detect cylinder faces in three.js

Question

I've created a scene with textured and meshed cylinder and a grid with line and vertices of three.js. The cylinder has a spinning effect by dragging mouse left or right.

<!DOCTYPE html>
<html lang="en">
    <head>
        <title>3d Model using HTML5 and three.js</title>
        <meta charset="utf-8">
        <meta name="viewport" content="width=device-width, user-scalable=no, minimum-scale=1.0, maximum-scale=1.0">
        <style>
            body {
                font-family: Monospace;
                background-color: #f0f0f0;
                margin: 0px;
                overflow: hidden;
            }
        </style>
    </head>
    <body>

        <script src="three.min.js" type="text/javascript"></script>
        <script src="Stats.js" type="text/javascript"></script>
        <script src="Detector.js" type="text/javascript"></script>

        <script>
            if ( ! Detector.webgl ) Detector.addGetWebGLMessage();

            var container, stats;

            var camera, scene, renderer, light, projector;
            var particleMaterial;
            var cylinder, line, geometry, geometry1;

            var targetRotation = 0;
            var targetRotationOnMouseDown = 0;

            var mouseX = 0;
            var mouseXOnMouseDown = 0;

            var windowHalfX = window.innerWidth / 2;
            var windowHalfY = window.innerHeight / 2;

            var objects = [];

            var radious = 1600, theta = 45, onMouseDownTheta = 45, phi = 60, onMouseDownPhi = 60, isShiftDown = false;
            init();                     
            animate();

            function init() {

                container = document.createElement( 'div' );
                document.body.appendChild( container );

                var info = document.createElement( 'div' );
                info.style.position = 'absolute';
                info.style.top = '10px';
                info.style.width = '100%';
                info.style.textAlign = 'center';
                info.innerHTML = 'Drag to spin the cylinder<br/>Objective: By spining left, cylinder should go into the surface and by spining right it should come out.';
                container.appendChild( info );

                // camera

                camera = new THREE.PerspectiveCamera( 70, window.innerWidth / window.innerHeight, 1, 10000 );
                camera.position.y = 200;                
                camera.position.z = 800;


                // scene

                scene = new THREE.Scene();

                // light

                scene.add( new THREE.AmbientLight( 0x404040 ) );
                light = new THREE.DirectionalLight( 0xffffff );
                light.position.set( 0, 1, 0 );
                scene.add( light );

                // texture              

                var materials = [];

                for ( var i = 0; i < 6; i ++ ) {

                    materials.push( new THREE.MeshBasicMaterial( { color: Math.random() * 0xffffff } ) );

                }//alert(materials.length);

                // Grid

                geometry = new THREE.Geometry();
                geometry.vertices.push( new THREE.Vector3( - 500, 0, 0 ) );
                geometry.vertices.push( new THREE.Vector3( 500, 0, 0 ) );

                for ( var i = 0; i <= 20; i ++ ) {

                    line = new THREE.Line( geometry, new THREE.LineBasicMaterial( { color: 0x000000, opacity: 0.2 } ) );
                    line.position.z = ( i * 50 ) - 500;
                    scene.add( line );

                    line = new THREE.Line( geometry, new THREE.LineBasicMaterial( { color: 0x000000, opacity: 0.2 } ) );
                    line.position.x = ( i * 50 ) - 500;
                    line.rotation.y = 90 * Math.PI / 180;
                    scene.add( line );
                }


                // cylinder                                         
                geometry1 = new THREE.CylinderGeometry(100, 100, 300, 16, 4, false);

                cylinder = new THREE.Mesh(geometry1 ,new THREE.MeshLambertMaterial( { color: 0x2D303D, wireframe: true, shading: THREE.FlatShading } ));
                //cylinder.position.x = 100;
                cylinder.position.y = -50;
                //cylinder.overdraw = true;
                scene.add(cylinder);
                alert(geometry1.faces.length);
                objects.push(cylinder);


                // projector
                projector = new THREE.Projector();

                // renderer

                renderer = new THREE.CanvasRenderer();
                renderer.setSize( window.innerWidth, window.innerHeight );
                container.appendChild( renderer.domElement );               

