Develop Reference

Update HTML canvas width and height attributes automatically

I want to use HTML5 canvas with flexbox. I need to set canvas.width and canvas.height automatically when user resizes window. I have tried to use jQuery for it:
$(".cnvs").attr("width", $(".cnvs").width());
$(".cnvs").attr("height", $(".cnvs").height());
but it keeps increasing actual width of canvas so it almost fills entire screen. I have put it on jsfiddle - try to resize output window with separator.
Is there any reasonable way how to do it? Thanks.
Edit: Just to be clear: I don't want to fill entire screen with that canvas. I want UI where I have:
<div class="container">
<div class="control"></div>
<canvas></canvas>
<div class="control"></div>
</div>
then use flexbox to put those three elements beside eachother, while canvas will be twice as wide as the other. This works without problem, but canvas.width and canvas.height doesn't get updated, so whenever I render something onto that canvas, it is rendered as if that canvas was 320x140 px.
Edit 2: I am sorry, but (perhaps because of my poor English) I am not clear enough. I will try to explain it once again:
Actual width of canvas element is correct (even without using any JavaScript code) only by using flexbox. My problem is that although width is correct (and $(".cnvs").width() returns correct value of width), it doesn't have any "width" attribute, it is still:
<canvas>
</canvas>
and I need to provide width argument by myself (because it renders badly when it's not set). When I try to use my code or proposed:
...
var rect = canvas.parentNode.getBoundingClientRect();
canvas.width = rect.width;
canvas.height = rect.height;
...
it behaves weirdly, canvas's width keeps increasing with every resize, but too much (it erases both control divs almost immediately).

Update 2
If I understand the question correct: the canvas has a flex CSS set (not shown in the question right now). It defines the canvas to be 2x the size of the other two elements, but since the canvas is resized and not its bitmap, the drawings are stretches as well and you want the bitmap to adopt dynamically.
If so, do this change to the code -
This update will leave the CSS rules of the canvas element alone and let flexbox handle it. It will read the actual pixel size of the element and then apply it to the bitmap so that data isn't stretched:
$(window).on("resize", function() {
var cnvs = $(".cnvs")[0]; // cache canvas element
var rect = cnvs.getBoundingClientRect(); // actual size of canvas el. itself
cnvs.width = rect.width;
cnvs.height = rect.height;
// ... redraw content here ...
}
Additionally, since resizing the window can produce a lot of events, you may want to consider "debouncing" so that you only deal with the more recent resize:
var timerID;
$(window).on("resize", function() {
clearTimeout(timerID);
timerID = setTimeout(function() {
var cnvs = $(".cnvs")[0]; // cache canvas element
var rect = cnvs.getBoundingClientRect(); // actual size of canvas el. itself
cnvs.width = rect.width;
cnvs.height = rect.height;
// ... redraw content here ...
}, 180); // adjust at will
}
This will delay the resizing/redrawing until 180ms has passed.

You don't even have to load jQuery, it is simple:
var canvas = document.querySelector("canvas");
window.addEventListener("resize", function(){
canvas.setAttribute("width", window.innerWidth);
canvas.setAttribute("height", window.innerHeight)
})

You have to add a listener to the resize event on the window object.
If your looking for a jquery solution.
$( window ).resize(function() {
$(".cnvs").attr("width", $(window).width());
$(".cnvs").attr("height", $(window).height());
});
This should work just fine.

Canvas line width changes as browser window size changes, any idea to work around?

I have a canvas inside a div and it fills the whole div. I draw a line on canvas but when I resize browser window, width of the line on the canvas changes, as I increase browser window size its thickness increases and adds some blur to the line.
I figured out that problem occurs here; I dont use a fixed size for canvas instead I use %100 for the width and height.
#myCanvas{
margin:0;
padding: 0;
border:0;
width: 100%;
height: 100%;
background-color: white;
}
When I used fixed size in pixeel for width and height, it's ok. But I want my canvas to resize without disturbing the drawing inside.
You can play around here: http://codepen.io/ertugrulmurat/pen/nxhof
Just try to change the heigth of browser window.
Any idea would be appreciated.

