我有一个Web应用程序,您可以在画布上绘制一个矩形。我使用两个画布元素:一个用于绘制时的预览,另一个用于精确地放置在另一个下,以进行绘制。
我的问题是,在Internet Explorer中,canvas2.width = canvas2.width
不会清除canvas2的内容,这是必需的,因为每次鼠标移动都会再次绘制矩形。我也尝试过context2.clearRect(0,0,canvas2.width,canvas2.height)
,但是,预览矩形根本无法绘制。在http://jsfiddle.net/Y389a/2/上试用
HTML:
<canvas id="canvas" width="600" height="400"></canvas>
<canvas id="canvas2" width="600" height="400" onmouseup="return drawLine()" onmousedown="return startLine()"></canvas>
CSS:
#canvas, #canvas2 {
position:absolute;
left:0px;
top:0px;
border-width:1px;
border-style:solid;
border-color:#666666;
cursor:default !important;
}
Javascript:
var x; var xStart;
var y; var yStart;
var clicked = false;
var canvas = document.getElementById("canvas");
var context = canvas.getContext("2d");
var canvas2 = document.getElementById("canvas2");
var context2 = canvas2.getContext("2d");
context.strokeStyle = "black";
context.lineCap = "round";
canvas2.addEventListener('mousemove', function (evt) {
var rect = canvas2.getBoundingClientRect();
x = evt.clientX - rect.left;
y = evt.clientY - rect.top;
if (clicked) {
canvas2.width = canvas2.width;
context2.rect(xStart, yStart, x - xStart, y - yStart);
context2.stroke();
}
}, false);
function startLine() {
context.beginPath();
xStart = x; yStart = y;
clicked = true;
}
function drawLine() {
clicked = false;
context.rect(xStart, yStart, x - xStart, y - yStart);
context.stroke();
}
问题
您正在绘制带有context2.rect
路径命令的矩形。
路径命令“记住”的画布,直到一个新的context2.beginPath
发
因此,当您执行操作时,您以前的所有区域都将被记住并重新绘制 context2.stroke
使固定
只需将context2.beginPath放在mousemove事件处理程序中:http : //jsfiddle.net/m1erickson/A8ge6/
canvas2.addEventListener("mousedown",startLine);
canvas2.addEventListener("mouseup",drawLine);
canvas2.addEventListener('mousemove', function (evt) {
var rect = canvas2.getBoundingClientRect();
x = evt.clientX - rect.left;
y = evt.clientY - rect.top;
if (clicked) {
canvas2.width = canvas2.width;
console.log(xStart);
// add beginPath so previous context2.rect's are dismissed
context2.beginPath();
context2.rect(xStart, yStart, x - xStart, y - yStart);
context2.stroke();
}
}, false);
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