A CodePen by suffick. Physics Playground - A small physics engine (dubbed JSVerlet). The engine is a work in progress, this a demo of what it can do at the moment. Instructions: - Left click to drag points (corners, joints etc). - Right click to delete a point. - Hold ctrl to enter edit mode, release to exit. When in edit mode: - Click to add po…

index.html

<canvas id = "c"></canvas>

<div id="info">
  <div id="top">
  <a target="_blank" id="site" href="http://lonely-pixel.com">my website</a>
  <a id="close" href="">close</a>
  </div>
  <p>
    - Left click to drag points (corners, joints etc).<br>
    - Right click to delete a point.<br>
    - Hold ctrl to enter edit mode, release to exit.<br><br>
      &nbsp;&nbsp;<strong>When in edit mode:</strong><br>
    - Click to add points, hold alt to make fixed points.<br>
    - New points will join to the last point by default.<br>
    - Press shift to unselect a point.<br>
    - Select any two points to join them.<br>
  </p>
</div>

script.js

/*
Copyright (c) 2013 lonely-pixel.com, Stuffit at codepen.io (http://codepen.io/stuffit)

Originally posted at http://codepen.io/stuffit/pen/fhjvk

View this and others at http://lonely-pixel.com

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
*/

document.getElementById('close').onmousedown = function(e) {
  e.preventDefault();
  document.getElementById('info').style.display = 'none';
  return false;
};

var two_PI = Math.PI * 2;

window.requestAnimFrame =
    window.requestAnimationFrame ||
    window.webkitRequestAnimationFrame ||
    window.mozRequestAnimationFrame ||
    window.oRequestAnimationFrame ||
    window.msRequestAnimationFrame ||
    function(callback) {
        window.setTimeout(callback, 1000 / 60);
    };

//-------------------------------------------------

var Vector = function(x, y) {

    this.x = x || 0;
    this.y = y || 0;
}

Vector.prototype.sub = function(v) {

    if (v.x != null && v.y != null) {

        this.x -= v.x;
        this.y -= v.y;

    } else {

        this.x -= v;
        this.y -= v;
    }

    return this;
};

Vector.prototype.add = function(v) {

    if (v.x != null && v.y != null) {

        this.x += v.x;
        this.y += v.y;

    } else {

        this.x += v;
        this.y += v;
    }

    return this;
};

Vector.prototype.mul = function(v) {

    if (v.x != null && v.y != null) {

        this.x *= v.x;
        this.y *= v.y;

    } else {

        this.x *= v;
        this.y *= v;
    }

    return this;
};

Vector.prototype.div = function(v) {

    if (v.x != null && v.y != null) {

        this.x /= v.x;
        this.y /= v.y;

    } else {

        this.x /= v;
        this.y /= v;
    }

    return this;
};

Vector.prototype.normalize = function() {

    var length = this.length();

    if (length > 0) {
        this.x /= length;
        this.y /= length;
    }

    return this;
};

Vector.prototype.length = function() {

    return Math.sqrt(this.x * this.x + this.y * this.y);
};

Vector.prototype.distance = function(v) {

    var x = this.x - v.x;
    var y = this.y - v.y;

    return Math.sqrt(x * x + y * y);
};

Vector.prototype.reset = function() {

    this.x = 0;
    this.y = 0;

    return this;
};

Vector.prototype.neg = function() {

    this.x *= -1;
    this.y *= -1;

    return this;
};

Vector.add = function(v1, v2) {

    if (v2.x != null && v2.y != null) {

        return new Vector(
        v1.x + v2.x,
        v1.y + v2.y);

    } else {

        return new Vector(
        v1.x + v2,
        v1.y + v2);
    }
};

Vector.sub = function(v1, v2) {

    if (v2.x != null && v2.y != null) {

        return new Vector(
        v1.x - v2.x,
        v1.y - v2.y);

    } else {

        return new Vector(
        v1.x - v2,
        v1.y - v2);
    }
};

Vector.mul = function(v1, v2) {

    if (v2.x != null && v2.y != null) {

        return new Vector(
        v1.x * v2.x,
        v1.y * v2.y);

    } else {

        return new Vector(
        v1.x * v2,
        v1.y * v2);
    }
};

Vector.div = function(v1, v2) {

    if (v2.x != null && v2.y != null) {

        return new Vector(
        v1.x / v2.x,
        v1.y / v2.y);

    } else {

        return new Vector(
        v1.x / v2,
        v1.y / v2);
    }
};

//-------------------------------------------------

var Point = function(x, y, fixed){

    this.pos = new Vector(x, y);
    this.pre = new Vector(x, y);
    this.acc = new Vector();

    this.fixed = fixed;
};

