CanvasRenderingContext2D.lineDashOffset 是 Canvas 2D API 设置虚线偏移量的属性，例如可以实现 “蚂蚁线“ 的效果。
ctx.lineDashOffset = value;
value0.0。lineDashOffset 属性这是一段简单的代码片段，使用 lineDashOffset 属性绘制虚线。
<canvas id="canvas"></canvas>
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
ctx.setLineDash([4, 16]);
ctx.lineDashOffset = 2;
ctx.beginPath();
ctx.moveTo(0,100);
ctx.lineTo(400, 100);
ctx.stroke();
修改下面的代码并在线查看 canvas 变化：
<canvas id="canvas" width="400" height="200" class="playable-canvas"></canvas> <div class="playable-buttons"> <input id="edit" type="button" value="Edit" /> <input id="reset" type="button" value="Reset" /> </div> <textarea id="code" class="playable-code" style="height: 120px"> ctx.setLineDash([4, 16]); ctx.lineDashOffset = 2; ctx.beginPath(); ctx.moveTo(0,100); ctx.lineTo(400, 100); ctx.stroke();</textarea>
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var textarea = document.getElementById("code");
var reset = document.getElementById("reset");
var edit = document.getElementById("edit");
var code = textarea.value;
function drawCanvas() {
ctx.clearRect(0, 0, canvas.width, canvas.height);
eval(textarea.value);
}
reset.addEventListener("click", function() {
textarea.value = code;
drawCanvas();
});
edit.addEventListener("click", function() {
textarea.focus();
})
textarea.addEventListener("input", drawCanvas);
window.addEventListener("load", drawCanvas);
{{ EmbedLiveSample('Playable_code', 700, 380) }}
”蚂蚁线“效果是一种动画技巧，经常出现在计算机绘图程序的套索工具中。 它能帮助用户根据图片背景动态变化的边界来区分选择的边界。
<canvas id="canvas" class="playable-canvas"></canvas>
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var offset = 0;
function draw() {
ctx.clearRect(0,0, canvas.width, canvas.height);
ctx.setLineDash([4, 2]);
ctx.lineDashOffset = -offset;
ctx.strokeRect(10,10, 100, 100);
}
function march() {
offset++;
if (offset > 16) {
offset = 0;
}
draw();
setTimeout(march, 20);
}
march();
{{ EmbedLiveSample('Marching_ants', 700, 200) }}
| Specification | Status | Comment |
|---|---|---|
| {{SpecName('HTML WHATWG', "scripting.html#dom-context-2d-linedashoffset", "CanvasRenderingContext2D.lineDashOffset")}} | {{Spec2('HTML WHATWG')}} |
{{Compat("api.CanvasRenderingContext2D.lineDashOffset")}}