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Setting canvas size does not work with CSS but works with JS attribute, when using putImageData [duplicate]

I have 2 canvases, one uses HTML attributes width and height to size it, the other uses CSS:
<canvas id="compteur1" width="300" height="300" onmousedown="compteurClick(this.id);"></canvas>
<canvas id="compteur2" style="width: 300px; height: 300px;" onmousedown="compteurClick(this.id);"></canvas>
Compteur1 displays like it should, but not compteur2. The content is drawn using JavaScript on a 300x300 canvas.
Why is there a display difference?

It seems that the width and height attributes determine the width or height of the canvas’s coordinate system, whereas the CSS properties just determine the size of the box in which it will be shown.
This is explained in the HTML specification:
The canvas element has two attributes to control the size of the element’s bitmap: width and height. These attributes, when specified, must have values that are valid non-negative integers. The rules for parsing non-negative integers must be used to obtain their numeric values. If an attribute is missing, or if parsing its value returns an error, then the default value must be used instead. The width attribute defaults to 300, and the height attribute defaults to 150.

To set the width and height on a canvas, you may use:
canvasObject.setAttribute('width', '150');
canvasObject.setAttribute('height', '300');

For <canvas> elements, the CSS rules for width and height set the actual size of the canvas element that will be drawn to the page. On the other hand, the HTML attributes of width and height set the size of the coordinate system or 'grid' that the canvas API will use.
For example, consider this (jsfiddle):
var ctx = document.getElementById('canvas1').getContext('2d');
ctx.fillStyle = "red";
ctx.fillRect(10, 10, 30, 30);
var ctx2 = document.getElementById('canvas2').getContext('2d');
ctx2.fillStyle = "red";
ctx2.fillRect(10, 10, 30, 30);
canvas {
border: 1px solid black;
}
<canvas id="canvas1" style="width: 50px; height: 100px;" height="50" width="100"></canvas>
<canvas id="canvas2" style="width: 100px; height: 100px;" height="50" width="100"></canvas>
Both have had the same thing drawn on them relative to the internal coordinates of the canvas element. But in the second canvas, the red rectangle will be twice as wide because the canvas as a whole is being stretched across a bigger area by the CSS rules.
Note: If the CSS rules for width and/or height aren't specified then the browser will use the HTML attributes to size the element such that 1 unit of these values equals 1px on the page. If these attributes aren't specified then they will default to a width of 300 and a height of 150.

The canvas will be stretched if you set the width and height in your CSS. If you want to dynamically manipulate the dimension of the canvas you have to use JavaScript like so:
canvas = document.getElementById('canv');
canvas.setAttribute('width', '438');
canvas.setAttribute('height', '462');

The browser uses the css width and height, but the canvas element scales based on the canvas width and height. In javascript, read the css width and height and set the canvas width and height to that.
var myCanvas = $('#TheMainCanvas');
myCanvas[0].width = myCanvas.width();
myCanvas[0].height = myCanvas.height();

Shannimal correction
var el = $('#mycanvas');
el.attr('width', parseInt(el.css('width')))
el.attr('height', parseInt(el.css('height')))

Canvas renders image by buffer, so when you specify the width and height HTML attributes the buffer size and length changes, but when you use CSS, the buffer's size is unchanged. Making the image stretched.
Using HTML sizing.
Size of canvas is changed -> buffer size is changed -> rendered
Using CSS sizing
Size of canvas is changed -> rendered
Since the buffer length is kept unchanged, when the context renders the image,
the image is displayed in resized canvas (but rendered in unchanged buffer).

CSS sets the width and height of the canvas element so it affects the coordinate space leaving everything drawn skewed
Here's my way on how to set the width and height with Vanilla JavaScript
canvas.width = numberForWidth
canvas.height = numberForHeight