Point.prototype.move = function(v) {

    if(this.fixed) return;

    this.pos.add(v);
};

Point.prototype.add_force = function(v) {

    if(this.fixed) return;

    this.acc.add(v);
};

Point.prototype.update = function(delta) {

    if(this.fixed) return;

    delta *= delta;

    var x = this.pos.x,
        y = this.pos.y;

    this.acc.mul(delta);

    this.pos.x += x - this.pre.x + this.acc.x;
    this.pos.y += y - this.pre.y + this.acc.y;

    this.acc.reset();
  
    this.pre.x = x;
    this.pre.y = y;
};

Point.prototype.check_walls = function(x, y, w, h) {

    this.pos.x = Math.max(x + 1, Math.min(w - 1, this.pos.x));
    this.pos.y = Math.max(y + 1, Math.min(h - 1, this.pos.y));

    if (this.pos.y >= h - 1)
        this.pos.x -= (this.pos.x - this.pre.x + this.acc.x);
};

Point.prototype.draw = function(ctx, size) {

    if(this.fixed) {

        ctx.fillStyle = 'rgba(255,255,255,0.2)';
        ctx.beginPath();
        ctx.arc(this.pos.x, this.pos.y, size * 3, 0, two_PI, false);
        ctx.fill();
    }

    ctx.fillStyle = (this.fixed) ? '#EDEA26' : '#aaa';
    ctx.beginPath();
    ctx.arc(this.pos.x, this.pos.y, size, 0, two_PI, false);
    ctx.fill();
};

//-------------------------------------------------

var Constraint = function(p1, p2) {

    this.p1 = p1;
    this.p2 = p2;
    this.length = p1.pos.distance(p2.pos);
    this.stretch = this.length * 0.15;
}

Constraint.prototype.resolve = function() {

    var dists  = Vector.sub(this.p2.pos, this.p1.pos),
        length = dists.length(),
        diff   = length - this.length;

    dists.normalize();

    var f = dists.mul(diff * 0.5);

    this.p1.move(f);
    this.p2.move(f.neg());
};

Constraint.prototype.draw = function(ctx, stress) {

    if(stress) {

        var diff = this.length - this.p1.pos.distance(this.p2.pos);

        var per = Math.round(Math.min(Math.abs(diff / this.stretch), 1) * 255);

        ctx.strokeStyle = 'rgba(255, '+(255 - per)+', '+(255 - per)+', 0.8)';

    } else ctx.strokeStyle = 'rgba(255,255,255,0.8)';

    ctx.beginPath();
    ctx.moveTo(this.p1.pos.x, this.p1.pos.y);
    ctx.lineTo(this.p2.pos.x, this.p2.pos.y);
    ctx.stroke();
};

//-------------------------------------------------

var JSVerlet = function(canvas, options) {

    this.canvas = canvas;
    this.ctx = canvas.getContext('2d');
    this.ctx.lineWidth = 1;

    this.width = canvas.width;
    this.height = canvas.height;
    this.options = options || {};

    this.constraints = [];
    this.points = [];
    this.draw_points = [];

    this.mouse = new Vector();
    this.gravity = this.options.gravity || new Vector(0, 0.98);
    this.point_size = this.options.point_size || 2;
    this.show_stress = this.options.show_stress;

    this.key = {
        ctrl: false,
        alt: false
    };

    canvas.oncontextmenu = function(e) {

        e.preventDefault();
    };

    var _this = this;

    if (this.options.edit) {

        document.onkeydown = function(e) {

            if (e.keyCode == 17) {

                _this.key.ctrl = true;

            } else if (e.keyCode == 16) {

                _this.draw_points = [];

            } else if (e.keyCode == 18) {

                _this.key.alt = true;
            }
        };

        document.onkeyup = function(e) {

            if (e.keyCode == 17) {

                _this.key.ctrl = false;
                _this.draw_points = [];

            } else if (e.keyCode == 18) {

                _this.key.alt = false;
            }
        };
    }

    if (this.options.edit || this.options.drag) {

        canvas.onmousedown = function(e) {

            var rect = _this.canvas.getBoundingClientRect();
            _this.mouse.x = e.clientX - rect.left;
            _this.mouse.y = e.clientY - rect.top;

            _this.mouse.down = true;

            if (_this.options.edit) {

                if (e.which == 3) {

                    if (_this.get_mouse_point())
                        _this.remove_point(_this.get_mouse_point())

                } else {

                    if (_this.key.ctrl) {

                        var p = _this.get_mouse_point();

                        if (!p) {

                            p = new Point(_this.mouse.x, _this.mouse.y, _this.key.alt);
                            _this.points.push(p);
                        }

