segment.js

window.int_x = 0;
window.int_y = 0;

var DONT_INTERSECT = 0;
var COLLINEAR = 1;
var DO_INTERSECT = 2;

function same_sign(a, b){ return ( ( a * b ) >= 0 ); }

var Segment = function(x, y, vecx, vecy)
{
	this.x = x;
	this.y = y;
	this.vecx = vecx;
	this.vecy = vecy;
	
//	var that = this;

	this.draw = function(width, color)
	{
		gfx.beginPath();
		gfx.lineWidth = width;
		gfx.moveTo(this.x, this.y);
		gfx.lineTo(this.x + this.vecx, this.y + this.vecy);
		gfx.strokeStyle = color;
		gfx.stroke();
	}

	// draw with an offset
	this.drawAt = function(offset_x, offset_y, width, color)
	{
		gfx.beginPath();
		gfx.lineWidth = width;
		gfx.moveTo(offset_x + this.x, offset_y + this.y);
		gfx.lineTo(offset_x + this.x + this.vecx, offset_y + this.y + this.vecy);
		gfx.strokeStyle = color;
		gfx.stroke();
		gfx.fill();
	}

	this.length = function() {
		var dx = this.vecx;
		var dy = this.vecy;
		return Math.sqrt(dx * dx + dy * dy);
	}
	
	this.normal = function() {
		var x1 = this.y;
		var y1 = this.x + this.vecx;
		var y2 = this.x;
		var x2 = this.y + this.vecy;
		return new Segment(x1, y1, x2-x1, y2-y1);
	}
	
	this.center = function() {
		var _x = this.x + this.x + this.vecx;
		var _y = this.y + this.y + this.vecy;
		_x /= 2;
		_y /= 2;
		return new Point(_x, _y);	
	}
	
	this.unit = function() {
		return new Segment(0, 0, this.vecx / this.length(), this.vecy / this.length());
	}
	
	this.multiply = function(multiplier)
	{
		return new Segment(0, 0, this.vecx*multiplier, this.vecy*multiplier);
	}
	
	this.project = function( seg_onto )
	{
		var vec = new Vector(this.vecx, this.vecy);
		var onto = new Vector(seg_onto.vecx, seg_onto.vecy);
		var d = onto.dot(onto);
		if (0 < d) {
			var dp = vec.dot(onto);
			var multiplier = dp / d;
			var rx = onto.x * multiplier;
			var ry = onto.y * multiplier;
			return new Point(rx, ry);
		}
		return new Point(0, 0);
	}
	
	this.intersect = function( segment )
	{
		// a
	    var x1 = this.x;
	    var y1 = this.y;
	    var x2 = this.x + this.vecx;
	    var y2 = this.y + this.vecy;
	    
	    	// b
	    var x3 = segment.x;
	    var y3 = segment.y;
	    var x4 = segment.x + segment.vecx;
	    var y4 = segment.y + segment.vecy;	    
	
	    var a1, a2, b1, b2, c1, c2;
	    var r1, r2 , r3, r4;
	    var denom, offset, num;

	    a1 = y2 - y1;
	    b1 = x1 - x2;
	    c1 = (x2 * y1) - (x1 * y2);

	    r3 = ((a1 * x3) + (b1 * y3) + c1);
	    r4 = ((a1 * x4) + (b1 * y4) + c1);

	    if ((r3 != 0) && (r4 != 0) && same_sign(r3, r4))
	  	return DONT_INTERSECT;

	    a2 = y4 - y3; // Compute a2, b2, c2
	    b2 = x3 - x4;
	    c2 = (x4 * y3) - (x3 * y4);
	    r1 = (a2 * x1) + (b2 * y1) + c2; // Compute r1 and r2
	    r2 = (a2 * x2) + (b2 * y2) + c2;

	    if ((r1 != 0) && (r2 != 0) && (same_sign(r1, r2)))
	  	return DONT_INTERSECT;

	    denom = (a1 * b2) - (a2 * b1); //Line segments intersect: compute intersection point.

	    if (denom == 0)
	  	return COLLINEAR;

	    if (denom < 0) offset = -denom / 2; else offset = denom / 2;

	    num = (b1 * c2) - (b2 * c1);
	    if (num < 0) window.int_x = (num - offset) / denom; else window.int_x = (num + offset) / denom;

	    num = (a2 * c1) - (a1 * c2);
	    if (num < 0)  window.int_y = ( num - offset) / denom; else window.int_y = (num + offset) / denom;

	    return DO_INTERSECT;
	}
}


function vec2ang(x,y)
{
	angleInRadians = Math.atan2(y, x);
	angleInDegrees = (angleInRadians / Math.PI) * 180.0;
	return angleInDegrees;
}

function ang2vec(angle)
{
	var radians = angle * (Math.PI / 180.0);
	var x = Math.cos(radians);
	var y = Math.sin(radians);
	
	var a = new Segment(0, 0, x, y);
	var u = a.normal().unit();

	return [u.vecx, u.vecy];
}