Develop Reference

faster method of copying image data

i have the following script
// copy image data to secondary canvas
var pixelData = contextSource.getImageData(x - (lineWidth/2), y - (lineWidth/2), lineWidth, lineWidth);
var tmpCanvas = document.createElement('canvas');
tmpCanvas.width = tmpCanvas.height = lineWidth;
var tmpContext = tmpCanvas.getContext('2d');
tmpContext.putImageData(pixelData, 0, 0);
contextDest.save();
contextDest.arc(x, y, (lineWidth/2), 0, 2*Math.PI);
contextDest.clip();
contextDest.drawImage(tmpCanvas, x - (lineWidth/2), y - (lineWidth/2));
contextDest.restore();
the script is sampling image data from canvas source when mouse is moving over the source, then copy it to destination. the script is working good, but a bit slow. here is the result when i move the mouse pointer a bit faster.
is there any faster method than mine? please help

#Banana has a good point.
Instead of drawing just the mouse positions, try connecting the positions into a single continuous line..
If you want the rounded effect in your illustration, you can set:
context.lineCap='round';
context.lineJoin='round';
As to masking faster...
A much faster way of masking your image is to:
draw your single line on the second canvas.
Use compositing to make all new draws be visible only where existing pixels are opaque. This compositing is 'source-in' and is set using: context.globalCompositeOperation='source-in'
Draw your image onto the second canvas. The image will show only where the line was.
Using compositing is much faster because compositing is hardware accelerated and is optimized by the browser.
That compositing code might look like this:
function draw(){
// clear the second canvas
ctx1.clearRect(0,0,cw,ch);
// draw your continuous line
ctx1.beginPath();
ctx1.moveTo(points[0].x,points[0].y);
for(var i=1;i<points.length;i++){
p=points[i];
ctx1.lineTo(p.x,p.y);
}
ctx1.stroke();
// set compositing so new draws only appear in
// existing opaque pixels
ctx1.globalCompositeOperation='source-in';
// draw the image
// the image will only be visible in the exising line
ctx1.drawImage(img,0,0);
// be tidy! return compositing to its default mode
ctx1.globalCompositeOperation='source-over';
}
Here's an example and Demo:
var canvas1=document.getElementById("canvas1");
var ctx1=canvas1.getContext("2d");
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;
var points=[];
var cw,ch;
var img=new Image();
img.onload=start;
img.src="https://dl.dropboxusercontent.com/u/139992952/stackoverflow/tempHouse%201.jpg";
function start(){
cw=canvas.width=canvas1.width=img.width;
ch=canvas.height=canvas1.height=img.height;
ctx1.lineCap = "round";
ctx1.lineJoin = "round";
ctx1.lineWidth=20;
ctx.drawImage(img,0,0);
$("#canvas").mousedown(function(e){handleMouseDown(e);});
$("#canvas").mousemove(function(e){handleMouseMove(e);});
$("#canvas").mouseup(function(e){handleMouseUp(e);});
$("#canvas").mouseout(function(e){handleMouseOut(e);});
}
function draw(){
// clear the second canvas
ctx1.clearRect(0,0,cw,ch);
// draw your continuous line
ctx1.beginPath();
ctx1.moveTo(points[0].x,points[0].y);
for(var i=1;i<points.length;i++){
p=points[i];
ctx1.lineTo(p.x,p.y);
}
ctx1.stroke();
// set compositing so new draws only appear in
// existing opaque pixels
ctx1.globalCompositeOperation='source-in';
// draw the image
// the image will only be visible in the exising line
ctx1.drawImage(img,0,0);
// be tidy! return compositing to its default mode
ctx1.globalCompositeOperation='source-over';
}
function handleMouseDown(e){
e.preventDefault();
e.stopPropagation();
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
points.length=0;
points.push({x:mouseX,y:mouseY});
isDown=true;
}
function handleMouseUp(e){
e.preventDefault();
e.stopPropagation();
isDown=false;
draw();
}
function handleMouseOut(e){
e.preventDefault();
e.stopPropagation();
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
points.push({x:mouseX,y:mouseY});
isDown=false;
draw();
}
function handleMouseMove(e){
if(!isDown){return;}
e.preventDefault();
e.stopPropagation();
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
points.push({x:mouseX,y:mouseY});
draw();
}
body{ background-color: ivory; }
canvas{border:1px solid red;}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<canvas id="canvas" width=300 height=300></canvas><br>
<canvas id="canvas1" width=300 height=300></canvas>

