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JavaScript - how to calculate intersection point of two lines for given 4 points?
6
points
Intersection point formula for given two points on each line should be calculated in following way:
Where:
L1andL2represent 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:
// ONLINE-RUNNER:browser;
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.');
}
// upper 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;
}
// Usage example:
// line 1
var p1 = { x: 85, y: 45 }; // P1: (x1, y1)
var p2 = { x: 260, y: 180 }; // P2: (x2, y2)
// line 2
var p3 = { x: 225, y: 75 }; // P3: (x4, y4)
var p4 = { x: 65, y: 260 }; // P4: (x4, y4)
// intersection point
var p = calculateIntersection(p1, p2, p3, p4); // intersection point
console.log('P=(' + p.x + '; ' + p.y + ')');
1. Intersection point formula example
In this section we can see example calculatation and visualization of cross point on canvas.
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.');
}
// upper 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)
// line 2
var p3 = { x: 225, y: 75 }; // P3: (x4, y4)
var p4 = { x: 65, y: 260 }; // P4: (x4, y4)
// intersection point
var p = calculateIntersection(p1, p2, p3, p4); // intersection point
drawLine(context, p1, p2, 'red');
drawPoint(context, p1.x, p1.y, 'P1', 'red', 5);
drawPoint(context, p2.x, p2.y, 'P2', 'red', 5);
drawLine(context, p3, p4, 'blue');
drawPoint(context, p3.x, p3.y, 'P3', 'blue', 5);
drawPoint(context, p4.x, p4.y, 'P4', 'blue', 5);
drawPoint(context, p.x, p.y, 'P', 'green', 5);
console.log('P1=(' + p1.x + '; ' + p1.y + ')');
console.log('P2=(' + p2.x + '; ' + p2.y + ')');
console.log('P3=(' + p3.x + '; ' + p3.y + ')');
console.log('P4=(' + p4.x + '; ' + p4.y + ')');
console.log('P=(' + p.x + '; ' + p.y + ')');
</script>
</body>
</html>