                // stats

                stats = new Stats();
                stats.domElement.style.position = 'absolute';
                stats.domElement.style.top = '0px';
                container.appendChild( stats.domElement );

                document.addEventListener( 'mousedown', onDocumentMouseDown, false );
                document.addEventListener( 'touchstart', onDocumentTouchStart, false );
                document.addEventListener( 'touchmove', onDocumentTouchMove, false );               

                window.addEventListener( 'resize', onWindowResize, false );

            }

            function onWindowResize() {

                camera.left = window.innerWidth / - 2;
                camera.right = window.innerWidth / 2;
                camera.top = window.innerHeight / 2;
                camera.bottom = window.innerHeight / - 2;
                camera.aspect = window.innerWidth / window.innerHeight;
                //camera.updateProjectionMatrix();

                renderer.setSize( window.innerWidth, window.innerHeight );

            }

            function onDocumentMouseDown( event ) {         

                event.preventDefault();

                document.addEventListener( 'mousemove', onDocumentMouseMove, false );
                document.addEventListener( 'mouseup', onDocumentMouseUp, false );
                document.addEventListener( 'mouseout', onDocumentMouseOut, false );

                mouseXOnMouseDown = event.clientX - windowHalfX;
                targetRotationOnMouseDown = targetRotation;



            }

            function onDocumentMouseMove( event ) {

                mouseX = event.clientX - windowHalfX;

                targetRotation = targetRotationOnMouseDown + ( mouseX - mouseXOnMouseDown ) * 0.02;

            }

            function onDocumentMouseUp( event ) {

                document.removeEventListener( 'mousemove', onDocumentMouseMove, false );
                document.removeEventListener( 'mouseup', onDocumentMouseUp, false );
                document.removeEventListener( 'mouseout', onDocumentMouseOut, false );

            }

            function onDocumentMouseOut( event ) {

                document.removeEventListener( 'mousemove', onDocumentMouseMove, false );
                document.removeEventListener( 'mouseup', onDocumentMouseUp, false );
                document.removeEventListener( 'mouseout', onDocumentMouseOut, false );

            }

            function onDocumentTouchStart( event ) {

                if ( event.touches.length === 1 ) {

                    event.preventDefault();

                    mouseXOnMouseDown = event.touches[ 0 ].pageX - windowHalfX;
                    targetRotationOnMouseDown = targetRotation;

                }

            }

            function onDocumentTouchMove( event ) {
                if ( event.touches.length === 1 ) {
                    event.preventDefault();
                    mouseX = event.touches[ 0 ].pageX - windowHalfX;
                    targetRotation = targetRotationOnMouseDown + ( mouseX - mouseXOnMouseDown ) * 0.05;
                }
            }

            function animate() {
                requestAnimationFrame( animate );
                render();
                stats.update();

            }

            function render() {             
                cylinder.rotation.y += ( targetRotation - cylinder.rotation.y ) * 0.05;
                renderer.render( scene, camera );               
            }           


        </script>

    </body>
</html>

How do I know which face of cylinder is selected by clicking on a face of it ?

Answer 1

You have to shoot a virtual ray from your viewpoint to the cube. You can then read out the face or faceIndex from the intersected object.

function onDocumentMouseDown( event ) {

            event.preventDefault();

            // position of mouse + screen calc
            var vector = new THREE.Vector3( ( event.clientX / window.innerWidth ) * 2 - 1, - ( event.clientY / window.innerHeight ) * 2 + 1, 0.5 );
            projector.unprojectVector( vector, camera );

            // create a ray from the camera thru the vector
            var ray = new THREE.Ray( camera.position, vector.subSelf( camera.position ).normalize() );

            // array of intersecting object (front to back)
            var intersects = ray.intersectObjects( objects );

            // are we hitting something?
            if ( intersects.length > 0 ) {

                // first intersection should be the object (Mesh,...)
                console.log(intersects[0].object);

                // gimme face as THREE.Face3/4 object
                console.log(intersects[0].face);

                // tell me face index (int)
                console.log(intersects[0].faceIndex);

                // lets change color of the intersected object for fun
                intersects[ 0 ].object.material.color.setHex( Math.random() * 0xffffff );

        }
}

Also check out the canvas_interactive_cubes example