Draw a line on mouse drag on a canvas loaded with an image

I have a canvas loaded with an image.
Now, I want to mark lines on some part of the image.
So, I wrote some code which draws a line on mouse drag.
function drawLine(x, y) {
ctx.beginPath();
ctx.moveTo(startX, startY);
ctx.lineTo(x, y);
ctx.closePath();
ctx.stroke();
ctx.restore();
}
canvas.onmousedown = function (e) {
ctx.save();
e.preventDefault();
e.stopPropagation();
startX = parseInt(e.clientX - offsetX);
startY = parseInt(e.clientY - offsetY);
isDown = true;
}
canvas.onmousemove = function (e) {
if (!isDown) {
return;
}
e.preventDefault();
e.stopPropagation();
mouseX = parseInt(e.clientX - offsetX);
mouseY = parseInt(e.clientY - offsetY);
drawLine(mouseX, mouseY);
}
canvas.onmouseup = function (e) {
if (!isDown) {
return;
}
e.preventDefault();
e.stopPropagation();
isDown = false;
}
The issue now is that, the drag creates a series of lines instead of one.
I have tried to save the context before drawing and restore it after drawing, but didn't help.
Note that, I cannot clear the canvas using clearRect(), as my image data will be lost.
Here, I have used a dummy image.
I have created a fiddle.
Any help would be appreciated.
Regards,
Rohith

One way of doing it is to save all your drag-lines in an array of drag-lines.
That way you can clear the canvas and redraw the image and redraw all the accumulated lines.
Example code and a Demo: http://jsfiddle.net/m1erickson/bH38a/
The green lines are my (totally non-medical !) additions.
BTW, this example reduces the opacity of the background image so the lines are more visible (a handy technique).
<!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>
$(function(){
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var canvasOffset=$("#canvas").offset();
var offsetX=canvasOffset.left;
var offsetY=canvasOffset.top;
var startX,startY,mouseX,mouseY;
var isDown=false;
var lines=[];
var imageOpacity=0.33;
var img=new Image();
img.crossOrigin="anonymous";
img.onload=start;
img.src="https://dl.dropboxusercontent.com/u/139992952/multple/heart.jpg";
function start(){
canvas.width=canvas.width=img.width;
canvas.height=img.height;
ctx.strokeStyle="green";
ctx.lineWidth=3;
$("#canvas").mousedown(function(e){handleMouseDown(e);});
$("#canvas").mousemove(function(e){handleMouseMove(e);});
$("#canvas").mouseup(function(e){handleMouseUp(e);});
$("#canvas").mouseout(function(e){handleMouseUp(e);});
// redraw the image
drawTheImage(img,imageOpacity);
}
function drawLines(toX,toY){
// clear the canvas
ctx.clearRect(0,0,canvas.width,canvas.height);
// redraw the image
drawTheImage(img,imageOpacity);
// redraw all previous lines
for(var i=0;i<lines.length;i++){
drawLine(lines[i]);
}
// draw the current line
drawLine({x1:startX,y1:startY,x2:mouseX,y2:mouseY});
}
function drawTheImage(img,opacity){
ctx.globalAlpha=opacity;
ctx.drawImage(img,0,0);
ctx.globalAlpha=1.00;
}
function drawLine(line){
ctx.beginPath();
ctx.moveTo(line.x1, line.y1);
ctx.lineTo(line.x2, line.y2);
ctx.stroke();
}
function handleMouseDown(e){
e.stopPropagation();
e.preventDefault();
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
// Put your mousedown stuff here
startX=mouseX;
startY=mouseY;
isDown=true;
}
function handleMouseUp(e){
e.stopPropagation();
e.preventDefault();
// Put your mouseup stuff here
isDown=false;
lines.push({x1:startX,y1:startY,x2:mouseX,y2:mouseY});
}
function handleMouseMove(e){
if(!isDown){return;}
e.stopPropagation();
e.preventDefault();
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
// Put your mousemove stuff here
drawLines(mouseX,mouseY);
}
$("#save").click(function(){
var html="<p>Right-click on image below and Save-Picture-As</p>";
html+="<img src='"+canvas.toDataURL()+"' alt='from canvas'/>";
var tab=window.open();
tab.document.write(html);
});
}); // end $(function(){});
</script>
</head>
<body>
<h4>Drag-draw lines.</h4>
<button id="save">Export Image</button><br>
<canvas id="canvas"></canvas>
</body>
</html>

