triangleContains

triangleContains demo

by Joseph Merdrignac

HTML

<p><span class="red">red triangle</span> is clockwise</p>
<p><span class="blue">blue triangle</span> is couter clockwise</p>
<br>
<div class="wrapper">
	<canvas id="dots"></canvas>
	<canvas id="interactive"></canvas>
</div>

CSS

* {
	margin: 0;
}

html {
	font-family: monospace;
}

body {
	padding: 32px;
}

span.red {
	color: #f00;
}

span.blue {
	color: #00f;
}

canvas {
	position: absolute;
	border: solid 1px #ddd;
}

JavaScript

function triangleContains(ax, ay, bx, by, cx, cy, x, y) {

    let det = (bx - ax) * (cy - ay) - (by - ay) * (cx - ax)
	
    return  det * ((bx - ax) * (y - ay) - (by - ay) * (x - ax)) > 0 &&
            det * ((cx - bx) * (y - by) - (cy - by) * (x - bx)) > 0 &&
            det * ((ax - cx) * (y - cy) - (ay - cy) * (x - cx)) > 0 

}






let width = 500, height = 500

// clockwise
let triangle1 = {

	A : { x: 10, y: -10 },
	C : { x: 20, y: 100 },
	B : { x: -90, y: 10 },
	
	color: '#f00',

}

// counter clockwise
let triangle2 = {

	A : { x: 20, y: -60 },
	B : { x: 90, y: 20 },
	C : { x: 20, y: 60 },

	color: '#00f',
	
}


let scale = 2
let mouse = { x: 0, y: 0 }






// DRAW >

let wrapper = document.querySelector('div.wrapper')

wrapper.onmousemove = ({ layerX:x, layerY:y }) => {
	
	x -= width / 2
	y -= height / 2
	x /= scale
	y /= scale
	
	mouse.x = x
	mouse.y = y
	
	drawInteractive()

}

function drawArrow(ctx, A, B) {

	let v = normalize(sub(B, A), 3)
	let I = center(A, B)
	
	let p
	
	p = add(I, rotate(v, 90), v)
	ctx.moveTo(p.x, p.y)
	ctx.lineTo(I.x, I .y)
	p = add(I, rotate(v, -90), v)
	ctx.lineTo(p.x, p.y)

}

function drawTriangle(ctx, { A, B, C, color }) {

	ctx.beginPath()
	ctx.moveTo(A.x, A.y)
	ctx.lineTo(B.x, B.y)
	ctx.lineTo(C.x, C.y)
	ctx.closePath()
	
	ctx.fillStyle = color + '6'
	ctx.strokeStyle = color
	ctx.fill()
	
	drawArrow(ctx, A, B)
	drawArrow(ctx, B, C)
	drawArrow(ctx, C, A)
	
	ctx.stroke()

}

function contains({ A, B, C }, P) {

	return triangleContains(A.x, A.y, B.x, B.y, C.x, C.y, P.x, P.y)

}

function resetCanvas(canvas) {

	canvas.width = width
	canvas.height = height
	
	let ctx = canvas.getContext('2d')

	ctx.resetTransform()
	ctx.clearRect(0, 0, width, height)
	ctx.setTransform(scale, 0, 0, scale, width/2, height/2)
	
}

function drawDots() {

	let canvas = document.querySelector('canvas#dots')
	let ctx = canvas.getContext('2d')

	resetCanvas(canvas)
	
	let count = 1000

	for (let i = 0; i < count; i++) {

		let x = width *...