I believe CSS has much better machinery for specifying the size of the canvas and CSS must decide styling, not JavaScript or HTML. Having said that, setting width and height in HTML is important for working around the issue with canvas.
CSS has !important rule that allows to override other styling rules for the property, including those in HTML. Usually, its usage is frowned upon but here the use is a legitimate hack.
In Rust module for WebAssembly you can do the following:
fn update_buffer(canvas: &HtmlCanvasElement) {
canvas.set_width(canvas.client_width() as u32);
canvas.set_height(canvas.client_height() as u32);
}
//..
#[wasm_bindgen(start)]
pub fn start() -> Result<(), JsValue> {
// ...
let canvas: Rc<_> = document
.query_selector("canvas")
.unwrap()
.unwrap()
.dyn_into::<HtmlCanvasElement>()
.unwrap()
.into();
update_buffer(&canvas);
// ...
// create resizing handler for window
{
let on_resize = Closure::<dyn FnMut(_)>::new(move |_event: Event| {
let canvas = canvas.clone();
// ...
update_buffer(&canvas);
// ...
window.add_event_listener_with_callback("resize", on_resize.as_ref().unchecked_ref())?;
on_resize.forget();
}
}
There we update the canvas buffer once the WASM module is loaded and then whenever the window is resized. We do it by manually specifying width and height of canvas as values of clientWidth and clientHeight. Maybe there are better ways to update the buffer but I believe this solution is better than those suggested by #SamB, #CoderNaveed, #Anthony Gedeon, #Bluerain, #Ben Jackson, #Manolo, #XaviGuardia, #Russel Harkins, and #fermar because
The element is styled by CSS, not HTML.
Unlike elem.style.width & elem.style.height trick used by #Manolo or its JQuery equivalent used by #XaviGuardia, it will work for canvas whose size is specified by usage as flex or grid item.
Unlike the solution by #Russel Harkings, this also handles resizing. Though I like his answer because it is really clean and easy.
WASM is the future! Haha :D
P.S. there's a ton of .unwrap() because Rust explicitly handles possible failures.
P.P.S.
{
let on_resize = Closure::<dyn FnMut(_)>::new(move |_event: Event| {
let canvas = canvas.clone();
// ...
update_buffer(&canvas);
// ...
window.add_event_listener_with_callback("resize", on_resize.as_ref().unchecked_ref())?;
on_resize.forget();
}
can be done much cleaner with better libraries. E.g.
add_resize_handler(&window, move |e: ResizeEvent| {
let canvas = canvas.clone();
// ...
update_buffer(&canvas);
})

If you want a dynamic behaviour based on, e.g. CSS media queries, don't use canvas width and height attributes. Use CSS rules and then, before getting the canvas rendering context, assign to width and height attributes the CSS width and height styles:
var elem = document.getElementById("mycanvas");
elem.width = elem.style.width;
elem.height = elem.style.height;
var ctx1 = elem.getContext("2d");
...

Why canvas objects are re-scaled when using CSS to style width and dimensions? [duplicate]

I have 2 canvases, one uses HTML attributes width and height to size it, the other uses CSS:
<canvas id="compteur1" width="300" height="300" onmousedown="compteurClick(this.id);"></canvas>
<canvas id="compteur2" style="width: 300px; height: 300px;" onmousedown="compteurClick(this.id);"></canvas>
Compteur1 displays like it should, but not compteur2. The content is drawn using JavaScript on a 300x300 canvas.
Why is there a display difference?

It seems that the width and height attributes determine the width or height of the canvas’s coordinate system, whereas the CSS properties just determine the size of the box in which it will be shown.
This is explained in the HTML specification:
The canvas element has two attributes to control the size of the element’s bitmap: width and height. These attributes, when specified, must have values that are valid non-negative integers. The rules for parsing non-negative integers must be used to obtain their numeric values. If an attribute is missing, or if parsing its value returns an error, then the default value must be used instead. The width attribute defaults to 300, and the height attribute defaults to 150.

To set the width and height on a canvas, you may use:
canvasObject.setAttribute('width', '150');
canvasObject.setAttribute('height', '300');

For <canvas> elements, the CSS rules for width and height set the actual size of the canvas element that will be drawn to the page. On the other hand, the HTML attributes of width and height set the size of the coordinate system or 'grid' that the canvas API will use.
For example, consider this (jsfiddle):
var ctx = document.getElementById('canvas1').getContext('2d');
ctx.fillStyle = "red";
ctx.fillRect(10, 10, 30, 30);
var ctx2 = document.getElementById('canvas2').getContext('2d');
ctx2.fillStyle = "red";
ctx2.fillRect(10, 10, 30, 30);
canvas {
border: 1px solid black;
}
<canvas id="canvas1" style="width: 50px; height: 100px;" height="50" width="100"></canvas>
<canvas id="canvas2" style="width: 100px; height: 100px;" height="50" width="100"></canvas>
Both have had the same thing drawn on them relative to the internal coordinates of the canvas element. But in the second canvas, the red rectangle will be twice as wide because the canvas as a whole is being stretched across a bigger area by the CSS rules.
Note: If the CSS rules for width and/or height aren't specified then the browser will use the HTML attributes to size the element such that 1 unit of these values equals 1px on the page. If these attributes aren't specified then they will default to a width of 300 and a height of 150.