How to differentiate click and mousedown event in JavaScript?

I have a canvas, I want to draw dots when user clicked and draw a line when clicked and dragged.
In order to identify whether I should generate a line when mouse is moving on the canvas, I set a variable 'isDrawing' to tell if the user has clicked on the canvas before moving on it. I bind 'mousedown' event to the canvas and set 'isDrawing' to true when the event is triggered. If it is true I will start drawing a line, otherwise I will do nothing to this behavior. But the problem is when user clicked to draw dots, the 'isDrawing' is also set to true because the 'mousedown' event is triggered by the click. My question is how to differentiate the click and mousedown event so that when user just clicked somewhere the 'mousedown' event will not be triggered? thanks.

#Aaron has the start of a good idea...Add your dot in mouseup instead of mousedown.
In mouseup if the mouse has been dragged less than 5 total pixels then treat the mouseup as a click rather than a drag. (5 pixels is an example--adjust for your desired tolerances).
In mousemove, delay drawing your line until the mouse has been dragged at least 5 pixels.
Here's example code and a Demo: http://jsfiddle.net/m1erickson/ZTuKP/
<!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 lastX,lastY;
var dragHash;
function handleMouseDown(e){
e.preventDefault();
e.stopPropagation();
lastX=parseInt(e.clientX-offsetX);
lastY=parseInt(e.clientY-offsetY);
// Put your mousedown stuff here
dragHash=0;
isDown=true;
}
function handleMouseUp(e){
e.preventDefault();
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
if(dragHash<5){
alert("It's a click...add a dot");
}else{
alert("You've been dragging");
}
// Put your mouseup stuff here
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
var dx=mouseX-lastX;
var dy=mouseY-lastY;
lastX=mouseX;
lastY=mouseY;
// accumulate the drag distance
// (used in mouseup to see if this is a drag or click)
dragHash+=Math.abs(dx)+Math.abs(dy);
if(dragHash>4){
// it's a drag operation, draw the line
}
}
$("#canvas").mousedown(function(e){handleMouseDown(e);});
$("#canvas").mousemove(function(e){handleMouseMove(e);});
$("#canvas").mouseup(function(e){handleMouseUp(e);});
}); // end $(function(){});
</script>
</head>
<body>
<canvas id="canvas" width=300 height=300></canvas>
</body>
</html>