                        if (_this.draw_points.length) {

                            _this.constraints.push(
                                new Constraint(
                                    p,
                                    _this.draw_points[_this.draw_points.length - 1]
                                )
                            );
                        }

                        _this.draw_points.push(p);

                    } else if (_this.options.drag)
                        _this.mouse_point = _this.get_mouse_point();
                }

            } else if (_this.options.drag)
                _this.mouse_point = _this.get_mouse_point();
        };

        canvas.onmouseup = function(e) {

            _this.mouse.down = false;
            _this.mouse_point = null;
        };

        canvas.onmousemove = function(e) {

            var rect = _this.canvas.getBoundingClientRect();
            _this.mouse.x = e.clientX - rect.left;
            _this.mouse.y = e.clientY - rect.top;
        };
    }
};

JSVerlet.prototype.draw = function(ctx) {

    ctx.clearRect(0, 0, this.canvas.width, this.canvas.height);

    var i = this.constraints.length;
    while(i--) this.constraints[i].draw(ctx, this.show_stress);

    var i = this.points.length;
    while(i--) this.points[i].draw(ctx, this.point_size);

    if(this.mouse_point) {

        ctx.beginPath();
        ctx.arc(this.mouse_point.pos.x, this.mouse_point.pos.y, this.point_size * 3, 0, two_PI);
        ctx.fillStyle = 'rgba(255, 255, 255, 0.2)';
        ctx.fill();

        ctx.beginPath();
        ctx.arc(this.mouse_point.pos.x, this.mouse_point.pos.y, this.point_size, 0, two_PI);
        ctx.fillStyle = (this.mouse_point.fixed) ? '#EDEA26' : '#aaa';
        ctx.fill();
    }

    if(this.draw_points.length) {

        var point = this.draw_points[this.draw_points.length - 1];

        ctx.beginPath();
        ctx.arc(point.pos.x, point.pos.y, this.point_size * 3, 0, two_PI);
        ctx.fillStyle = 'rgba(255, 255, 255, 0.2)';
        ctx.fill();

        ctx.beginPath();
        ctx.arc(point.pos.x, point.pos.y, this.point_size, 0, two_PI);
        ctx.fillStyle = '#aaa';
        ctx.fill();
    }
};

JSVerlet.prototype.update = function(iter) {

    if(this.key.ctrl) return;

    iter = iter || 6;

    var delta = 1 / iter;
    
    var n = iter;
    while(n--) {

        if (this.mouse_point) {

            this.mouse_point.pos.x += (this.mouse.x - this.mouse_point.pos.x)/iter;
            this.mouse_point.pos.y += (this.mouse.y - this.mouse_point.pos.y)/iter;
        }

        var i = this.points.length;
        while(i--) {

            var point = this.points[i];
            point.add_force(this.gravity);
            point.update(delta);
            point.check_walls(0, 0, this.width, this.height);
        }

        var i = this.constraints.length;
        while(i--) this.constraints[i].resolve();
    }
};

JSVerlet.prototype.remove_point = function(point) {

    var i = this.constraints.length;
    while (i--) {

        var constraint = this.constraints[i];
        if (constraint.p1 == point || constraint.p2 == point)
            this.constraints.splice(this.constraints.indexOf(constraint), 1);
    }

    if (this.points.indexOf(point) != -1)
        this.points.splice(this.points.indexOf(point), 1);
};

JSVerlet.prototype.get_mouse_point = function() {

    var closest;

    var i = this.points.length;

    while(i--) {

        var point = this.points[i];

        if(point.pos.distance(this.mouse) < 10) {

            closest = point;
        }
    }

    return closest;
};

JSVerlet.prototype.add_point = function(x, y, fixed) {

    var point = new Point(x, y, fixed);

    this.points.push(point);

    return point;
};

JSVerlet.prototype.add_constraint = function(p1, p2) {

    this.constraints.push(new Constraint(p1, p2));
};

JSVerlet.prototype.add_shape = function(shapes) {

    if(!(shapes instanceof Array)) {

        if(!(shapes instanceof Shape)) {

            console.log('Error: shape is not an instance of Shape.');
            return false;
        }

        this.points = this.points.concat(shapes.points);
        this.constraints = this.constraints.concat(shapes.constraints);

        return true;
    }

    var i = shapes.length;

    while(i--) {

        var shape = shapes[i];

        if(!(shape instanceof Shape)) {

            console.log('Error: shape[' + i + '] is not an instance of Shape.');
            return false;
        }

        this.points = this.points.concat(shape.points);
        this.constraints = this.constraints.concat(shape.constraints);
    }

    return true;
};

//-------------------------------------------------

var Shape = function() {};