The canvas will be stretched if you set the width and height in your CSS. If you want to dynamically manipulate the dimension of the canvas you have to use JavaScript like so:
canvas = document.getElementById('canv');
canvas.setAttribute('width', '438');
canvas.setAttribute('height', '462');

The browser uses the css width and height, but the canvas element scales based on the canvas width and height. In javascript, read the css width and height and set the canvas width and height to that.
var myCanvas = $('#TheMainCanvas');
myCanvas[0].width = myCanvas.width();
myCanvas[0].height = myCanvas.height();

Shannimal correction
var el = $('#mycanvas');
el.attr('width', parseInt(el.css('width')))
el.attr('height', parseInt(el.css('height')))

Canvas renders image by buffer, so when you specify the width and height HTML attributes the buffer size and length changes, but when you use CSS, the buffer's size is unchanged. Making the image stretched.
Using HTML sizing.
Size of canvas is changed -> buffer size is changed -> rendered
Using CSS sizing
Size of canvas is changed -> rendered
Since the buffer length is kept unchanged, when the context renders the image,
the image is displayed in resized canvas (but rendered in unchanged buffer).

CSS sets the width and height of the canvas element so it affects the coordinate space leaving everything drawn skewed
Here's my way on how to set the width and height with Vanilla JavaScript
canvas.width = numberForWidth
canvas.height = numberForHeight

I believe CSS has much better machinery for specifying the size of the canvas and CSS must decide styling, not JavaScript or HTML. Having said that, setting width and height in HTML is important for working around the issue with canvas.
CSS has !important rule that allows to override other styling rules for the property, including those in HTML. Usually, its usage is frowned upon but here the use is a legitimate hack.
In Rust module for WebAssembly you can do the following:
fn update_buffer(canvas: &HtmlCanvasElement) {
canvas.set_width(canvas.client_width() as u32);
canvas.set_height(canvas.client_height() as u32);
}
//..
#[wasm_bindgen(start)]
pub fn start() -> Result<(), JsValue> {
// ...
let canvas: Rc<_> = document
.query_selector("canvas")
.unwrap()
.unwrap()
.dyn_into::<HtmlCanvasElement>()
.unwrap()
.into();
update_buffer(&canvas);
// ...
// create resizing handler for window
{
let on_resize = Closure::<dyn FnMut(_)>::new(move |_event: Event| {
let canvas = canvas.clone();
// ...
update_buffer(&canvas);
// ...
window.add_event_listener_with_callback("resize", on_resize.as_ref().unchecked_ref())?;
on_resize.forget();
}
}
There we update the canvas buffer once the WASM module is loaded and then whenever the window is resized. We do it by manually specifying width and height of canvas as values of clientWidth and clientHeight. Maybe there are better ways to update the buffer but I believe this solution is better than those suggested by #SamB, #CoderNaveed, #Anthony Gedeon, #Bluerain, #Ben Jackson, #Manolo, #XaviGuardia, #Russel Harkins, and #fermar because
The element is styled by CSS, not HTML.
Unlike elem.style.width & elem.style.height trick used by #Manolo or its JQuery equivalent used by #XaviGuardia, it will work for canvas whose size is specified by usage as flex or grid item.
Unlike the solution by #Russel Harkings, this also handles resizing. Though I like his answer because it is really clean and easy.
WASM is the future! Haha :D
P.S. there's a ton of .unwrap() because Rust explicitly handles possible failures.
P.P.S.
{
let on_resize = Closure::<dyn FnMut(_)>::new(move |_event: Event| {
let canvas = canvas.clone();
// ...
update_buffer(&canvas);
// ...
window.add_event_listener_with_callback("resize", on_resize.as_ref().unchecked_ref())?;
on_resize.forget();
}
can be done much cleaner with better libraries. E.g.
add_resize_handler(&window, move |e: ResizeEvent| {
let canvas = canvas.clone();
// ...
update_buffer(&canvas);
})

If you want a dynamic behaviour based on, e.g. CSS media queries, don't use canvas width and height attributes. Use CSS rules and then, before getting the canvas rendering context, assign to width and height attributes the CSS width and height styles:
var elem = document.getElementById("mycanvas");
elem.width = elem.style.width;
elem.height = elem.style.height;
var ctx1 = elem.getContext("2d");
...

