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JavaScript - how to calculate intersection point of two lines for given 4 points?

3 points
Created by:
Marcin
2544

Intersection point formula for given two points on each line should be calculated in following way:

Intersection point formula for given two points on each line
Intersection point formula for given two points on each line

Where: L1 and L2 represent points on line 1 and line 2 calculated with linear parametric equation. Points (x1, y1) and (x2, y2) are located on line 1(x3, y3) and (x4, y4) are located on line 2.

Using JavaScript it is possible to calculate intersection point using above formula in following way.

1. Intersection point formula example

Animated version of this article has been presented here.

// ONLINE-RUNNER:browser;

<!doctype html>
<html>
<head>
  <style>
    
    #my-canvas { border: 1px solid gray; }
    
  </style>
  <script>
    
    function drawLine(context, p1, p2, color) {
      	context.strokeStyle = color;
      
        context.beginPath();
        context.moveTo(p1.x, p1.y);
        context.lineTo(p2.x, p2.y);
        context.stroke();
    }

    function drawPoint(context, x, y, label, color, size) {
      	if (color == null) {
        	color = '#000';
        }
      
        if (size == null) {
            size = 5;
        }
      
      	// to increase smoothing for numbers with decimal part
      	var pointX = Math.round(x);
        var pointY = Math.round(y);

        context.beginPath();
        context.fillStyle = color;
        context.arc(pointX, pointY, size, 0 * Math.PI, 2 * Math.PI);
        context.fill();
      
      	if (label) {
            var textX = pointX;
          	var textY = Math.round(pointY - size - 3);
          
          	var text = label + '=(' + x + '; ' + y + ')';
          
            context.font = 'Italic 14px Arial';
            context.fillStyle = color;
            context.textAlign = 'center';
            context.fillText(text, textX, textY);
        }
    }

  </script>
</head>
<body>
  <canvas id="my-canvas" width="350" height="300"></canvas>
  <script>

    var canvas = document.querySelector('#my-canvas');
    var context = canvas.getContext('2d');

    function calculateIntersection(p1, p2, p3, p4) {
    	
      	// down part of intersection point formula
      	var d1 = (p1.x - p2.x) * (p3.y - p4.y); // (x1 - x2) * (y3 - y4)
      	var d2 = (p1.y - p2.y) * (p3.x - p4.x); // (y1 - y2) * (x3 - x4)
      	var d  = (d1) - (d2);
      
      	if(d == 0) {
        	throw new Error('Number of intersection points is zero or infinity.');
        }
      
      	// down part of intersection point formula
      	var u1 = (p1.x * p2.y - p1.y * p2.x); // (x1 * y2 - y1 * x2)
      	var u4 = (p3.x * p4.y - p3.y * p4.x); // (x3 * y4 - y3 * x4)
          
      	var u2x = p3.x - p4.x; // (x3 - x4)
      	var u3x = p1.x - p2.x; // (x1 - x2)
      	var u2y = p3.y - p4.y; // (y3 - y4)
      	var u3y = p1.y - p2.y; // (y1 - y2)
      
      	// intersection point formula
      	
      	var px = (u1 * u2x - u3x * u4) / d;
      	var py = (u1 * u2y - u3y * u4) / d;
      	
      	var p = { x: px, y: py };
      
      	return p;
    }
    
    // line 1
    var p1 = { x:  85, y:  45 }; // P1: (x1, y1)
    var p2 = { x: 260, y: 180 }; // P2: (x2, y2)
    
    drawLine(context, p1, p2, 'red');
    drawPoint(context, p1.x, p1.y, 'P1', 'red', 5);
    drawPoint(context, p2.x, p2.y, 'P2', 'red', 5);
    
    console.log('P1=(' + p1.x + '; ' + p1.y + ')');
    console.log('P2=(' + p2.x + '; ' + p2.y + ')');
    
    // line 2
    var p3 = { x: 225, y:  75 }; // P3: (x4, y4)
    var p4 = { x:  65, y: 260 }; // P4: (x4, y4)

    drawLine(context, p3, p4, 'blue');
    drawPoint(context, p3.x, p3.y, 'P3', 'blue', 5);
    drawPoint(context, p4.x, p4.y, 'P4', 'blue', 5);
    
    console.log('P3=(' + p3.x + '; ' + p3.y + ')');
    console.log('P4=(' + p4.x + '; ' + p4.y + ')');
    
    // intersection point
    var p = calculateIntersection(p1, p2, p3, p4); // intersection point
    drawPoint(context, p.x, p.y, 'P', 'green', 5);
    
    console.log('P=(' + p.x + '; ' + p.y + ')');

  </script>
</body>
</html>

 

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