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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