Here's an example using pure javascript small and compact: http://jsfiddle.net/kychan/2t97S/
function e(id) { return document.getElementById(id); }
var box = e('box'),
ctx = box.getContext('2d'),
w = box.width,
h = box.height,
mx = 0,
my = 0
;
ctx.fillStyle = '#333';
ctx.fillRect(0,0,w,h);
ctx.fillStyle = '#FF0000';
ctx.strokeStyle= '#FF0000';
box.addEventListener('mousedown', function(e) {
mx = e.pageX - box.offsetLeft,
my = e.pageY - box.offsetTop;
}, false);
// reduces dender.
function d(i,c) {
return (c-10<i && c+10>i);
}
box.addEventListener('mouseup', function(e) {
var nx = e.pageX - box.offsetLeft,
ny = e.pageY - box.offsetTop;
ctx.beginPath();
if (d(mx,nx) && d(my,ny)) {
ctx.arc(mx,my,1, 0, Math.PI*2, false);
}else{
ctx.moveTo(mx, my);
ctx.lineTo(nx, ny);
}
ctx.closePath();
ctx.stroke();
mx=nx, my=ny;
}, false);

How to draw a rectangle on canvas like we do on paint?

Say i want to draw a rectangle on canvas. I want to be able to get the co-ordinates from user's mouse. Ideal scenario is user clicks at a point and drags down to another end like those rectangles we draw using paint. How can i draw a rectangle like we do in paint by dragging mouse? (how to get the co-ordinates of the mouse when he clicks mouse and leaves at?)

Here's a outline of how to drag-draw a rectangle on canvas:
In mousedown:
save the starting mouse position
set a flag indicating the drag has begun
In mousemove:
clear the canvas of the previous rectangle
calculate the rectangle width/height based on the starting vs current mouse position
draw a rectangle from the starting XY to the current mouse position
In mouseup:
clear the dragging flag because the drag is over
Here's example code and a Demo: http://jsfiddle.net/m1erickson/6E2yd/
<!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(){
// get references to the canvas and context
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
// style the context
ctx.strokeStyle = "blue";
ctx.lineWidth=3;
// calculate where the canvas is on the window
// (used to help calculate mouseX/mouseY)
var $canvas=$("#canvas");
var canvasOffset=$canvas.offset();
var offsetX=canvasOffset.left;
var offsetY=canvasOffset.top;
var scrollX=$canvas.scrollLeft();
var scrollY=$canvas.scrollTop();
// this flage is true when the user is dragging the mouse
var isDown=false;
// these vars will hold the starting mouse position
var startX;
var startY;
function handleMouseDown(e){
e.preventDefault();
e.stopPropagation();
// save the starting x/y of the rectangle
startX=parseInt(e.clientX-offsetX);
startY=parseInt(e.clientY-offsetY);
// set a flag indicating the drag has begun
isDown=true;
}
function handleMouseUp(e){
e.preventDefault();
e.stopPropagation();
// the drag is over, clear the dragging flag
isDown=false;
}
function handleMouseOut(e){
e.preventDefault();
e.stopPropagation();
// the drag is over, clear the dragging flag
isDown=false;
}
function handleMouseMove(e){
e.preventDefault();
e.stopPropagation();
// if we're not dragging, just return
if(!isDown){return;}
// get the current mouse position
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
// Put your mousemove stuff here
// clear the canvas
ctx.clearRect(0,0,canvas.width,canvas.height);
// calculate the rectangle width/height based
// on starting vs current mouse position
var width=mouseX-startX;
var height=mouseY-startY;
// draw a new rect from the start position
// to the current mouse position
ctx.strokeRect(startX,startY,width,height);
}
// listen for mouse events
$("#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>
<h4>Drag the mouse to create a rectangle</h4>
<canvas id="canvas" width=300 height=300></canvas>
</body>
</html>
clear the

See this, move you mouse over the square, and witness the awesomeness of Process.js
http://processingjs.org/learning/topic/pattern/

