Home >Web Front-end >H5 Tutorial >How to draw a circle using canvas? Three dynamic implementation methods for drawing circles using canvas
We all should know that canvas can be used to draw a variety of pictures on the browser. So, how to use canvas to draw a circle? This article will introduce to you three dynamic implementation methods of using canvas to draw a circle.
First let’s look at the first method of drawing a circle on canvas: using the canvas arc() method to dynamically draw a circle
arc() method creates an arc/curve ( for creating circles or partial circles).
Idea: Limit it by setting the starting angle and ending angle, and then achieve the animation effect by accumulating temporary angle variables.
Syntax: context.arc(x,y,r,sAngle,eAngle,counterclockwise);
Canvas circle drawing implementation code:
<!DOCTYPE html> <html> <head> <meta charset="UTF-8"> <style type="text/css"> #myCanvas{ margin: 0 auto; display: block; } </style> </head> <body> <canvas id="myCanvas">当前浏览器不支持canvas组件请升级!</canvas> <script type="text/javascript"> //方法一:arc 动态画圆 var c = document.getElementById('myCanvas'); var ctx = c.getContext('2d'); var mW = c.width = 300; var mH = c.height = 300; var lineWidth = 5; var r = mW / 2; //中间位置 var cR = r - 4 * lineWidth; //圆半径 var startAngle = -(1 / 2 * Math.PI); //开始角度 var endAngle = startAngle + 2 * Math.PI; //结束角度 var xAngle = 1 * (Math.PI / 180); //偏移角度量 var fontSize = 35; //字号大小 var tmpAngle = startAngle; //临时角度变量 //渲染函数 var rander = function(){ if(tmpAngle >= endAngle){ return; }else if(tmpAngle + xAngle > endAngle){ tmpAngle = endAngle; }else{ tmpAngle += xAngle; } ctx.clearRect(0, 0, mW, mH); //画圈 ctx.beginPath(); ctx.lineWidth = lineWidth; ctx.strokeStyle = '#1c86d1'; ctx.arc(r, r, cR, startAngle, tmpAngle); ctx.stroke(); ctx.closePath(); //写字 ctx.fillStyle = '#1d89d5'; ctx.font= fontSize + 'px Microsoft Yahei'; ctx.textAlign='center'; ctx.fillText( Math.round((tmpAngle - startAngle) / (endAngle - startAngle) * 100) + '%', r, r + fontSize / 2); requestAnimationFrame(rander); }; rander(); </script> </body> </html>
canvas drawing circle renderings:
## Next, let’s take a look at the second canvas drawing circle Method : Use the canvas rotate() method to draw a circle
<!DOCTYPE html> <html> <head> <meta charset="UTF-8"> <style type="text/css"> #myCanvas{ margin: 0 auto; display: block; } </style></head><body> <canvas id="myCanvas">当前浏览器不支持canvas组件请升级!</canvas> <script type="text/javascript"> //方法二:rotate() 动态画圆 var c = document.getElementById('myCanvas'); var ctx = c.getContext('2d'); var mW = c.width = 300; var mH = c.height = 300; var lineWidth = 5; var r = mW / 2; //中间位置 var cR = r - 4 * lineWidth; //圆半径 var startAngle = -(1 / 2 * Math.PI); //开始角度 var endAngle = startAngle + 2 * Math.PI; //结束角度 var xAngle = 1 * (Math.PI / 180); //偏移角度量 var fontSize = 35; //字号大小 var tmpAngle = startAngle; //临时角度变量 //渲染函数 var rander = function(){ if(tmpAngle >= endAngle){ return; }else if(tmpAngle + xAngle > endAngle){ tmpAngle = endAngle; }else{ tmpAngle += xAngle; } ctx.clearRect(0, 0, mW, mH); //画圈 ctx.save(); ctx.beginPath(); ctx.lineWidth = lineWidth; ctx.strokeStyle = '#1c86d1'; ctx.translate(r, r); //重定义圆点 ctx.rotate(-Math.PI); //最上方为起点 for(var i = 0; i <= tmpAngle - startAngle; i += xAngle){ //绘图 ctx.moveTo(0, cR - lineWidth); ctx.lineTo(0, cR); ctx.rotate(xAngle); //通过旋转角度和画点的方式绘制圆 } ctx.stroke(); ctx.closePath(); ctx.restore(); //写字 ctx.fillStyle = '#1d89d5'; ctx.font= fontSize + 'px Microsoft Yahei'; ctx.textAlign='center'; ctx.fillText( Math.round((tmpAngle - startAngle) / (endAngle - startAngle) * 100) + '%', r, r + fontSize / 2); requestAnimationFrame(rander); }; rander(); </script> </body> </html>
Canvas drawing circle rendering:
Finally let’s look at the third method of drawing a circle on canvas: Use to get the circle Draw a circle using coordinates
Idea: Use sin() and cos() to store the coordinate position between the start angle and the end angle in an array according to a certain angle offset, and then follow the array The coordinate points in are drawn.<!DOCTYPE html> <html> <head> <meta charset="UTF-8"> <style type="text/css"> #myCanvas{ margin: 0 auto; display: block; } </style></head><body> <canvas id="myCanvas">当前浏览器不支持canvas组件请升级!</canvas> <script type="text/javascript"> //方法三:获取圆坐标方式 动态画圆 var c = document.getElementById('myCanvas'); var ctx = c.getContext('2d'); var mW = c.width = 300; var mH = c.height = 300; var lineWidth = 5; var r = mW / 2; //中间位置 var cR = r - 4 * lineWidth; //圆半径 var startAngle = -(1 / 2 * Math.PI); //开始角度 var endAngle = startAngle + 2 * Math.PI; //结束角度 var xAngle = 2 * (Math.PI / 180); //偏移角度量 var cArr = []; //圆坐标数组 //初始化圆坐标数组 for(var i = startAngle; i <= endAngle; i += xAngle){ //通过sin()和cos()获取每个角度对应的坐标 var x = r + cR * Math.cos(i); var y = r + cR * Math.sin(i); cArr.push([x, y]); } //移动到开始点 var startPoint = cArr.shift(); ctx.beginPath(); ctx.moveTo(startPoint[0], startPoint[1]); //渲染函数 var rander = function(){ //画圈 if(cArr.length){ ctx.lineWidth = lineWidth; ctx.strokeStyle = '#1c86d1'; var tmpPoint = cArr.shift(); ctx.lineTo(tmpPoint[0], tmpPoint[1]); ctx.stroke(); }else{ cArr = null; return; } requestAnimationFrame(rander); }; rander(); </script> </body> </html>
Canvas drawing circle rendering:
HTML5 Development Manual.
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