HTML5 Canvas中繪製橢圓的幾種方法
阿新 • • 發佈:2018-11-07
1.canvas自帶的繪製橢圓的方法
ellipse(x, y, radiusX, radiusY, rotation, startAngle, endAngle, anticlockwise)是後來新增的,
引數的意思:(起點x.起點y,半徑x,半徑y,旋轉的角度,起始角,結果角,順時針還是逆時針)
1 <!DOCTYPE html> 2 <html> 3 <head lang="en"> 4 <meta charset="UTF-8"> 5 <title>橢圓</title>6 </head> 7 <body> 8 <canvas id="canvas" style="border:1px solid #aaa;display:block;margin:50px auto;"> 9 當前瀏覽器不支援Canvas,請更換瀏覽器後再試 10 </canvas> 11 12 <script> 13 window.onload = function(){ 14 var canvas = document.getElementById("canvas");15 var ctx=canvas.getContext('2d'); 16 canvas.width = 800; 17 canvas.height = 800; 18 if(ctx.ellipse){ 19 ctx.ellipse(400,400,300,200,0,0,Math.PI*2); 20 ctx.fillStyle="#058"; 21 ctx.strokeStyle="#000"; 22 ctx.fill(); 23 ctx.stroke();24 }else{ 25 alert("no ellipse!"); 26 } 27 } 28 </script> 29 </body> 30 </html>
目前谷歌可以支援,其他瀏覽器還未存在ellipse();
下面這幾種方法是我在網上看到的,在此也做個總結:
2.用引數方程繪製橢圓
//函式的引數x,y為橢圓中心;a,b分別為橢圓橫半軸、
//縱半軸長度,不可同時為0
//該方法的缺點是,當lineWidth較寬,橢圓較扁時
//橢圓內部長軸端較為尖銳,不平滑,效率較低
1 <!DOCTYPE html> 2 <html> 3 <head lang="en"> 4 <meta charset="UTF-8"> 5 <title>橢圓</title> 6 </head> 7 <body> 8 <canvas id="canvas" style="border:1px solid #aaa;display:block;margin:50px auto;"> 9 當前瀏覽器不支援Canvas,請更換瀏覽器後再試 10 </canvas> 11 <script> 12 var canvas = document.getElementById("canvas"); 13 canvas.width = 600; 14 canvas.height = 600; 15 var context = canvas.getContext("2d"); 16 context.lineWidth = 10; 17 context.strokeStyle="black"; 18 ParamEllipse(context, 130, 80, 100, 20); //橢圓 19 function ParamEllipse(context, x, y, a, b){ 20 //max是等於1除以長軸值a和b中的較大者 21 //i每次迴圈增加1/max,表示度數的增加 22 //這樣可以使得每次迴圈所繪製的路徑(弧線)接近1畫素 23 var step = (a > b) ? 1 / a : 1 / b; 24 context.beginPath(); 25 context.moveTo(x + a, y); //從橢圓的左端點開始繪製 26 for (var i = 0; i < 2 * Math.PI; i += step) 27 { 28 //引數方程為x = a * cos(i), y = b * sin(i), 29 //引數為i,表示度數(弧度) 30 context.lineTo(x + a * Math.cos(i), y + b * Math.sin(i)); 31 } 32 context.closePath(); 33 context.stroke(); 34 }; 35 </script> 36 </body> 37 </html>
3.均勻壓縮法繪製橢圓
//其方法是用arc方法繪製圓,結合scale進行
//橫軸或縱軸方向縮放(均勻壓縮)
//這種方法繪製的橢圓的邊離長軸端越近越粗,長軸端點的線寬是正常值
//邊離短軸越近、橢圓越扁越細,甚至產生間斷,這是scale導致的結果
//這種缺點某些時候是優點,比如在表現環的立體效果(行星光環)時
//對於引數a或b為0的情況,這種方法不適用
1 <!DOCTYPE html> 2 <html> 3 <head lang="en"> 4 <meta charset="UTF-8"> 5 <title>橢圓</title> 6 </head> 7 <body> 8 <canvas id="canvas" style="border:1px solid #aaa;display:block;margin:50px auto;"> 9 當前瀏覽器不支援Canvas,請更換瀏覽器後再試 10 </canvas> 11 <script> 12 var canvas = document.getElementById("canvas"); 13 canvas.width = 600; 14 canvas.height = 600; 15 var context = canvas.getContext("2d"); 16 context.lineWidth = 10; 17 context.strokeStyle="black"; 18 EvenCompEllipse(context, 130, 200, 100, 20); //橢圓 19 function EvenCompEllipse(context, x, y, a, b){ 20 