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Editorial Team
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Asked: 2026-06-10T14:29:31+00:00

I am currently working on a project involving the canvas element and have come

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I am currently working on a project involving the canvas element and have come across a problem when I resize the canvas. I’m trying to draw an image at the center of the canvas by dividing the width and height by 2, but whenever I resize the canvas, it only changes the HTML, so when I go to redraw the image, it still draws at half the width and height of the old canvas. I’m not sure if this is a DOM issue or what, but would appreciate any help. I have tested in Safari, FF, and Chrome

Edit: Code

<body>
<div id="holder">
<div id="dropdown">
  <div id="ddcontents" style="display: none;">
    <button onclick="fade()">Options</button>
    <button onclick="getSize()">Canvas Size</button>
    <button onclick="redraw()" disabled="true">Redraw</button>
  </div>
</div>
<canvas id="c" height="480" width="640">Your browser does not support HTML5.</canvas>
<div id="options" style="display: none">
 <div id="optcontent" style="display: none">
    <center><h1>Options</h1></center>       
    <div id="sound">
        <div>Volume:
            <form oninput="volumenumber.value = parseInt(volume.value)">
                <input type="range" name="volume" min="0" max="100" value="80" disabled="true">
                <input type="hidden" name="hello" value="%">
                <output name="volumenumber" for="volume">80</output>
            </form>
            Resolution:
            <select id="resolution">
                <option value="480p">480x640</option>
                <option value="720p">720x1280</option>
            </select>
        </div>
    </div>
    <div id="closeoptions">
        <button onclick="apply()">Apply</button>
        </div>
    </div>
</div>

</div>


<script src="options.js"></script>
</body>
</html>

And for the JS:

var c = document.getElementById('c');
var ctx=c.getContext("2d");
var img=new Image();
var w = this.c.clientWidth; // stores the width for later use
var h = this.c.clientHeight;
this.c.width = w;
this.c.height = h;
var cwidth = c.width;
var cheight = c.height;

img.onload = function(){
ctx.drawImage(img,w/2 - 14,h/2 - 19);
};
img.src="image_preview.png";


function apply()
{
var reso = document.getElementById("resolution");
var resol = reso.options[reso.selectedIndex].value;
//alert(resol)
if (resol == "720p")
 {
    //alert("You changed to 720p");
    h = 720;
    w = 1280;
    cheight = 720;
    cwidth = 1280;
 }
else if (resol == "480p")
 {
    //alert("You changed to 480p");
    h = 480;
    w = 640;
    cheight = 480;
    cheight = 640;
 }
getSize();
redraw();
}

function redraw()
{
ctx.drawImage(img,w/2 - 14,h/2 - 19);
img.src="image_preview.png";
}

function getSize()
{
alert(h + "x" + w);
}
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1 Answer

  1. Editorial Team

    When using a canvas you have to ensure its internal structure has the same size of the rendering html zone.

    You can do that :

    this.canvas.width = this.canvas.clientWidth;
this.canvas.height = this.canvas.clientHeight;

    Usually I also store those dimensions for my rendering computations :

    var w = this.canvas.clientWidth; // stores the width for later use
var h = this.canvas.clientHeight;
this.canvas.width = w;
this.canvas.height = h;

    EDIT : to change the element size, do this kind of thing before the precedent code :

    $('#mycanvas').width(newWidthInPx);
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