Drawing on mouse move with Canvas, HMTL5?
Question
What I am trying to achieve is a simple functionality to draw lines on a canvas when the mouse is clicked.
I have looked at code online and am trying to implement it myself but it won't work.
So far:
<html>
<canvas id="myCanvas" width="400" height="500"> </canvas>
</html>
<script type="text/javascript">
var el = document.getElementById('myCanvas');
var ctx = el.getContext('2d');
var isDrawing;
el.onmousedown = function(e) {
isDrawing = true;
ctx.moveTo(e.clientX, e.clientY);
};
el.onmousemove = function(e) {
if (isDrawing) {
ctx.lineTo(e.clientX, e.clientY);
ctx.stroke();
}
};
el.onmouseup = function() {
isDrawing = false;
};
</script>
This doesn't output anything on the canvas however. I don't fully understand the event handling aspect of the code (ie, e.clientX, e.clientY) I suspect I must add to this code for these parts to have the desired effect?
2 answers
solution1
0 2014-04-17 18:27:26
粗略浏览一下，您会在第2行的末尾缺少“>”。
solution2
0 2014-04-17 19:07:13
A couple of issues:
Adjust your mouse positions by the offset of the canvas (unless your canvas is at top-left of the browser)
do all drawing commands in mousemove (otherwise you will restroke the line with every ctx.stroke)
Here's example code and a Demo: http://jsfiddle.net/m1erickson/kkLrT/
<!doctype html>
<html>
<head>
<link rel="stylesheet" type="text/css" media="all" href="css/reset.css" /> <!-- reset css -->
<script type="text/javascript" src="http://code.jquery.com/jquery.min.js"></script>
<style>
body{ background-color: ivory; }
canvas{border:1px solid red;}
</style>
<script>
$(function(){
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var BB=canvas.getBoundingClientRect();
var offsetX=BB.left;
var offsetY=BB.top;
var lastX,lastY;
var isDown=false;
canvas.onmousedown=handleMousedown;
canvas.onmousemove=handleMousemove;
canvas.onmouseup=handleMouseup;
function handleMousedown(e){
e.preventDefault();
e.stopPropagation();
lastX=e.clientX-offsetX;
lastY=e.clientY-offsetY;
isDown=true;
}
function handleMouseup(e){
e.preventDefault();
e.stopPropagation();
isDown=false;
}
function handleMousemove(e){
e.preventDefault();
e.stopPropagation();
if(!isDown){return;}
var mouseX=e.clientX-offsetX;
var mouseY=e.clientY-offsetY;
ctx.beginPath();
ctx.moveTo(lastX,lastY);
ctx.lineTo(mouseX,mouseY);
ctx.stroke();
lastX=mouseX;
lastY=mouseY;
}
}); // end $(function(){});
</script>
</head>
<body>
<h4>Drag mouse to draw.</h4>
<canvas id="canvas" width=300 height=300></canvas>
</body>
</html>
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