This is unfortunately quite involved to fix. Essentially you need to:
Add an event listener to the window for the resize event and
change the width/height attributes on the canvas tag (not the css, that stays at 100%) every time the window resizes
You will also need to redraw the line every time the resize happens.
Remember the width/height attributes are unitless so make sure you set it to something like width="100" and not width="100px"
Here is the relevant section of your code with the corrections in place:
if (canvas.getContext){
var ctx = canvas.getContext('2d');
ctx.canvas.width = ctx.canvas.clientWidth;
ctx.canvas.height = ctx.canvas.clientHeight;
var draw = function(){
ctx.lineWidth = 0.5;
ctx.shadowBlur = 0;
ctx.strokeStyle="#FF0000";
ctx.beginPath();
ctx.lineWidth=0.5;
ctx.lineCap="round";
ctx.moveTo(20,20);
ctx.lineTo(200,20);
ctx.stroke();
ctx.closePath();
}
window.addEventListener('resize', function(){
ctx.canvas.width = ctx.canvas.clientWidth;
ctx.canvas.height = ctx.canvas.clientHeight;
draw();
});
draw();
}

I've taken a look around and it seems that the canvas element doesn't deal well with CSS, and it's best to define the width and height as part of the tag, such as
<canvas width="200" height="100"></canvas>
Of course the only problem is that those values are only in px. My work around would be to use javascript to therefore collect the parent size and enter the new value on the resize event. I have wrote all this out in a jsfiddle for you and it works brilliantly if you resize.
Note that I call the redraw function every time it is resized. If you fail to do this, it goes blank until the next click.
http://jsfiddle.net/N23w2/1/

Canvas images and Click event

I have currently two circles in a <canvas> tag with HTML5 & JavaScript.
Now I'm trying to add an image (done) that changes based on mouse-over and click.
It's basically an implementation of a play / pause button with an extra color change when the user mouse-overs the button.
I can't seem to figure out how events work on shapes in HTML5 since they are not objects ... Here is my code at the moment :
window.onload = function() {
var canvas = document.getElementsByTagName('canvas')[0];
var context = canvas.getContext('2d');
var centerX = canvas.width / 2;
var centerY = canvas.height / 2;
//Outer circle
context.beginPath();
context.arc(centerX, centerY, 150, 0, Math.PI * 2, false);
context.fillStyle = "#000";
context.fill();
context.stroke();
//Inner cicle
context.beginPath();
context.arc(centerX, centerY, 75, 0, Math.PI * 2, false);
context.fillStyle = "#fff";
context.fill();
context.stroke();
//Play / Pause button
var imagePlay = new Image();
var imageHeight = 48/2;
imagePlay.onload = function() {
context.drawImage(imagePlay, centerX - imageHeight, centerY - imageHeight);
};
imagePlay.src = "images/play.gif";
}
How to handle events on shapes created with <canvas>?
How to clean-up / remove images on the <canvas> when replacing it with another one?

There is technically no way to register mouse events on canvas-drawn shapes. However, if you use a library, like Raphael (http://raphaeljs.com/), it can keep track of shape positions and thus figure out what shape is receiving the mouse event. here's an example:
var circle = r.circle(50, 50, 40);
circle.attr({fill: "red"});
circle.mouseover(function (event) {
this.attr({fill: "red"});
});
As you can see, it's very simple this way. For modifying shapes, this library will also come in handy. Without it you would need to remember how to redraw everything each time you make a change

Well The simple answer is you can't. You either will have to find the coordinates of the click event and calculate whether you want to perform an option or not or you can use area and map tags and overlay the canvas element with it. To change a canvas use the clearRect function to draw paint a rectangle over everything and then redraw what you want.

There is no "built-in" way of keeping track of shapes drawn on the canvas. They are not treated as objects, but rather just pixels in an area. If you want to handle events on shapes drawn on the canvas, you would need to keep track of the area each shape covers, and then determine which shape you're triggering the event for based on the mouse position.
You can just draw over other shapes if you want to replace it with something. You might want to take a look at globalCompositeOperation.
If you want to treat your drawings as objects, I would recommend using SVG instead of canvas.
Another option is to use buttons, and then style them using CSS.
Basically, what you're doing now really wasn't the intended purpose or use of the canvas. It's like using a pencil to hammer in nails - you're using the wrong tool for the job.

While it's true that you cannot create click events for objects drawn on the canvas there is a workaround: Wrap the canvas in a DIV tag and then add the images within the DIV tag above the CANVAS tag.
<div id="wrapper">
<img src="img1.jpg" id="img1"></img>
<canvas id="thecanvas"></canvas>
</div>
Then use CSS to make the images position:absolute and use left:*px and top:*px attributes to place the image over the canvas where you would have normally drawn it.
#img1{
position:absolute;
left: 10px;
top: 10px;
}
You can then add click events to the image which is placed over the canvas giving the impression that you are clicking on the canvas(the below example uses the jQuery click() function)
$( "#img1" ).click(function(){
alert("Thanks for clicking me");
});

You can cast a ray into the canvas and manually test your images for intersection with the ray. You should look at it like you press, and send a ray into the screen. You should write a
objectsUnderPoint( x, y );
function that returns an array of all the images that intersect with the ray at x, y.
This is the only real right answer, and this is how it is usually done in 3D engines as well.

creating custom drawing with jquery

Here is my code:
$('#add_shape').click(function() {
var rectHeight = $('#rect_height_input').val();
var rectWidth = $('#width_input').val();
$('<canvas>').attr({
id: 'canvas'
}).css({
height: function() {
if (rectHeight > 0) {
return rectHeight + 'px';
}
else {
return rectWidth + 'px';
}
},
width: rectWidth + 'px'
}).appendTo('#work_area');
var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
ctx.fillStyle = $('#color_list option:selected').val();
ctx.fillRect(0, 0, rectWidth, rectHeight);
});
When the #add_shape button is clicked, the rectangle does not show up. What am I missing here? Please help.