context.save(); 21 //選擇a、b中的較大者作為arc方法的半徑引數 22 var r = (a > b) ? a : b; 23 var ratioX = a / r; //橫軸縮放比率 24 var ratioY = b / r; //縱軸縮放比率 25 context.scale(ratioX, ratioY); //進行縮放(均勻壓縮) 26 context.beginPath(); 27 //從橢圓的左端點開始逆時針繪製 28 context.moveTo((x + a) / ratioX, y / ratioY); 29 context.arc(x / ratioX, y / ratioY, r, 0, 2 * Math.PI); 30 context.closePath(); 31 context.stroke(); 32 context.restore(); 33 }; 34 </script> 35 </body> 36 </html>
4.使用三次貝塞爾曲線模擬橢圓1
//此方法也會產生當lineWidth較寬,橢圓較扁時,
//長軸端較尖銳,不平滑的現象
1 <!DOCTYPE html> 2 <html> 3 <head lang="en"> 4 <meta charset="UTF-8"> 5 <title>橢圓</title> 6 </head> 7 <body> 8 <canvas id="canvas" style="border:1px solid #aaa;display:block;margin:50px auto;"> 9 當前瀏覽器不支援Canvas,請更換瀏覽器後再試 10 </canvas> 11 <script> 12 var canvas = document.getElementById("canvas"); 13 canvas.width = 600; 14 canvas.height = 600; 15 var context = canvas.getContext("2d"); 16 context.lineWidth = 10; 17 context.strokeStyle="black"; 18 BezierEllipse1(context, 470, 80, 100, 20); //橢圓 19 function BezierEllipse1(context, x, y, a, b){ 20 //關鍵是bezierCurveTo中兩個控制點的設定 21 //0.5和0.6是兩個關鍵係數(在本函式中為試驗而得) 22 var ox = 0.5 * a, 23 oy = 0.6 * b; 24 context.save(); 25 context.translate(x, y); 26 context.beginPath(); 27 //從橢圓縱軸下端開始逆時針方向繪製 28 context.moveTo(0, b); 29 context.bezierCurveTo(ox, b, a, oy, a, 0); 30 context.bezierCurveTo(a, -oy, ox, -b, 0, -b); 31 context.bezierCurveTo(-ox, -b, -a, -oy, -a, 0); 32 context.bezierCurveTo(-a, oy, -ox, b, 0, b); 33 context.closePath(); 34 context.stroke(); 35 context.restore(); 36 }; 37 </script> 38 </body> 39 </html>
5.使用三次貝塞爾曲線模擬橢圓2
//此方法也會產生當lineWidth較寬,橢圓較扁時
//,長軸端較尖銳,不平滑的現象
//這種方法比前一個貝塞爾方法精確度高,但效率稍差
1 <!DOCTYPE html> 2 <html> 3 <head lang="en"> 4 <meta charset="UTF-8"> 5 <title>橢圓</title> 6 </head> 7 <body> 8 <canvas id="canvas" style="border:1px solid #aaa;display:block;margin:50px auto;"> 9 當前瀏覽器不支援Canvas,請更換瀏覽器後再試 10 </canvas> 11 <script> 12 var canvas = document.getElementById("canvas"); 13 canvas.width = 600; 14 canvas.height = 600; 15 var context = canvas.getContext("2d"); 16 context.lineWidth = 10; 17 context.strokeStyle="black"; 18 BezierEllipse2(context, 470, 200, 100, 20); //橢圓 19 function BezierEllipse2(ctx, x, y, a, b){ 20 var k = .5522848, 21 ox = a * k, // 水平控制點偏移量 22 oy = b * k; // 垂直控制點偏移量</p> <p> ctx.beginPath(); 23 //從橢圓的左端點開始順時針繪製四條三次貝塞爾曲線 24 ctx.moveTo(x - a, y); 25 ctx.bezierCurveTo(x - a, y - oy, x - ox, y - b, x, y - b); 26 ctx.bezierCurveTo(x + ox, y - b, x + a, y - oy, x + a, y); 27 ctx.bezierCurveTo(x + a, y + oy, x + ox, y + b, x, y + b); 28 ctx.bezierCurveTo(x - ox, y + b, x - a, y + oy, x - a, y); 29 ctx.closePath(); 30 ctx.stroke(); 31 }; 32 </script> 33 </body> 34 </html>
原文:https://blog.csdn.net/gao_xu_520/article/details/58588020