Dimensions of a canvas

I found this pretty awesome blog post that explains how to resize a canvas to fit any screen http://www.html5rocks.com/en/tutorials/casestudies/gopherwoord-studios-resizing-html5-games/ I implemented it and it is sizing it flawlessly but I get this problem that my objects appear off screen when the canvas is too little.
For example for my game I define a world with dimensions width 480 and height 1024, if I put an element on x axis at about 400 but the display that is currently used is 360px wide then the canvas would have correct ratio of width to height 0.46875 but the object will not be visible.
To solve this problem I suppose I need to define a ratio between the absolute world dimensions and the screen dimensions and multiply by that when I'm calculating the object's position, however my question is - is there a way for the canvas to do that automatically?

Both the canvas itself and the canvas element have a width and height. If they're not the same, the content of the canvas is automatically scaled to fit the element. So yes, you could define your canvas to be the size you want, and define the element such that it scales. Naturally, though, everything scales, and if the aspect ratio is different, your shapes get distorted.
The width and height of the canvas within the canvas element are controlled by the width and height attributes of the canvas element. (They default to 300 and 150, respectively.)
The width and height of the canvas element are controlled by CSS; they default to the width and height of the canvas, but you can override that with CSS.
For example: Here's a canvas that's 300x300 (thanks to the CSS), within a canvas element that's only 300x150 (because those are the default width and height of the canvas element). You can see the effect of scaling when I draw a circle in it — it comes out oval instead:
// Get the canvas element
const canvas = document.getElementById("clock");
// Get the 2D rendering context for it
const context = canvas.getContext("2d");
// Show the canvas's dimensions (NOT the dimensions of the element)
document.getElementById("dimensions").innerHTML = canvas.width + "x" + canvas.height;
// Draw a circle -- it comes out oval because of scaling
const path = new Path2D();
path.arc(75, 75, 50, 0, Math.PI * 2, true);
context.fillStyle = context.strokeStyle = "blue";
context.fill(path);
canvas#clock {
width: 300px;
height: 300px;
background: #1E1E1E;
}
<canvas id="clock"></canvas>
<div id="dimensions"></div>
Here's an example where the canvas and its container are the same width and height, so there's no scaling; the code is identical, it's just I've added width="300" height="300" to the canvas HTML:
// Get the canvas element
const canvas = document.getElementById("clock");
// Get the 2D rendering context for it
const context = canvas.getContext("2d");
// Show the canvas's dimensions (NOT the dimensions of the element)
document.getElementById("dimensions").innerHTML = canvas.width + "x" + canvas.height;
// Draw a circle
const path = new Path2D();
path.arc(75, 75, 50, 0, Math.PI * 2, true);
context.fillStyle = context.strokeStyle = "blue";
context.fill(path);
canvas#clock {
/* We could comment out the width and height properties
since they'll default to the values from the attributes
*/
width: 300px;
height: 300px;
background: #1E1E1E;
}
<canvas id="clock" width="300" height="300"></canvas>
<div id="dimensions"></div>
The automatic scaling is handy provided you ensure the aspect ratio of the element and its canvas are the same. If you don't directly set the size of the canvas element, on most browsers it will default to the aspect ratio of the canvas thanks to the width and height attributes. From MDN's coverage of aspect-ratio:
Browsers have added an internal aspect-ratio property that applies to replaced elements and other related elements that accept width and height attributes. This appears in the browser's internal UA stylesheet.
In Firefox, the internal stylesheet rule looks like this:
img, input[type="image"], video, embed, iframe, marquee, object, table {
aspect-ratio: attr(width) / attr(height);
}

Relatively sizing HTML Canvas

The HTML5 <canvas> element does not accept relative sizes (percent) for its width and height properties.
What I'm trying to accomplish is to have my canvas sized relative to the window. This is what I've come up with so far, but I'm wondering if there is a better way that is:
Simpler
Does not require wrapping the <canvas> in a <div>.
Not dependent on jQuery (I use it to get the width/height of the parent div)
Ideally, doesn't redraw on browser resize (but I think that might be a requirement)
See below for the code, which draws a circle in the middle of the screen, 40% wide up to a maximum of 400px.
Live demo: http://jsbin.com/elosil/2
Code:
<!DOCTYPE html>
<html>
<head>
<title>Canvas of relative width</title>
<style>
body { margin: 0; padding: 0; background-color: #ccc; }
#relative { width: 40%; margin: 100px auto; height: 400px; border: solid 4px #999; background-color: White; }
</style>
<script language="javascript" type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.6.2/jquery.min.js"></script>
<script>
function draw() {
// draw a circle in the center of the canvas
var canvas = document.getElementById('canvas');
var relative = document.getElementById('relative');
canvas.width = $(relative).width();
canvas.height = $(relative).height();
var w = canvas.width;
var h = canvas.height;
var size = (w > h) ? h : w; // set the radius of the circle to be the lesser of the width or height;
var ctx = canvas.getContext('2d');
ctx.beginPath();
ctx.arc(w / 2, h / 2, size/2, 0, Math.PI * 2, false);
ctx.closePath();
ctx.fill();
}
$(function () {
$(window).resize(draw);
});
</script>
</head>
<body onload="draw()">
<div id="relative">
<canvas id="canvas"></canvas>
</div>
</body>
</html>