In response to previous revision:
You retrieved existing canvas element using document.getElementById() and then you created another one using jQuery $('<canvas>') and appended it to #work_area.
Change $('<canvas>') into $(canvas) to use the already existing canvas.
Did you really want to call document.getElementById('canvas') instead of document.createElement('canvas')? In the 1st case there must be already a canvas element in the DOM with the id="canvas" which looks suspicious.
Edit #1 (in response to current revision):
If that was just a wrote code in the answer then check your HTML (which you should also provide). Your code is working as demonstrated in this fiddle.
Make sure that IDs are correct - You are using #rect_height_input for height, but #width_input for width and double-check values for the color options - maybe the rectangle is drawin using white color.
Edit #2 (in response to the fiddle in comments):
For each shape you create a new canvas element and all those elements have same (!) id. This is incorrect. Attribute id of an element should be unique. In your code you will always get the first canvas and draw on it - now matter how many shapes you crated. Rest of canvas elements are empty.
Your code is drawing rectangles correctly (apart from the "same canvas" problem). Try to draw rectangle with big height and width - it is working. When the width/height are very small then the canvas is too small to show the rectangle. Setting minimum width/height or using one big canvas and drawing shapes onto it is a way to go.

Draw a cross at the mouses location on a canvas, non persistently

I have this bound to the mousemove event of my canvas:
function(e){
var contDiv = $('#current_system_map');
var offset = contDiv.offset();
x = e.clientX-offset.left;
y = e.clientY-offset.top;
context.beginPath();
context.moveTo(0,y);
context.lineTo(595,y);
context.moveTo(x,0);
context.lineTo(x,595);
context.strokeStyle = "rgb(255,255,255)";
context.stroke();
}
and it works fine, to a point. The drawn cross is persistent, so when the mouse moves a new cross is drawn but the old one remains. I've tried essentially re-drawing the canvas but that cause the cross to be laggy and remain quite away behind the mouse.
So i need to know how to draw the cross and make it dis-appear without having to re-draw everything on the canvas

http://jsfiddle.net/PgKEt/2/
This is the best that I can do.
If you try to use setInterval and such to animate it, it will keep redrawing even when it does not need to. So by doing this, you essentially only redraw when the mouse moves, and only draw 2 lines, instead of whatever content you want it on top.
In addition, if you have any detection such as mousedown and such, it has to be on whatever canvas is on the top, otherwise it will not detect them anymore.

Usually if you draw something on the canvas you will have to redrawn the canvas contents to erase it. I suggest you use an image element as a cursor and position it absolutely above the
Or you could try the old trick and draw the cursor in the canvas with globalCompositeOperation='xor', then draw it again in the same place to erase it. Afterwards you will need to restore globalCompositeOperation to source-over.

This approach works fast enough for me in Firefox 3.6.8 to do in a mousemove event. Save the image before you draw the crosshair and then restore it to erase:
To save:
savedImage = new Image()
savedImage.src = canvas.toDataURL("image/png")
The to restore:
ctx = canvas.getContext('2d')
ctx.drawImage(savedImage,0,0)

If you do not want to store it persistently, you can also take a look at SVG.

Try
ctx.clearRect(0,0,YourCanvasHeight,YourCanvasWidth);
In my case I implemented a circle and every time the user clicks inside it, this instruction returns and deletes the previous points.
This is the complete code:
function getMousePosition(canvas, event) {
let rect = canvas.getBoundingClientRect();
let x = event.offsetX; //event.clientX - rect.left;
let y = event.offsetY; //event.clientY - rect.top;
drawPoint(canvas,x,y);
};
function drawPoint(canvas,x,y) {
if (canvas.getContext){
var ctx = canvas.getContext('2d');
ctx.clearRect(0,0,200,200);
ctx.beginPath();
ctx.arc(x, y, 5, 0, 2 * Math.PI);
ctx.fillStyle = "white";
ctx.fill();
}
};
$(document).ready(function(){
let canvasElem = document.getElementById("circle");
canvasElem.addEventListener("click", function(e)
{
getMousePosition(canvasElem, e);
});
});