HTML canvas - drawing disappear on resizing

Question

I have created a basic shape in HTML canvas element which works fine.

The problem occurs when I resize the canvas, all the drawing in the canvas disappears. Is this

Answer 1

You need to redraw the scene when you resize.

setting the width or height of a canvas, even if you are setting it to the same value as before, not only clears the canvas but resets the entire canvas context. Any set properties (fillStyle, lineWidth, the clipping region, etc) will also be reset.

If you do not have the ability to redraw the scene from whatever data structures you might have representing the canvas, you can always save the entire canvas itself by drawing it to an in-memory canvas, setting the original width, and drawing the in-memory canvas back to the original canvas.

Here's a really quick example of saving the canvas bitmap and putting it back after a resize:

http://jsfiddle.net/simonsarris/weMbr/

Answer 2

I also met this problem.but after a experiment, I found that Resizing the canvas element will automatically clear all drawings off the canvas! just try the code below

<canvas id = 'canvas'></canvas>
<script>
    var canvas1 = document.getElementById('canvas')
    console.log('canvas size',canvas1.width, canvas1.height)
    var ctx = canvas1.getContext('2d')
    ctx.font = 'Bold 48px Arial'
    var f = ctx.font
    canvas1.width = 480
    var f1 = ctx.font
    alert(f === f1) //false
</script>

Answer 3

I had the same problem when I had to resize the canvas to adjust it to the screen. But I solved it with this code:

var c = document.getElementById('canvas');
ctx = c.getContext('2d');
ctx.fillRect(0,0,20,20);
// Save canvas settings
ctx.save();
// Save canvas context
var dataURL = c.toDataURL('image/jpeg');
// Resize canvas
c.width = 50;
c.height = 50;
// Restore canvas context
var img = document.createElement('img');
img.src = dataURL;
img.onload=function(){
  ctx.drawImage(img,20,20);
}
// Restote canvas settings
ctx.restore();

<canvas id=canvas width=40 height=40></canvas>

Answer 4

Everytime you resize the canvas it will reset itself to transparant black, as defined in the spec.

You will either have to:

• redraw when you resize the canvas, or,
• don't resize the canvas

Answer 5

I had the same problem. Try following code

var wrapper = document.getElementById("signature-pad");
var canvas = wrapper.querySelector("canvas");
var ratio = Math.max(window.devicePixelRatio || 1, 1);
canvas.width = canvas.offsetWidth * ratio;
canvas.height = canvas.offsetHeight * ratio;

It keeps the drawing as it is

Answer 6

One thing that worked for me was to use requestAnimationFrame().

let height = window.innerHeight;
let width = window.innerWidth;

function handleWindowResize() {
    height = window.innerHeight;
    width = window.innerWidth;
}

function render() {
    // Draw your fun shapes here
    // ...

    // Keep this on the bottom
    requestAnimationFrame(render);
}

// Canvas being defined at the top of the file.
function init() {
    ctx = canvas.getContext("2d");

    render();
}