//-------------------------------------------------

var Rectangle = function(x, y, w, h) {

    var p1 = new Point(x, y);
    var p2 = new Point(x + w, y);
    var p3 = new Point(x, y + h);
    var p4 = new Point(x + w, y + h);

    var c1 = new Constraint(p1, p2);
    var c2 = new Constraint(p2, p3);
    var c3 = new Constraint(p3, p4);
    var c4 = new Constraint(p4, p1);
    var c5 = new Constraint(p1, p3);
    var c6 = new Constraint(p2, p4);

    this.points = [p1,p2,p3,p4];
    this.constraints = [c1,c2,c3,c4,c5,c6];
};

Rectangle.prototype = new Shape();

//-------------------------------------------------

window.onload = function() {

    var canvas = document.getElementById('c');
    canvas.width = 1000;//window.innerWidth;
    canvas.height = 340;//window.innerHeight;
    var ctx = canvas.getContext('2d');

    var jsv = new JSVerlet(canvas, {drag: true, edit: true, show_stress: true});

    // demo scene ------------------------------

    var p1  = jsv.add_point(90, 40, false),
        p2  = jsv.add_point(160, 40, false),
        p3  = jsv.add_point(90, 110, false),
        p4  = jsv.add_point(160, 110, false),
        p5  = jsv.add_point(90, 180, false),
        p6  = jsv.add_point(160, 180, false),
        p7  = jsv.add_point(90, 250, false),
        p8  = jsv.add_point(160, 250, false),
        p9  = jsv.add_point(90, 320, true),
        p10 = jsv.add_point(160, 320, true),
        p11 = jsv.add_point(300, 40, false),
        p12 = jsv.add_point(365, 198, false),
        p13 = jsv.add_point(345, 218, false),
        p14 = jsv.add_point(385, 218, false);

    jsv.add_constraint(p1, p2);
    jsv.add_constraint(p3, p4);
    jsv.add_constraint(p5, p6);
    jsv.add_constraint(p7, p8);
    jsv.add_constraint(p1, p3);
    jsv.add_constraint(p3, p5);
    jsv.add_constraint(p5, p7);
    jsv.add_constraint(p7, p9);
    jsv.add_constraint(p2, p4);
    jsv.add_constraint(p4, p6);
    jsv.add_constraint(p6, p8);
    jsv.add_constraint(p8, p10);
    jsv.add_constraint(p1, p4);
    jsv.add_constraint(p2, p3);
    jsv.add_constraint(p3, p6);
    jsv.add_constraint(p4, p5);
    jsv.add_constraint(p5, p8);
    jsv.add_constraint(p6, p7);
    jsv.add_constraint(p7, p10);
    jsv.add_constraint(p8, p9);
    jsv.add_constraint(p2, p11);
    jsv.add_constraint(p4, p11);
    jsv.add_constraint(p11, p12);
    jsv.add_constraint(p12, p13);
    jsv.add_constraint(p12, p14);
    jsv.add_constraint(p13, p14);

    jsv.add_shape(new Rectangle(500, 70, 70, 70));
  
    var square = new Rectangle(630, 70, 50, 50);

    square.points[1].fixed = true;

    jsv.add_shape(square);

    //----------------------------------------

    var Loop = function() {

        //var time = new Date().getTime();

        jsv.update(16);
        jsv.draw(ctx);

        //console.log(new Date().getTime() - time);

        requestAnimFrame(Loop);
    };

    Loop();
};

style.css

* {
	 margin: 0;
  overflow:hidden;
  -webkit-user-select: none;
  -moz-user-select: none;
  -ms-user-select: none;
  -o-user-select: none;
  user-select: none;
}

body {
  background:#333;
}

canvas {
  background:#333;
  width:1000px;
  height:340px;
  margin:0 auto;
  display:block;
  border:1px dashed #555;
  border-top:none;
}

#info {
  position:absolute;
  left:-1px;
  top:-1px;
  width:auto;
  max-width:380px;
  height:auto;
  background:#f2f2f2;
  border-bottom-right-radius:10px;
}

#top {
  background:#fff;
  width:100%;
  height:auto;
  position:relative;
  border-bottom:1px solid #eee;
}

p {
  font-family:Arial, sans-serif;
  color:#666;
  text-align:justify;
  font-size: 16px;
  line-height:22px;
  margin:10px;
}

a {
  font-family:sans-serif;
  color:#444;
  text-decoration:none;
  font-size: 20px;
}

#site {
  float:left;
  margin: 10px;
  color: #38a;
  border-bottom: 1px dashed #bbf;
  padding-bottom:3px
}

#close {
  float:right;
  margin: 10px;
}