椭圆

注意：浏览器正在添加内置的 context.ellipse 绘图命令，但此命令并未被普遍采用（特别是在 IE 中）。以下方法适用于所有浏览器。

根据所需的左上角坐标绘制一个椭圆：

// draws an ellipse based on x,y being top-left coordinate
function drawEllipse(x,y,width,height){
    var PI2=Math.PI*2;
    var ratio=height/width;
    var radius=Math.max(width,height)/2;
    var increment = 1 / radius;
    var cx=x+width/2;
    var cy=y+height/2;
    
    ctx.beginPath();
    var x = cx + radius * Math.cos(0);
    var y = cy - ratio * radius * Math.sin(0);
    ctx.lineTo(x,y);

    for(var radians=increment; radians<PI2; radians+=increment){ 
        var x = cx + radius * Math.cos(radians);
        var y = cy - ratio * radius * Math.sin(radians);
        ctx.lineTo(x,y);
     }

    ctx.closePath();
    ctx.stroke();
}

根据所需的中心点坐标绘制一个椭圆：

// draws an ellipse based on cx,cy being ellipse's centerpoint coordinate
function drawEllipse2(cx,cy,width,height){
    var PI2=Math.PI*2;
    var ratio=height/width;
    var radius=Math.max(width,height)/2;
    var increment = 1 / radius;

    ctx.beginPath();
    var x = cx + radius * Math.cos(0);
    var y = cy - ratio * radius * Math.sin(0);
    ctx.lineTo(x,y);

    for(var radians=increment; radians<PI2; radians+=increment){ 
        var x = cx + radius * Math.cos(radians);
        var y = cy - ratio * radius * Math.sin(radians);
        ctx.lineTo(x,y);
     }

    ctx.closePath();
    ctx.stroke();
}