I am trying to draw a line on canvas by using mouse events
Question
I am trying to draw a line on canvas using mousedown , mousemove , mouseup events, but I am unable to set coordinates for drawing a line on Canvas. Here I am using the following JavaScript code:
function line()
{
canvas = document.getElementById("drawingCanvas");
context = canvas.getContext("2d");
canvas.onmousedown = startLine;
canvas.onmouseup = drawLine;
canvas.onmouseout = stopLine;
//canvas.onmousemove =drawLine;
};
function startLine(e)
{
isLine = true;
context.beginPath();
context.moveTo(e.pageX - canvas.offsetLeft, e.pageY - canvas.offsetTop);
}
function drawLine(e)
{
if (isLine==true)
{
var x = e.pageX - canvas.offsetLeft;
var y = e.pageY - canvas.offsetTop;
context.lineTo(x ,y);
context.stroke();
context.closePath();
}
cPush();
}
function stopLine()
{
isLine = false;
}
When I am using mousemove event for drawLine() method it is drawing multiple lines. Can you lease tell me what wrong in my code?
2 answers
solution1
4 2014-01-31 05:21:52
Basically, you need to refactor your code to do beginPath+moveTo+lineTo+stroke inside mousemove.
Otherwise you will get those multiple lines...
In mouseDown: Save the startX/startY and set the isDown flag to indicate the drag has started:
function handleMouseDown(e){
e.preventDefault();
startX=parseInt(e.clientX-offsetX);
startY=parseInt(e.clientY-offsetY);
isDown=true;
}
In mouseMove: Draw a new line from startX/Y to mouseX/Y and reset startX/Y=mouseX/Y
function handleMouseMove(e){
if(!isDown){return;}
e.preventDefault();
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
ctx.beginPath(); // use beginPath for every segment of the line
ctx.moveTo(startX,startY);
ctx.lineTo(mouseX,mouseY);
ctx.stroke();
startX=mouseX;
startY=mouseY;
}
In mouseUP: Clear the isDown flag to indicate the drag has ended:
function handleMouseUp(e){
e.preventDefault();
isDown=false;
}
Here's code and a Fiddle: http://jsfiddle.net/m1erickson/hzNg4/
<!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 $canvas=$("#canvas");
var canvasOffset=$canvas.offset();
var offsetX=canvasOffset.left;
var offsetY=canvasOffset.top;
var scrollX=$canvas.scrollLeft();
var scrollY=$canvas.scrollTop();
var isDown=false;
var startX;
var startY;
function handleMouseDown(e){
e.preventDefault();
startX=parseInt(e.clientX-offsetX);
startY=parseInt(e.clientY-offsetY);
isDown=true;
}
function handleMouseUp(e){
e.preventDefault();
isDown=false;
}
function handleMouseOut(e){
e.preventDefault();
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
isDown=false;
}
function handleMouseMove(e){
if(!isDown){return;}
e.preventDefault();
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
// Put your mousemove stuff here
ctx.beginPath();
ctx.moveTo(startX,startY);
ctx.lineTo(mouseX,mouseY);
ctx.stroke();
startX=mouseX;
startY=mouseY;
}
$("#canvas").mousedown(function(e){handleMouseDown(e);});
$("#canvas").mousemove(function(e){handleMouseMove(e);});
$("#canvas").mouseup(function(e){handleMouseUp(e);});
$("#canvas").mouseout(function(e){handleMouseOut(e);});
}); // end $(function(){});
</script>
</head>
<body>
<canvas id="canvas" width=300 height=300></canvas>
</body>
</html>
solution2
1 2014-01-31 05:27:33
Try this way ( DEMO ) :
function line() {
canvas = document.getElementById("drawingCanvas");
context = canvas.getContext("2d");
canvas.onmousedown = startLine;
canvas.onmouseup = canvas.onmouseout = stopLine;
canvas.onmousemove = drawLine;
};
function startLine(e) {
isLine = true;
context.beginPath();
context.moveTo(startX = (e.pageX - canvas.offsetLeft),
startY = (e.pageY - canvas.offsetTop));
}
function drawLine(e) {
if (isLine) {
var x = e.pageX - canvas.offsetLeft;
var y = e.pageY - canvas.offsetTop;
context.clearRect(0, 0, 300, 150); // width = 300, height = 150
context.beginPath();
context.moveTo(startX, startY);
context.lineTo(x, y);
context.stroke();
context.closePath();
}
//cPush();
}
function stopLine() {
isLine = false;
}
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