The canvas width and height attributes are separate from the same canvas's width and height styles. The width and height attributes are the size of the canvas's rendering surface, in pixels, whereas its styles choose a location in the document where the browser should draw the content from the rendering surface. It just so happens that the default value for the width and height styles, if they're not specified, is the rendering surface's width and height. So you're right about #1: there's no reason to wrap it in a div. You can set percentage values for all of the styles on your canvas element, just like any other element.
For #3, it's pretty easy (and cross-browser) to get the size of things with clientWidth and clientHeight, as long as you're not using padding on your canvas element.
I coded up the slightly simplified version here.
For #4, you're right about being out of luck. It's possible to check before setting width and height and leave the canvas alone if it doesn't need resizing, which would eliminate some of the redraws, but you can't get rid of all of them.
EDIT: Portman pointed out I messed up the centering style. Updated version here.

Like said by sethobrien a canvas element has TWO pairs width/height of attributes.
canvas.width / canvas.height are about the size in pixel of the buffer that will contains the result of drawing commands.
canvas.style.width / canvas.style.height are about the size used to show the canvas object in the browser window and they can be in any of the units supported by css.
You can indeed set canvas.width and canvas.height just once, do the drawing in the canvas, setting the style size parameters in percentage and then forget about redrawing the canvas content. Of course this means that the browser will just do the scaling itself like for a regular image loaded from the network so the visible result will show pixel scaling artifacts.
You need to redraw the canvas content after the resize of the canvas element only if you want pixel-perfect results.

Alright. Here is the technique that i ve used to implement the same.
Suppose you have the canvas height=400, for the window's height=480, and you want to change the height of it relatively if the window's height changes to 640.
canvas = document.getElementById("canvas");
canvas.height=window.innerHeight*400/480;
p.s: do not initialize the height of the canvas inside the html tag.
Make use of 'window.innerHeight' (which returns the height of the browser's window.. similarly 'window.innerWidth') any where you want to calculate the relative positions on the window.
Hope you got what you needed.

HTML5 canvas stroke() thick and fuzzy

I'm trying to allow the user to draw a rectangle on the canvas (like a selection box). I'm getting some ridiculous results, but then I noticed that even just trying the code from my reference here, I get huge fuzzy lines and don't know why.
it's hosted at dylanstestserver.com/drawcss. the javascript is inline so you can check it out. I am using jQuery to simplify getting the mouse coordinates.

The blurry problem will happen if you use css to set the canvas height and width instead of setting height and width in the canvas element.
<style>
canvas { height: 800px; width: 1200px; } WRONG WAY -- BLURRY LINES
</style>
<canvas height="800" width="1200"></canvas> RIGHT WAY -- SHARP LINES

For some reason, your canvas is stretched. Because you have its css property width set to 100%, it is stretching it from some sort of native size. It's the difference between using the css width property and the width attribute on the <canvas> tag. You might want to try using a bit of javascript to make it fill the viewport (see jQuery .width()):
jQuery(document).ready(function(){
var canvas = document.getElementById('drawing');
canvas.width(($(window).width()).height($(window).height());
var context = canvas.getContext('2d');
//...

The way I do it is to set the canvas element to a width and height in the css, and then set the canvas.width = canvas.clientWidth and canvas.height = canvas.clientHeight
var canvas = document.getElementById("myCanvas");
canvas.width = canvas.clientWidth;
canvas.height = canvas.clientHeight;
var context = canvas.getContext("2d");

You haven't indicated canvas size in pixels, so it is scaled up. It is 300x150 here. Try setting the width, height

On retina displays you also need to scale (in addition to the accepted answer):
var context = canvas.getContext('2d');
context.scale(2,2);

The css sizing issue mentioned in these comments is correct, but another more subtle thing that can cause blurred lines is forgetting to call make a call to context.beginPath() before drawing a line. Without calling this, you will still get a line, but it won't be smoothed which makes the line looks like a series of steps.

I found the reason mine was blurry was that there was a slight discrepancy between the inline width and the CSS width.
I have both inline width/height parameters AND css width/height assigned to my canvas, because I need to keep its physical dimensions static, while increasing its inline dimensions for retina screens.
Check yours are the same if you have a situation like mine.