如何在画布元素中正确绘制带有间隙的圆?

如果您想要一个稍微灵活的解决方案,即,如果您想要更改段的数量和每个段的大小,那么您可以创建一个通用函数,例如:

/**
 * ctx      = context
 * x / y    = center
 * radius   = of circle
 * offset   = rotation in angle (radians)
 * segments = How many segments circle should be in
 * size     = size of each segment (of one segment) [0.0, 1.0]
*/
function dashedCircle(ctx, x, y, radius, offset, segments, size) {

    var pi2 = 2 * Math.PI,             /// cache 2*Math.PI = 360 degrees in rads
        segs = pi2 / segments,         /// calc. size of each segment in rads
        len = segs * size,             /// calc. length of segment in rads
        i = 0,
        ax, ay;

    ctx.save();
    ctx.translate(x, y);
    ctx.rotate(offset);                /// rotate canvas
    ctx.translate(-x, -y);

    for(; i < pi2; i += segs) {
        ax = x + radius * Math.cos(i); /// calculate start position of arc
        ay = y + radius * Math.sin(i);
        ctx.moveTo(ax, ay);            /// make sure to move to beginning of arc
        ctx.arc(x, y, radius, i, i + len); /// draw arc
    }

    ctx.restore();                     /// remove rotation
}

然后在代码中,只需调用:

ctx.beginPath();

dashedCircle(ctx, 250, 250, 100, 0, 5, 0.7);

ctx.lineWidth = 15;
ctx.strokeStyle = 'hsla(111, 56%, 50%, 0.67)';
ctx.stroke();
/// don't closePath here

Modified fiddle here

使用偏移参数可以轻松旋转:

var offset = 0;
var step = 0.03;
var pi2 = 2 * Math.PI;

(function loop() {
    ctx.clearRect(0, 0, canvas.width, canvas.height);
    ctx.beginPath();

    dashedCircle(ctx, 250, 250, 100, offset % pi2, 5, 0.7);
    ctx.stroke();

    offset += step;

    requestAnimationFrame(loop);
})();

Rotating circle fiddle

moveTo需要位于弧的起点,而不是中心。这最容易通过平移到圆的中心来实现,如下所示。 http://jsfiddle.net/utWdM/1

function draw() {
    var canvas = document.getElementById("canvas");
    if (canvas.getContext) {
        var ctx = canvas.getContext("2d");
        ctx.lineWidth = 15;
        ctx.strokeStyle = 'hsla(111, 56%, 50%, 0.67)';
        ctx.beginPath();

        ctx.translate(250,250);
        ctx.arc(0, 0, 100, 0, Math.PI * 8 / 30);
        ctx.moveTo(100*Math.cos(Math.PI*12/30), 100*Math.sin(Math.PI*12/30));
        ctx.arc(0, 0, 100, Math.PI * 12 / 30, Math.PI * 20 / 30);
        ctx.moveTo(100*Math.cos(Math.PI*24/30), 100*Math.sin(Math.PI*24/30));
        ctx.arc(0, 0, 100, Math.PI * 24 / 30, Math.PI * 32 / 30);
        ctx.moveTo(100*Math.cos(Math.PI*36/30), 100*Math.sin(Math.PI*36/30));
        ctx.arc(0, 0, 100, Math.PI * 36 / 30, Math.PI * 44 / 30);
        ctx.moveTo(100*Math.cos(Math.PI*48/30), 100*Math.sin(Math.PI*48/30));
        ctx.arc(0, 0, 100, Math.PI * 48 / 30, Math.PI * 56 / 30);

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

避免复杂数学的最简单方法是调用 .beginPath() 和 .stroke(); 对于每个段。我知道这很重复,但它看起来是这样的:

function draw() {
    var canvas = document.getElementById("canvas");
    if (canvas.getContext) {
        var ctx = canvas.getContext("2d");
        ctx.lineWidth = 15;
        ctx.strokeStyle = 'hsla(111, 56%, 50%, 0.67)';

        ctx.beginPath();        
        ctx.arc(250, 250, 100, 0, Math.PI * 8 / 30);
        ctx.stroke();

        ctx.beginPath();
        ctx.arc(250, 250, 100, Math.PI * 12 / 30, Math.PI * 20 / 30);
        ctx.stroke();

        ctx.beginPath();
        ctx.arc(250, 250, 100, Math.PI * 24 / 30, Math.PI * 32 / 30);
        ctx.stroke();

        ctx.beginPath();
        ctx.arc(250, 250, 100, Math.PI * 36 / 30, Math.PI * 44 / 30);
        ctx.stroke();

        ctx.beginPath();
        ctx.arc(250, 250, 100, Math.PI * 48 / 30, Math.PI * 56 / 30);
        ctx.stroke();
    }
}

draw();

这是你的小提琴: http://jsfiddle.net/utWdM/13/