How do I rotate a triangle around an edge?

Question

How can a triangle, made with three.js, be rotated around one of its edges? When I try to rotate it, it does around its center as it seems.

The triangle is made by:

var triangleMesh; 
var triangleGeometry = new THREE.Geometry(); 
    triangleGeometry.vertices.push( new THREE.Vector3( 0.0, 1.0, 0.0 ) ); 
    triangleGeometry.vertices.push( new THREE.Vector3( -1.0, -1.0, 0.0 ) ); 
    triangleGeometry.vertices.push( new THREE.Vector3( 1.0, -1.0, 0.0 ) ); 

    triangleGeometry.faces.push( new THREE.Face3( 0, 1, 2 ) ); 

    triangleGeometry.faces[0].vertexColors[0] = new THREE.Color(0xFF0000); 
    triangleGeometry.faces[0].vertexColors[1] = new THREE.Color(0x00FF00); 
    triangleGeometry.faces[0].vertexColors[2] = new THREE.Color(0x0000FF); 

var triangleMaterial = new THREE.MeshBasicMaterial({ vertexColors:THREE.VertexColors, side:THREE.DoubleSide }); 

    triangleMesh = new THREE.Mesh( triangleGeometry, triangleMaterial ); 
    triangleMesh.position.set(-1.5, 0.0, 4.0 ); 

    triangleMesh.position.z     -=  5;
    triangleMesh.rotation.z     +=  angle * Math.PI / 180; // angle is from outer for loop

    parent.add( triangleMesh );

I would need to rotate it around one edge to build prisms/hexagons.

Answer 1

To rotate a triangle around one of its corners, that corner must be in the center of coordinates. For this purpose you can use .translate ( x, y, z ) method of THREE.Geometry() .

Take a look at the code snippet.

 var scene = new THREE.Scene(); var camera = new THREE.PerspectiveCamera(60, window.innerWidth / window.innerHeight, 1, 1000); camera.position.set(0, 0, 5); var renderer = new THREE.WebGLRenderer({ antialias: true }); renderer.setSize(window.innerWidth, window.innerHeight); document.body.appendChild(renderer.domElement); var ngon = 11; var radius = 2; var angle = Math.PI / ngon; var triHeight = Math.cos(angle) * radius; var triWidth = Math.sin(angle) * radius; var triangleMesh; var triangleGeometry = new THREE.Geometry(); triangleGeometry.vertices.push(new THREE.Vector3(0.0, triHeight, 0.0)); triangleGeometry.vertices.push(new THREE.Vector3(-triWidth, 0, 0.0)); triangleGeometry.vertices.push(new THREE.Vector3(triWidth, 0, 0.0)); triangleGeometry.faces.push(new THREE.Face3(0, 1, 2)); triangleGeometry.faces[0].vertexColors[0] = new THREE.Color(0xFF0000); triangleGeometry.faces[0].vertexColors[1] = new THREE.Color(0x00FF00); triangleGeometry.faces[0].vertexColors[2] = new THREE.Color(0x0000FF); triangleGeometry.translate(0, -triHeight, 0); // the upper vertex is at the center now var triangleMaterial = new THREE.MeshBasicMaterial({ vertexColors: THREE.VertexColors, side: THREE.DoubleSide }); triangleMesh = new THREE.Mesh(triangleGeometry, triangleMaterial); for (var i = 1; i < ngon; i++) { var newTri = triangleMesh.clone(); newTri.rotation.z = i * angle * 2; scene.add(newTri); } scene.add(triangleMesh); render(); function render() { requestAnimationFrame(render); renderer.render(scene, camera); } 

 body { overflow: hidden; margin: 0; } 

 <script src="https://threejs.org/build/three.min.js"></script>