【发布时间】：2017-08-22 10:03:44
【问题描述】：
我有一个页面，我在其中添加了一个画布，技术人员或客户将使用该画布进行签名。我已经设法让它工作并能够在画布内绘图。但是，当页面向下滚动一点时，绘图开始位置的位置高于鼠标点。当我再次将页面滚动回顶部时，绘图开始的位置再次直接在鼠标点上。
无论页面是否滚动，如何获得正确的 X 和 Y 位置？
这就是我所做的：
HTML：
<body onload="init();">
<div id="dialogSignature" runat="server" style="padding-top:10px; display:none; ">
<canvas id="myCanvas" width="400" height="200" style="border:2px solid;"></canvas>
<img id="canvasimg" style="position:absolute;top:10%;left:52%;" style="display:none;">
</body>
<table>
<tr>
<td>
<input type="button" value="Clear" id="clr" size="23" onclick="erase()" class="myButton" >
</td>
<td>
<button class="myButton" onclick="javascript:UploadPic();return false;">Sumbit</button>
</td>
</tr>
</table>
<asp:Label ID="lblText" runat="server" Text="Label" Visible="false"></asp:Label>
<asp:Label ID="lblScopeIdentity" runat="server" Text="sss" Visible="false"></asp:Label>
</div>
Javascript：
<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('myCanvas');
ctx = canvas.getContext("2d");
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);
canvas.addEventListener("touchstart", function (e) {
mousePos = getTouchPos(canvas, e);
var touch = e.touches[0];
var mouseEvent = new MouseEvent("mousedown", {
clientX: touch.clientX,
clientY: touch.clientY
});
canvas.dispatchEvent(mouseEvent);
}, false);
canvas.addEventListener("touchend", function (e) {
var mouseEvent = new MouseEvent("mouseup", {});
canvas.dispatchEvent(mouseEvent);
}, false);
canvas.addEventListener("touchmove", function (e) {
var touch = e.touches[0];
var mouseEvent = new MouseEvent("mousemove", {
clientX: touch.clientX,
clientY: touch.clientY
});
canvas.dispatchEvent(mouseEvent);
}, false);
// Prevent scrolling when touching the canvas
document.body.addEventListener("touchstart", function (e) {
if (e.target == canvas) {
e.preventDefault();
}
}, false);
document.body.addEventListener("touchend", function (e) {
if (e.target == canvas) {
e.preventDefault();
}
}, false);
document.body.addEventListener("touchmove", function (e) {
if (e.target == canvas) {
e.preventDefault();
}
}, false);
}
function getTouchPos(canvasDom, touchEvent) {
var rect = canvasDom.getBoundingClientRect();
return {
x: touchEvent.touches[0].clientX - rect.left,
y: touchEvent.touches[0].clientY - rect.top
};
}
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();
}
}
}
var scopeIdentityUse = "<%=scopeIdentityJS1%>";
var Pic = document.getElementById("myCanvas").toDataURL("image/png");
Pic = Pic.replace(/^data:image\/(png|jpg);base64,/, "")
$.ajax({
type: 'POST',
url: 'Save_Picture.aspx/UploadPic',
data: '{ "imageData" : "' + Pic + '", "param" : "' + scopeIdentityUse + '" }',
contentType: 'application/json; charset=utf-8',
dataType: 'json',
success: function (msg) {
document.location.href = 'CreatePDFReport.aspx'
//alert("Done, Picture Uploaded.");
}
});
</script>
【问题讨论】：
标签： javascript jquery html canvas