Created
March 6, 2021 04:31
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A sample.html rotating rect at center in Canvas
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<!DOCTYPE html> | |
<html> | |
<head> | |
<meta charset="utf-8"> | |
<title></title> | |
</head> | |
<body style="min-height: 100% height: 100%"> | |
<canvas id="drawingview" style="width: 100%; height: 100%"> | |
<script> | |
let canvas = document.getElementById('drawingview') | |
let ctx = canvas.getContext('2d') | |
ctx.fillStyle = "red" | |
ctx.fillRect(0, 0, canvas.width, canvas.height) | |
let rect = { x: 10, y: 10, w: 30, h: 30 } | |
ctx.fillStyle = "green" | |
ctx.fillRect(rect.x, rect.y, rect.w, rect.h) | |
let theta = Math.PI / 4.0 | |
drawRotatedBlueRect(ctx, rect, theta); | |
ctx.restore() | |
function drawRotatedBlueRect(ctx, rect, theta) { | |
let center = { x: rect.w / 2.0, y: rect.h / 2.0 } | |
// Anchor point | |
let ap = center | |
let rotatedAp = rotatePoint(ap, theta) | |
let dap = { x: ap.x - rotatedAp.x, y: ap.y - rotatedAp.y }; | |
ctx.save() | |
ctx.translate(rect.x, rect.y); | |
ctx.translate(dap.x, dap.y) | |
ctx.rotate(theta) | |
ctx.fillStyle = "blue" | |
ctx.fillRect(0, 0, rect.w, rect.h) | |
ctx.restore() | |
} | |
function rotatePoint(point, theta) { | |
let x2 = point.x * Math.cos(theta) - point.y * Math.sin(theta) | |
let y2 = point.x * Math.sin(theta) + point.y * Math.cos(theta) | |
return { x: x2, y: y2 } | |
} | |
</script> | |
</body> | |
</html> | |
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