stevemao · December 15, 2018 10:18
diff --git a/index.html b/index.html
 <head>
    <script src="//cdnjs.cloudflare.com/ajax/libs/d3/4.8.0/d3.min.js"></script>
    <script src="//unpkg.com/d3-force-bounce"></script>
    <script src="//unpkg.com/d3-force-constant"></script>

    <link rel="stylesheet" href="style.css">
 </head>

 <body>
    <svg id="canvas">
        <defs>
            <radialGradient id="sphere-gradient">
                <stop offset="0%" stop-color="slategrey"></stop>
                <stop offset="100%" stop-color="darkslategrey"></stop>
            </radialGradient>
        </defs>
    </svg>

    <script src="index.js"></script>
 </body>
diff --git a/index.js b/index.js
 const BALL_RADIUS = 20,
 	WIRE_LENGTH = window.innerHeight/ 2,
 	GRAVITY = 0.6;

 const canvasWidth = window.innerWidth,
 	canvasHeight = window.innerHeight;

 // DOM nodes
 const svgCanvas = d3.select('svg#canvas')
 		.attr('width', canvasWidth)
 		.attr('height', canvasHeight),
 	wiresG = svgCanvas.append('g'),
 	ballsG = svgCanvas.append('g');

 svgCanvas.append('rect')
 	.attr('x', canvasWidth/2 - 4*BALL_RADIUS - 10)
 	.attr('y', canvasHeight*2/3 - WIRE_LENGTH - 5)
 	.attr('width', BALL_RADIUS*8 + 20)
 	.attr('height', 10)
 	.attr('rx', 5)
 	.attr('ry', 5)
 	.attr('fill', 'slategrey');

 const balls = [
 		{ id: '0', init: { x: canvasWidth/2 - 4*BALL_RADIUS, y: canvasHeight*2/3 } },
 		{ id: '1', init: { x: canvasWidth/2 - 2*BALL_RADIUS, y: canvasHeight*2/3 } },
 		{ id: '2', init: { x: canvasWidth/2 - 0*BALL_RADIUS, y: canvasHeight*2/3 } },
 		{ id: '3', init: { x: canvasWidth/2 + 2*BALL_RADIUS, y: canvasHeight*2/3 } },
 		{ id: '4', init: { x: canvasWidth/2 + 4*BALL_RADIUS, y: canvasHeight*2/3 } }
 	],
 	anchors = balls.map(ball => ({
 		id: 'a' + ball.id,
 		fx: ball.init.x,
 		fy: ball.init.y - WIRE_LENGTH
 	})),
 	wires = balls.map(ball => ({
 		source: ball.id,
 		target: 'a' + ball.id
 	}));

 	// swing it
 	balls[0].init.x -= WIRE_LENGTH;
 	balls[0].init.y -= WIRE_LENGTH;

 let init = false;

 const forceSim = d3.forceSimulation()
 	.alphaDecay(0)
 	.velocityDecay(0)
 	.nodes([...balls, ...anchors])
 	.force('gravity', d3.forceConstant()
 		.strength(GRAVITY)
 		.direction(90)
 	)
 	.force('wires', d3.forceLink(wires)
 		.id(node => node.id)
 		.distance(WIRE_LENGTH)
 		.strength(0.4)
 	)
 	.force('bounce', d3.forceBounce()
 		.radius(node => BALL_RADIUS)
 	)
 	.force('init', () => {
 		if (!init) {
 			balls.forEach((ball) => {
 				ball.x = ball.init.x;
 				ball.y = ball.init.y;
 				ball.vx = 0;
 				ball.vy = 0;
 			});
 			init = true;
 		}
 	})
 	.on('tick', () => { ballDigest(); wireDigest(); });

 //

 function ballDigest() {
 	let ball = ballsG.selectAll('circle.ball').data(balls);

 	ball.exit().remove();

 	ball.merge(
 		ball.enter().append('circle')
 			.classed('ball', true)
 			.attr('r', BALL_RADIUS)
 			.attr('fill', 'url(#sphere-gradient)')
 			.call(d3.drag()
 				.on("start", d => { d.fx = d.x; d.fy = d.y; })
 				.on("drag", d => { d.fx = d3.event.x; d.fy = d3.event.y; })
 				.on("end", d => { d.fx = null; d.fy = null; })
 			)
 	)
 		.attr('cx', d => d.x)
 		.attr('cy', d => d.y);
 }

 function wireDigest() {
 // 	let wire = wiresG.selectAll('line.wire').data(wires);

 // 	wire.exit().remove();

 // 	wire.merge(
 // 		wire.enter().append('line')
 // 			.classed('wire', true)
 // 			.attr('stroke', 'slategrey')
 // 			.attr('stroke-width', 1)
 // 	)
 // 		.attr('x1', d => d.source.x)
 // 		.attr('y1', d => d.source.y)
 // 		.attr('x2', d => d.target.x)
 // 		.attr('y2', d => d.target.y);
 }
diff --git a/preview.png b/preview.png
diff --git a/style.css b/style.css
 body {
    margin: 0;
    text-align: center;
 }

