我目前在画布上尝试使用虚线。
我尝试的是ctx.setLineDash([5,5])，如果我绘得非常快，它就有效，但这就是全部。当我绘制一条非常慢的线时，该方法似乎不起作用。所以在我的画布上，我可以用鼠标绘制自己，我希望绘制的线条能够破灭。只有当我快速移动鼠标时，它才有效，当我移动它的速度很慢时，我只是直接穿过它。
即使画得很慢，我还能做些什么才能让它发挥作用？
代码：
<html>
<script type="text/javascript">
var canvas, ctx, flag = false,
prevX = 0,
currX = 0,
prevY = 0,
currY = 0,
dot_flag = false;
var x = "black",
y = 2;
function init() {
canvas = document.getElementById('can');
ctx = canvas.getContext("2d");
ctx.setLineDash([5,5]); //here I try to set the line to a dashed line
w = canvas.width;
h = canvas.height;
canvas.addEventListener("mousemove", function (e) {
findxy('move', e)
}, false);
canvas.addEventListener("mousedown", function (e) {
findxy('down', e)
}, false);
canvas.addEventListener("mouseup", function (e) {
findxy('up', e)
}, false);
canvas.addEventListener("mouseout", function (e) {
findxy('out', e)
}, false);
}
function color(obj) {
switch (obj.id) {
case "green":
x = "green";
break;
case "blue":
x = "blue";
break;
case "red":
x = "red";
break;
case "yellow":
x = "yellow";
break;
case "orange":
x = "orange";
break;
case "black":
x = "black";
break;
case "white":
x = "white";
break;
}
if (x == "white") y = 14;
else y = 2;
}
function draw() {
ctx.beginPath();
ctx.moveTo(prevX, prevY);
ctx.lineTo(currX, currY);
ctx.strokeStyle = x;
ctx.lineWidth = y;
ctx.stroke();
ctx.closePath();
}
function erase() {
var m = confirm("Want to clear");
if (m) {
ctx.clearRect(0, 0, w, h);
document.getElementById("canvasimg").style.display = "none";
}
}
function save() {
document.getElementById("canvasimg").style.border = "2px solid";
var dataURL = canvas.toDataURL();
document.getElementById("canvasimg").src = dataURL;
document.getElementById("canvasimg").style.display = "inline";
}
function findxy(res, e) {
if (res == 'down') {
prevX = currX;
prevY = currY;
currX = e.clientX - canvas.offsetLeft;
currY = e.clientY - canvas.offsetTop;
flag = true;
dot_flag = true;
if (dot_flag) {
ctx.beginPath();
ctx.fillStyle = x;
ctx.fillRect(currX, currY, 2, 2);
ctx.closePath();
dot_flag = false;
}
}
if (res == 'up' || res == "out") {
flag = false;
}
if (res == 'move') {
if (flag) {
prevX = currX;
prevY = currY;
currX = e.clientX - canvas.offsetLeft;
currY = e.clientY - canvas.offsetTop;
draw();
}
}
}
</script>
<body onload="init()">
<canvas id="can" width="400" height="400" style="position:absolute;top:10%;left:10%;border:2px solid;"></canvas>
<div style="position:absolute;top:12%;left:43%;">Choose Color</div>
<div style="position:absolute;top:15%;left:45%;width:10px;height:10px;background:green;" id="green" onclick="color(this)"></div>
<div style="position:absolute;top:15%;left:46%;width:10px;height:10px;background:blue;" id="blue" onclick="color(this)"></div>
<div style="position:absolute;top:15%;left:47%;width:10px;height:10px;background:red;" id="red" onclick="color(this)"></div>
<div style="position:absolute;top:17%;left:45%;width:10px;height:10px;background:yellow;" id="yellow" onclick="color(this)"></div>
<div style="position:absolute;top:17%;left:46%;width:10px;height:10px;background:orange;" id="orange" onclick="color(this)"></div>
<div style="position:absolute;top:17%;left:47%;width:10px;height:10px;background:black;" id="black" onclick="color(this)"></div>
<div style="position:absolute;top:20%;left:43%;">Eraser</div>
<div style="position:absolute;top:22%;left:45%;width:15px;height:15px;background:white;border:2px solid;" id="white" onclick="color(this)"></div>
<img id="canvasimg" style="position:absolute;top:10%;left:52%;" style="display:none;">
<input type="button" value="save" id="btn" size="30" onclick="save()" style="position:absolute;top:55%;left:10%;">
<input type="button" value="clear" id="clr" size="23" onclick="erase()" style="position:absolute;top:55%;left:15%;">
</body>
</html>
答案 0 :(得分：3)
您的问题是您正在每次鼠标移动时绘制一个新路径。破折号是从路径的起点到终点 当您慢慢移动鼠标时，实际上会生成许多非常小的小块，小于短划线阵列的5px。 当您更快地移动鼠标时，两点之间的距离大于10px，因此，您可以看到短划线。
此问题的解决方案是将您的点存储在数组中，并在每次重绘时从这些存储点重绘单个路径。通过这种方式，您的修补程序将变得更长，并且您的破折号将正常工作。
在下面的示例中，我甚至在每个mouseup上保存一个新路径，因此它们可以拥有自己的dashed属性：
const ctx = c.getContext('2d');
const pathes = []; // this is where we will store all our pathes
let mouse_down = false; // shall we draw ?
c.onmousedown = e => {
// add a new path object
pathes.push({
pts: [], // an array of points
dashed: check.checked // boolean
});
mouse_down = true; // we should draw
}
c.onmouseup = c.onmouseleave = e => mouse_down = false;
c.onmousemove = throttle(e => {
if (!mouse_down) {
return;
} else {
const rec = c.getBoundingClientRect();
// add a new point
addPoint(e.clientX - rec.left, e.clientY - rec.top);
redraw(); // redraw everything
}
});
function redraw() {
ctx.clearRect(0, 0, c.width, c.height); // we clear everything
// and draw every pathes
pathes.forEach(path => {
ctx.setLineDash(path.dashed ? [5, 5] : [0]);
ctx.beginPath();
path.pts.forEach(pt => ctx.lineTo(pt.x, pt.y));
ctx.stroke();
})
}
function addPoint(x, y) {
// append to the last one
const points = pathes[pathes.length - 1].pts;
points.push({
x: x,
y: y
});
}
// just to avoid unnecessary drawings
function throttle(callback) {
if (typeof callback !== 'function')
throw 'A callback function must be passed';
var active = false;
var evt;
var handler = function() {
active = false;
callback(evt);
};
return function handleEvent(e) {
evt = e;
if (!active) {
active = true;
requestAnimationFrame(handler);
}
};
}canvas {
border: 1px solid
}<label>dashed : <input type="checkbox" id="check" checked></label><br>
<canvas id="c" width="500" height="500"></canvas>
答案 1 :(得分：0)
<canvas id='asdf' width='200px' height='200px'></canvas>
var canvas = document.getElementById("asdf");
var abc = canvas.getContext("2d");
abc.setLineDash([5, 3]);