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

/**
 * @author alteredq / http://alteredqualia.com/
 */

THREE.MaskPass = function ( scene, camera ) {

	this.scene = scene;
	this.camera = camera;

	this.enabled = true;
	this.clear = true;
	this.needsSwap = false;

};

THREE.MaskPass.prototype = {

	render: function ( renderer, writeBuffer, readBuffer, delta ) {

		var context = renderer.context;

		// don't update color or depth

		context.colorMask( false, false, false, false );
		context.depthMask( false );

		// set up stencil

		context.enable( context.STENCIL_TEST );
		context.stencilOp( context.REPLACE, context.REPLACE, context.REPLACE );
		context.stencilFunc( context.ALWAYS, 1, 0xffffffff );

		// draw into the stencil buffer

		renderer.render( this.scene, this.camera, readBuffer, this.clear );
		renderer.render( this.scene, this.camera, writeBuffer, this.clear );

		// re-enable update of color and depth

		context.colorMask( true, true, true, true );
		context.depthMask( true );

		// only render where stencil is set to 1

		context.stencilFunc( context.EQUAL, 1, 0xffffffff );  // draw if == 1
		context.stencilOp( context.KEEP, context.KEEP, context.KEEP );

	}

};


THREE.ClearMaskPass = function () {

	this.enabled = true;

};

THREE.ClearMaskPass.prototype = {

	render: function ( renderer, writeBuffer, readBuffer, delta ) {

		var context = renderer.context;

		context.disable( context.STENCIL_TEST );

	}

};