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
let triangle1 = {
A : { x: 10, y: -10 },
C : { x: 20, y: 100 },
B : { x: -90, y: 10 },
color: '#f00',
}
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 }
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 * (Math.random() - .5)
let y = width * (Math.random() - .5)
ctx.beginPath()
ctx.ellipse(x, y, 1, 1, 0, 0, 2 * Math.PI)
if (contains(triangle1, { x, y })) {
ctx.fillStyle = '#f00'
} else if (contains(triangle2, { x, y })) {
ctx.fillStyle = '#00f'
} else {
ctx.fillStyle = '#0003'
}
ctx.fill()
}
}
function drawInteractive() {
let canvas = document.querySelector('canvas#interactive')
let ctx = canvas.getContext('2d')
resetCanvas(canvas)
ctx.beginPath()
ctx.moveTo(0, -height/2)
ctx.lineTo(0, height/2)
ctx.moveTo(-width/2, 0)
ctx.lineTo(width/2, 0)
ctx.strokeStyle = '#0003'
ctx.stroke()
drawTriangle(ctx, triangle1)
drawTriangle(ctx, triangle2)
ctx.beginPath()
ctx.ellipse(mouse.x, mouse.y, 4, 4, 0, 0, 2 * Math.PI)
if (contains(triangle1, mouse)) {
ctx.fillStyle = triangle1.color + 'a'
ctx.fill()
} else if (contains(triangle2, mouse)) {
ctx.fillStyle = triangle2.color + 'a'
ctx.fill()
} else {
ctx.strokeStyle = 'black'
ctx.stroke()
}
}
drawDots()
drawInteractive()
function add(...points) {
let x = 0, y = 0
for (let point of points) {
x += point.x
y += point.y
}
return { x, y }
}
function center(...points) {
let x = 0, y = 0
for (let point of points) {
x += point.x
y += point.y
}
x /= points.length
y /= points.length
return { x, y }
}
function sub(A, B) {
let x = A.x - B.x
let y = A.y - B.y
return { x, y }
}
function normalize({ x, y }, length = 10) {
let r = length / Math.sqrt(x * x + y * y)
x *= r
y *= r
return { x, y }
}
function rotate({ x, y }, angle = 90) {
let length = Math.sqrt(x * x + y * y)
angle *= Math.PI / 180
angle += Math.atan2(y, x)
x = length * Math.cos(angle)
y = length * Math.sin(angle)
return { x, y }
}
* {
margin: 0;
}
html {
font-family: monospace;
}
body {
padding: 32px;
}
span.red {
color: #f00;
}
span.blue {
color: #00f;
}
canvas {
position: absolute;
border: solid 1px #ddd;
}
<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>
