transmission objects pass.
	const currentToneMapping = _this.toneMapping;
	_this.toneMapping = NoToneMapping;

	// Remove viewport from camera to avoid nested render calls resetting viewport to it (e.g Reflector).
	// Transmission render pass requires viewport to match the transmissionRenderTarget.
	const currentCameraViewport = camera.viewport;
	if ( camera.viewport !== undefined ) camera.viewport = undefined;

	currentRenderState.setupLightsView( camera );

	if ( _clippingEnabled === true ) clipping.setGlobalState( _this.clippingPlanes, camera );

	renderObjects( opaqueObjects, scene, camera );

	textures.updateMultisampleRenderTarget( transmissionRenderTarget );
	textures.updateRenderTargetMipmap( transmissionRenderTarget );

	if ( extensions.has( 'WEBGL_multisampled_render_to_texture' ) === false ) { // see #28131

		let renderTargetNeedsUpdate = false;

		for ( let i = 0, l = transmissiveObjects.length; i < l; i ++ ) {

			const renderItem = transmissiveObjects[ i ];

			const object = renderItem.object;
			const geometry = renderItem.geometry;
			const material = renderItem.material;
			const group = renderItem.group;

			if ( material.side === DoubleSide && object.layers.test( camera.layers ) ) {

				const currentSide = material.side;

				material.side = BackSide;
				material.needsUpdate = true;

				renderObject( object, scene, camera, geometry, material, group );

				material.side = currentSide;
				material.needsUpdate = true;

				renderTargetNeedsUpdate = true;

			}

		}

		if ( renderTargetNeedsUpdate === true ) {

			textures.updateMultisampleRenderTarget( transmissionRenderTarget );
			textures.updateRenderTargetMipmap( transmissionRenderTarget );

		}

	}

	_this.setRenderTarget( currentRenderTarget );

	_this.setClearColor( _currentClearColor, _currentClearAlpha );

	if ( currentCameraViewport !== undefined ) camera.viewport = currentCameraViewport;

	_this.toneMapping = currentToneMapping;

}

export default /* glsl */`
#ifdef USE_TRANSMISSION

	material.transmission = transmission;
	material.transmissionAlpha = 1.0;
	material.thickness = thickness;
	material.attenuationDistance = attenuationDistance;
	material.attenuationColor = attenuationColor;

	#ifdef USE_TRANSMISSIONMAP

		material.transmission *= texture2D( transmissionMap, vTransmissionMapUv ).r;

	#endif

	#ifdef USE_THICKNESSMAP

		material.thickness *= texture2D( thicknessMap, vThicknessMapUv ).g;

	#endif

	vec3 pos = vWorldPosition;
	vec3 v = normalize( cameraPosition - pos );
	vec3 n = inverseTransformDirection( normal, viewMatrix );

	vec4 transmitted = getIBLVolumeRefraction(
		n, v, material.roughness, material.diffuseColor, material.specularColor, material.specularF90,
		pos, modelMatrix, viewMatrix, projectionMatrix, material.dispersion, material.ior, material.thickness,
		material.attenuationColor, material.attenuationDistance );

	material.transmissionAlpha = mix( material.transmissionAlpha, transmitted.a, material.transmission );

	totalDiffuse = mix( totalDiffuse, transmitted.rgb, material.transmission );

#endif
`;