 .ball {
    cursor: grab;
    cursor: -webkit-grab;
 }

 .ball:active {
    cursor: grabbing;
    cursor: -webkit-grabbing;
 }
diff --git a/thumbnail.png b/thumbnail.png
	<head>
	<script src="//cdnjs.cloudflare.com/ajax/libs/d3/4.8.0/d3.min.js"></script>
	<script src="//unpkg.com/d3-force-bounce"></script>
	<script src="//unpkg.com/d3-force-constant"></script>

	<link rel="stylesheet" href="style.css">
	</head>

	<body>
	<svg id="canvas">
	<defs>
	<radialGradient id="sphere-gradient">
	<stop offset="0%" stop-color="slategrey"></stop>
	<stop offset="100%" stop-color="darkslategrey"></stop>
	</radialGradient>
	</defs>
	</svg>

	<script src="index.js"></script>
	</body>
	const BALL_RADIUS = 20,
	WIRE_LENGTH = window.innerHeight/ 2,
	GRAVITY = 0.6;

	const canvasWidth = window.innerWidth,
	canvasHeight = window.innerHeight;

	// DOM nodes
	const svgCanvas = d3.select('svg#canvas')
	.attr('width', canvasWidth)
	.attr('height', canvasHeight),
	wiresG = svgCanvas.append('g'),
	ballsG = svgCanvas.append('g');

	svgCanvas.append('rect')
	.attr('x', canvasWidth/2 - 4*BALL_RADIUS - 10)
	.attr('y', canvasHeight*2/3 - WIRE_LENGTH - 5)
	.attr('width', BALL_RADIUS*8 + 20)
	.attr('height', 10)
	.attr('rx', 5)
	.attr('ry', 5)
	.attr('fill', 'slategrey');

	const balls = [
	{ id: '0', init: { x: canvasWidth/2 - 4BALL_RADIUS, y: canvasHeight2/3 } },
	{ id: '1', init: { x: canvasWidth/2 - 2BALL_RADIUS, y: canvasHeight2/3 } },
	{ id: '2', init: { x: canvasWidth/2 - 0BALL_RADIUS, y: canvasHeight2/3 } },
	{ id: '3', init: { x: canvasWidth/2 + 2BALL_RADIUS, y: canvasHeight2/3 } },
	{ id: '4', init: { x: canvasWidth/2 + 4BALL_RADIUS, y: canvasHeight2/3 } }
	],
	anchors = balls.map(ball => ({
	id: 'a' + ball.id,
	fx: ball.init.x,
	fy: ball.init.y - WIRE_LENGTH
	})),
	wires = balls.map(ball => ({
	source: ball.id,
	target: 'a' + ball.id
	}));

	// swing it
	balls[0].init.x -= WIRE_LENGTH;
	balls[0].init.y -= WIRE_LENGTH;

	let init = false;

	const forceSim = d3.forceSimulation()
	.alphaDecay(0)
	.velocityDecay(0)
	.nodes([...balls, ...anchors])
	.force('gravity', d3.forceConstant()
	.strength(GRAVITY)
	.direction(90)
	)
	.force('wires', d3.forceLink(wires)
	.id(node => node.id)
	.distance(WIRE_LENGTH)
	.strength(0.4)
	)
	.force('bounce', d3.forceBounce()
	.radius(node => BALL_RADIUS)
	)
	.force('init', () => {
	if (!init) {
	balls.forEach((ball) => {
	ball.x = ball.init.x;
	ball.y = ball.init.y;
	ball.vx = 0;
	ball.vy = 0;
	});
	init = true;
	}
	})
	.on('tick', () => { ballDigest(); wireDigest(); });

	//

	function ballDigest() {
	let ball = ballsG.selectAll('circle.ball').data(balls);

	ball.exit().remove();

	ball.merge(
	ball.enter().append('circle')
	.classed('ball', true)
	.attr('r', BALL_RADIUS)
	.attr('fill', 'url(#sphere-gradient)')
	.call(d3.drag()
	.on("start", d => { d.fx = d.x; d.fy = d.y; })
	.on("drag", d => { d.fx = d3.event.x; d.fy = d3.event.y; })
	.on("end", d => { d.fx = null; d.fy = null; })
	)
	)
	.attr('cx', d => d.x)
	.attr('cy', d => d.y);
	}

	function wireDigest() {
	// let wire = wiresG.selectAll('line.wire').data(wires);

	// wire.exit().remove();

	// wire.merge(
	// wire.enter().append('line')
	// .classed('wire', true)
	// .attr('stroke', 'slategrey')
	// .attr('stroke-width', 1)
	// )
	// .attr('x1', d => d.source.x)
	// .attr('y1', d => d.source.y)
	// .attr('x2', d => d.target.x)
	// .attr('y2', d => d.target.y);
	}
	body {
	margin: 0;
	text-align: center;
	}

	.ball {
	cursor: grab;
	cursor: -webkit-grab;
	}

	.ball:active {
	cursor: grabbing;
	cursor: -webkit-grabbing;
	}