Created
April 7, 2023 05:25
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low latency canvas and predicted events
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| <!DOCTYPE html> | |
| <html lang="en"> | |
| <head> | |
| <style> | |
| * { | |
| touch-action: none; | |
| } | |
| canvas { | |
| width: 600px; | |
| height: 600px; | |
| border: 5px solid #333; | |
| position: absolute; | |
| left: 1rem; | |
| top: 1rem; | |
| } | |
| #canvas-1 { | |
| z-index: 100; | |
| } | |
| #canvas-2 { | |
| z-index: 10; | |
| } | |
| </style> | |
| <script> | |
| window.addEventListener('DOMContentLoaded', setup); | |
| function setup() { | |
| install(document.querySelector('#canvas-1'), document.querySelector('#canvas-2'), { desynchronized: true }); | |
| } | |
| function install(ele, eleBack, options) { | |
| const context = ele.getContext('2d', options || {}); | |
| const subcontext = eleBack.getContext('2d', options || {}); | |
| let isDrawing = false; | |
| let x = 0; | |
| let y = 0; | |
| ele.addEventListener('pointerdown', (e) => { | |
| console.log('move') | |
| x = e.offsetX; | |
| y = e.offsetY; | |
| isDrawing = true; | |
| }); | |
| ele.addEventListener('pointermove', (e) => { | |
| if (isDrawing) { | |
| clearArea(subcontext); | |
| e.getPredictedEvents().reduce((carry, e) => { | |
| drawLine(subcontext, carry.x, carry.y, e.offsetX, e.offsetY, '#aaa'); | |
| carry.x = e.offsetX; | |
| carry.y = e.offsetY; | |
| return carry; | |
| }, {x, y}); | |
| drawLine(context, x, y, e.offsetX, e.offsetY); | |
| x = e.offsetX; | |
| y = e.offsetY; | |
| } | |
| }); | |
| ele.addEventListener('pointerup', (e) => { | |
| if (isDrawing) { | |
| clearArea(subcontext); | |
| drawLine(context, x, y, e.offsetX, e.offsetY); | |
| x = 0; | |
| y = 0; | |
| isDrawing = false; | |
| } | |
| }); | |
| ele.addEventListener("dblclick", () => { | |
| clearArea(context); | |
| }) | |
| } | |
| function drawLine(context, x1, y1, x2, y2, color) { | |
| context.beginPath(); | |
| context.strokeStyle = color || '#000000'; | |
| context.lineWidth = 3; | |
| context.lineJoin = "round"; | |
| context.moveTo(x1 * 2, y1 * 2); | |
| context.lineTo(x2 * 2, y2 * 2); | |
| context.closePath(); | |
| context.stroke(); | |
| } | |
| function clearArea(context) { | |
| context.setTransform(1, 0, 0, 1, 0, 0); | |
| context.clearRect(0, 0, context.canvas.width, context.canvas.height); | |
| } | |
| </script> | |
| </head> | |
| <body> | |
| <canvas id="canvas-1" width="1200" height="1200"></canvas> | |
| <canvas id="canvas-2" width="1200" height="1200"></canvas> | |
| </body> | |
| </html> |
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