Created
August 7, 2013 14:01
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Chrome arc函数采用贝塞尔曲线拟合圆
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| <html> | |
| <head> | |
| <title>chrome arc</title> | |
| <meta charset='utf-8'> | |
| </head> | |
| <body> | |
| <canvas id='c' width='2000' height='1500'></canvas> | |
| <script> | |
| // javascript code | |
| function bezierArc(ctx, cx, cy, r) { | |
| var begin = {x: r, y: 0}; | |
| var middle = {x: r, y: r * Math.tan(Math.PI/8)}; | |
| var end = {x: r * Math.SQRT1_2, y: r * Math.SQRT1_2}; | |
| var a0, a1, a2, t; | |
| var lposx = r, lposy = 0; | |
| var cposx = r, cposy = 0; | |
| for(var i = 0; i <= 100; ++i) { | |
| t = i / 100; | |
| a0 = Math.pow(1 - t, 2); | |
| a1 = 2 * t * (1 - t); | |
| a2 = Math.pow(t, 2); | |
| cposx = a0*begin.x + a1*middle.x + a2*end.x; | |
| cposy = a0*begin.y + a1*middle.y + a2*end.y; | |
| ctx.moveTo(lposx + cx, lposy + cy); | |
| ctx.lineTo(cposx + cx, cposy + cy); | |
| ctx.moveTo(lposy + cx, lposx + cy); | |
| ctx.lineTo(cposy + cx, cposx + cy); | |
| // 仅仅拟合1/4个圆 | |
| // ctx.moveTo(-lposx + cx, lposy + cy); | |
| // ctx.lineTo(-cposx + cx, cposy + cy); | |
| // ctx.moveTo(lposy + cx, -lposx + cy); | |
| // ctx.lineTo(cposy + cx, -cposx + cy); | |
| // ctx.moveTo(lposx + cx, -lposy + cy); | |
| // ctx.lineTo(cposx + cx, -cposy + cy); | |
| // ctx.moveTo(-lposy + cx, lposx + cy); | |
| // ctx.lineTo(-cposy + cx, cposx + cy); | |
| // ctx.moveTo(-lposx + cx, -lposy + cy); | |
| // ctx.lineTo(-cposx + cx, -cposy + cy); | |
| // ctx.moveTo(-lposy + cx, -lposx + cy); | |
| // ctx.lineTo(-cposy + cx, -cposx + cy); | |
| lposx = cposx; | |
| lposy = cposy; | |
| } | |
| } | |
| function normalArc(ctx, cx, cy, r) { | |
| var delta = 0.5; | |
| var t = 1 / 180; | |
| ctx.moveTo(cx + r, cy); | |
| for(var i = 0; i < 360; i+= delta) { | |
| ctx.lineTo( | |
| cx + Math.cos(Math.PI * i * t) * r, | |
| cy + Math.sin(Math.PI * i * t) * r); | |
| } | |
| } | |
| var ctx = document.getElementById('c').getContext('2d'); | |
| // native | |
| ctx.beginPath(); | |
| ctx.arc(500, 0, 1000, 0, 2* Math.PI, true) | |
| ctx.closePath(); | |
| ctx.strokeStyle = "red"; | |
| ctx.stroke(); | |
| // bezier | |
| ctx.beginPath(); | |
| bezierArc(ctx, 500, 0, 1000); | |
| ctx.closePath(); | |
| ctx.strokeStyle = "green"; | |
| ctx.stroke(); | |
| // normal | |
| ctx.beginPath(); | |
| normalArc(ctx, 500, 0, 1000); | |
| ctx.closePath(); | |
| ctx.strokeStyle = "blue"; | |
| ctx.stroke(); | |
| ctx.fillText("此处红色和绿色重合，颜色呈灰色，说明Chrome采用贝塞尔拟合", 900, 950); | |
| ctx.fillText("蓝色为标准圆", 900, 970); | |
| ctx.fillText("红色为Chrome的arc", 900, 990); | |
| </script> | |
| </body> | |
| </html> |
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