calculating the point of intersection of two lines

Question

I have dynamically generated lines that animate and I want to detect when a lines hits another. I'm trying to implement some basic linear algebra to obtain the equation of the lines and then solving for x,y, but the results are erratic. At this point I'm testing only with two lines which means I should be getting one point of intersection, but I get two. I just want to make sure my math is ok and that I should be looking elsewhere for the problem.

function collision(boid1, boid2) {
  var x1 = boid1.initialX, y1 = boid1.initialY, x2 = boid1.x, y2 = boid1.y, x3 = boid2.initialX, y3 = boid2.initialY, x4 = boid2.x, y4 = boid2.y;
  slope1 = (y1 - y2)/(x1 - x2);
  slope2 = (y3 - y4)/(x3- x4);

  if(slope1 != slope2){
    var b1 = getB(slope1,x1,y1);
    var b2 = getB(slope2,x3,y3);

    if(slope2 >= 0){
      u = slope1 - slope2;
    }else{
      u = slope1 + slope2;
    }

    if(b1 >= 0){
      z = b2 - b1;
    }else{
      z = b2 + b1;
    }
    pointX = z / u;
    pointY = (slope1*pointX)+b1;
    pointYOther = (slope2*pointX)+b2;
    console.log("pointx:"+pointX+" pointy:"+pointY+" othery:"+pointYOther);
    context.beginPath();
    context.arc(pointX, pointY, 2, 0, 2 * Math.PI, false);
    context.fillStyle = 'green';
    context.fill();
    context.lineWidth = 1;
    context.strokeStyle = '#003300';
    context.stroke();
  }
  return false;
}

function getB(slope,x,y){
  var y = y, x = x, m = slope;
  a = m*x;
  if(a>=0){
    b = y - a;
  }else{
    b = y + a;
  }
  return b;
}

The problem is that I'm getting two different values for the point of intersection. There should only be one, which leads me to believe my calculations are wrong. Yes, x2,y2,x4,y4 are all moving, but they have a set angle and the consistent slopes confirm that.

Answer 1

I found a great solution by Paul Bourke. Here it is, implemented in JavaScript:

function line_intersect(x1, y1, x2, y2, x3, y3, x4, y4)
{
    var ua, ub, denom = (y4 - y3)*(x2 - x1) - (x4 - x3)*(y2 - y1);
    if (denom == 0) {
        return null;
    }
    ua = ((x4 - x3)*(y1 - y3) - (y4 - y3)*(x1 - x3))/denom;
    ub = ((x2 - x1)*(y1 - y3) - (y2 - y1)*(x1 - x3))/denom;
    return {
        x: x1 + ua * (x2 - x1),
        y: y1 + ua * (y2 - y1),
        seg1: ua >= 0 && ua <= 1,
        seg2: ub >= 0 && ub <= 1
    };
}

Answer 2

For line segment-line segment intersections, use Paul Bourke's solution:

// line intercept math by Paul Bourke http://paulbourke.net/geometry/pointlineplane/
// Determine the intersection point of two line segments
// Return FALSE if the lines don't intersect
function intersect(x1, y1, x2, y2, x3, y3, x4, y4) {

  // Check if none of the lines are of length 0
    if ((x1 === x2 && y1 === y2) || (x3 === x4 && y3 === y4)) {
        return false
    }

    denominator = ((y4 - y3) * (x2 - x1) - (x4 - x3) * (y2 - y1))

  // Lines are parallel
    if (denominator === 0) {
        return false
    }

    let ua = ((x4 - x3) * (y1 - y3) - (y4 - y3) * (x1 - x3)) / denominator
    let ub = ((x2 - x1) * (y1 - y3) - (y2 - y1) * (x1 - x3)) / denominator

  // is the intersection along the segments
    if (ua < 0 || ua > 1 || ub < 0 || ub > 1) {
        return false
    }

  // Return a object with the x and y coordinates of the intersection
    let x = x1 + ua * (x2 - x1)
    let y = y1 + ua * (y2 - y1)

    return {x, y}
}

For infinite line intersections, use Justin C. Round's algorithm:

function checkLineIntersection(line1StartX, line1StartY, line1EndX, line1EndY, line2StartX, line2StartY, line2EndX, line2EndY) {
    // if the lines intersect, the result contains the x and y of the intersection (treating the lines as infinite) and booleans for whether line segment 1 or line segment 2 contain the point
    var denominator, a, b, numerator1, numerator2, result = {
        x: null,
        y: null,
        onLine1: false,
        onLine2: false
    };
    denominator = ((line2EndY - line2StartY) * (line1EndX - line1StartX)) - ((line2EndX - line2StartX) * (line1EndY - line1StartY));
    if (denominator == 0) {
        return result;
    }
    a = line1StartY - line2StartY;
    b = line1StartX - line2StartX;
    numerator1 = ((line2EndX - line2StartX) * a) - ((line2EndY - line2StartY) * b);
    numerator2 = ((line1EndX - line1StartX) * a) - ((line1EndY - line1StartY) * b);
    a = numerator1 / denominator;
    b = numerator2 / denominator;

    // if we cast these lines infinitely in both directions, they intersect here:
    result.x = line1StartX + (a * (line1EndX - line1StartX));
    result.y = line1StartY + (a * (line1EndY - line1StartY));

    // if line1 is a segment and line2 is infinite, they intersect if:
    if (a > 0 && a < 1) {
        result.onLine1 = true;
    }
    // if line2 is a segment and line1 is infinite, they intersect if:
    if (b > 0 && b < 1) {
        result.onLine2 = true;
    }
    // if line1 and line2 are segments, they intersect if both of the above are true
    return result;
};