three.js/examples/js/postprocessing/MaskPass.js

( function () {

	var MaskPass = function ( scene, camera ) {

		THREE.Pass.call( this );
		this.scene = scene;
		this.camera = camera;
		this.clear = true;
		this.needsSwap = false;
		this.inverse = false;

	};

	MaskPass.prototype = Object.assign( Object.create( THREE.Pass.prototype ), {
		constructor: MaskPass,
		render: function ( renderer, writeBuffer, readBuffer
			/*, deltaTime, maskActive */
		) {

			var context = renderer.getContext();
			var state = renderer.state; // don't update color or depth

			state.buffers.color.setMask( false );
			state.buffers.depth.setMask( false ); // lock buffers

			state.buffers.color.setLocked( true );
			state.buffers.depth.setLocked( true ); // set up stencil

			var writeValue, clearValue;

			if ( this.inverse ) {

				writeValue = 0;
				clearValue = 1;

			} else {

				writeValue = 1;
				clearValue = 0;

			}

			state.buffers.stencil.setTest( true );
			state.buffers.stencil.setOp( context.REPLACE, context.REPLACE, context.REPLACE );
			state.buffers.stencil.setFunc( context.ALWAYS, writeValue, 0xffffffff );
			state.buffers.stencil.setClear( clearValue );
			state.buffers.stencil.setLocked( true ); // draw into the stencil buffer

			renderer.setRenderTarget( readBuffer );
			if ( this.clear ) renderer.clear();
			renderer.render( this.scene, this.camera );
			renderer.setRenderTarget( writeBuffer );
			if ( this.clear ) renderer.clear();
			renderer.render( this.scene, this.camera ); // unlock color and depth buffer for subsequent rendering

			state.buffers.color.setLocked( false );
			state.buffers.depth.setLocked( false ); // only render where stencil is set to 1

			state.buffers.stencil.setLocked( false );
			state.buffers.stencil.setFunc( context.EQUAL, 1, 0xffffffff ); // draw if == 1

			state.buffers.stencil.setOp( context.KEEP, context.KEEP, context.KEEP );
			state.buffers.stencil.setLocked( true );

		}
	} );

	var ClearMaskPass = function () {

		THREE.Pass.call( this );
		this.needsSwap = false;

	};

	ClearMaskPass.prototype = Object.create( THREE.Pass.prototype );
	Object.assign( ClearMaskPass.prototype, {
		render: function ( renderer
			/*, writeBuffer, readBuffer, deltaTime, maskActive */
		) {

			renderer.state.buffers.stencil.setLocked( false );
			renderer.state.buffers.stencil.setTest( false );

		}
	} );

	THREE.ClearMaskPass = ClearMaskPass;
	THREE.MaskPass = MaskPass;

} )();