Using this function you can get the mousecoordinates
function getMousePos(canvas, evt) {
var rect = canvas.getBoundingClientRect();
return {
x: evt.clientX - rect.left,
y: evt.clientY - rect.top
};
}
this function takes in the canvas object and the event.
Now you just have to add an eventHandler on mousedown and mouseup and you can get both the locations.
var canvas = document.getElementById('canvasId');
var ctx = canvas.getContext('2d');
var locA, locB;
document.addEventListener('mousedown', function(e) {
e.preventDefault();
locA = getMousePos(canvas, e);
});
document.addEventListener('mouseup', function(e) {
e.preventDefault();
locB = getMousePos(canvas, e);
ctx.fillStyle = '#000000';
ctx.fillRect(locA.x, locA.y, (locB.x - locA.x), (locB.y - locA.y));
});
function source: http://www.html5canvastutorials.com/advanced/html5-canvas-mouse-coordinates/
There are still some problems surrounding canvas coordinates vs document coordinates, but I'm sure you'll be able to fix that.

Show converted mouse coordinates of an element with javascript

I'd like to show a mouse tooltip like this:
with a coordinate system relative to its image.
Whenever the mouse is hovered over one of the 75x75 cells, the position is displayed in text. I can only show the mouse's raw coordinates, but can't figure out the math to display it like it is in the picture.
I'm open to HTML5 implementations as well.

Here’s how to convert mouse coordinates to cell coordinates and display a tooltip
This math calculates which 75x75 cell your mouse is inside:
var col=parseInt(mouseX/75);
var row=parseInt(mouseY/75);
And here is the math to calculate a tip rectangle in the upper-right of that cell:
var tipX=tipCol*75+75-tipWidth;
var tipY=tipRow*75;
You can use canvas to draw the tip inside the cell at your calculated coordinates:
function tip(x,y){
var tipX=tipCol*75+75-tipWidth;
var tipY=tipRow*75;
ctx.beginPath();
ctx.rect(tipX,tipY,tipWidth,tipHeight);
ctx.fillStyle="ivory";
ctx.fill();
ctx.fillStyle="blue";
ctx.fillText(tipCol+","+tipRow,tipX+2,tipY+17);
}
Here is code and a Fiddle: http://jsfiddle.net/m1erickson/9V5QK/
<!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; padding:25px;}
#canvas{border:1px solid red;}
</style>
<script>
$(function(){
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var canvasOffset=$("#canvas").offset();
var offsetX=canvasOffset.left;
var offsetY=canvasOffset.top;
var startX;
var startY;
var isDown=false;
var tipWidth=35;
var tipHeight=22;
var tipRow;
var tipCol;
ctx.font="14pt verdana";
draw();
function draw(){
// you would just draw your image here
// ctx.drawImage(0,0,image.width,image.height);
// but for illustration, this just recreates your image
ctx.beginPath();
ctx.rect(0,0,375,225);
for(var x=1;x<5;x++){ ctx.moveTo(x*75,0); ctx.lineTo(x*75,canvas.height); }
for(var y=1;y<3;y++){ ctx.moveTo(0,y*75); ctx.lineTo(canvas.width,y*75); }
ctx.fillStyle="black";
ctx.fill();
ctx.strokeStyle="gray";
ctx.lineWidth=2;
ctx.stroke();
}
function tip(x,y){
var tipX=tipCol*75+75-tipWidth;
var tipY=tipRow*75;
ctx.beginPath();
ctx.rect(tipX,tipY,tipWidth,tipHeight);
ctx.fillStyle="ivory";
ctx.fill();
ctx.fillStyle="blue";
ctx.fillText(tipCol+","+tipRow,tipX+2,tipY+17);
}
function handleMouseMove(e){
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
$("#movelog").html("Move: "+ mouseX + " / " + mouseY);
// Put your mousemove stuff here
var col=parseInt(mouseX/75);
var row=parseInt(mouseY/75);
if(!(row==tipRow && col==tipCol)){
tipCol=col;
tipRow=row;
draw();
tip();
}
}
$("#canvas").mousemove(function(e){handleMouseMove(e);});
}); // end $(function(){});
</script>
</head>
<body>
<p>Move mouse over grid to display current cell</p>
<p id="movelog">Move</p>
<canvas id="canvas" width=375 height=225></canvas>
</body>
</html>