Resize Canvas to fit parent boostrap cell in a grid

Question

I'm designing a dashboard using boostrap grid. Some cells have a canvas in it and I want to force it to match the size of its parent. I'm basically calculating the size this way:

var parent = canvas.parentElement;
if (parent.style) {
  var parentStyle = window.getComputedStyle(parent, null);

  var w = parseFloat(parentStyle.width) - (parseFloat(parentStyle.paddingLeft) + parseFloat(parentStyle.paddingRight) + parseFloat(parentStyle.borderLeftWidth) + parseFloat(parentStyle.borderRightWidth));
  var h = parseFloat(parentStyle.height) - (parseFloat(parentStyle.paddingTop) + parseFloat(parentStyle.paddingBottom) + parseFloat(parentStyle.borderTopWidth) + parseFloat(parentStyle.borderBottomWidth));

  canvas.width = w;
  canvas.height = h;
}

However, when I resize the page, the canvas, or the cell seems to keep growing in height and I don't understand why.

Run the following example in "full page" and resize the window horizontally. You'll see how the cells keep growing in height.

function resizeCanvas(canvas) {
  var parent = canvas.parentElement;
  if (parent.style) {
    var parentStyle = window.getComputedStyle(parent, null);

    var w = parseFloat(parentStyle.width) - (parseFloat(parentStyle.paddingLeft) + parseFloat(parentStyle.paddingRight) + parseFloat(parentStyle.borderLeftWidth) + parseFloat(parentStyle.borderRightWidth));
    var h = parseFloat(parentStyle.height) - (parseFloat(parentStyle.paddingTop) + parseFloat(parentStyle.paddingBottom) + parseFloat(parentStyle.borderTopWidth) + parseFloat(parentStyle.borderBottomWidth));

    canvas.width = w;
    canvas.height = h;
  }
}

function paintOnCanvas(canvas) {
  var ctx = canvas.getContext("2d");
  ctx.clearRect(0, 0, canvas.width, canvas.height);
  ctx.fillStyle = "#FF0000";
  ctx.fillRect(0, 0, canvas.width, canvas.height);
  ctx.font = "20px Verdana";
  ctx.fillStyle = "black";
  ctx.fillText("Paint on Canvas", 10, 50);
}

document.addEventListener("DOMContentLoaded", function(event) {
  paintOnCanvas(document.getElementById("canvas"));
});

window.addEventListener('resize', function(event) {
  resizeCanvas(document.getElementById("canvas"));
  paintOnCanvas(document.getElementById("canvas"));
});

canvas {
  background-color: #ce8483;
  width: 100%;
  height: 100%;
}

.col-sm-6 {
  border: 1px #2e6da4 solid;
  padding: 15px 15px;
}

<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.2/css/bootstrap.min.css" integrity="sha384-Smlep5jCw/wG7hdkwQ/Z5nLIefveQRIY9nfy6xoR1uRYBtpZgI6339F5dgvm/e9B" crossorigin="anonymous">

<div class="container-fluid">
  <div class="row">
    <div class="col-md-4 col-sm-6 col-xs-12">
    </div>
    <div class="col-md-4 col-sm-6 col-xs-12">
      <canvas id="canvas"></canvas>
    </div>
    <div class="col-md-4 col-sm-6 col-xs-12">
    </div>
  </div>
</div>

TessavWalstijn · Accepted Answer · 2018-07-19T12:21:02.167

You should use the .offsetWidth and .offsetHeight properties.
Note they belong to the element, not .style.
Quoted from: How do I retrieve an HTML element's actual width and height?

I added a minimal size div to give the canvas a minimal height. You can see what happens if you remove that div.

function resizeCanvas(canvas) {
  var parent = canvas.parentElement;
  
  canvas.width = parent.offsetWidth;
  canvas.height = parent.offsetHeight;
}

function paintOnCanvas(canvas) {
  var ctx = canvas.getContext("2d");
  ctx.clearRect(0, 0, canvas.width, canvas.height);
  ctx.fillStyle = "#FF0000";
  ctx.fillRect(0, 0, canvas.width, canvas.height);
  ctx.font = "20px Verdana";
  ctx.fillStyle = "black";
  ctx.fillText("Paint on Canvas", 10, 50);
}

document.addEventListener("DOMContentLoaded", function(event) {
  resizeCanvas(document.getElementById("canvas"));
  paintOnCanvas(document.getElementById("canvas"));
});

window.addEventListener('resize', function(event) {
  resizeCanvas(document.getElementById("canvas"));
  paintOnCanvas(document.getElementById("canvas"));
});

canvas {
  position:absolute;
  left: 0;
  top: 0;
}

.col-sm-6 {
  border: 1px #2e6da4 solid;
  padding: 15px 15px;
  height: 200px;
}

<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.2/css/bootstrap.min.css" integrity="sha384-Smlep5jCw/wG7hdkwQ/Z5nLIefveQRIY9nfy6xoR1uRYBtpZgI6339F5dgvm/e9B" crossorigin="anonymous">

<div class="container-fluid">
  <div class="row">
    <div class="col-md-4 col-sm-6 col-xs-12">
    </div>
    <div class="col-md-4 col-sm-6 col-xs-12">
      <canvas id="canvas"></canvas>
    </div>
    <div class="col-md-4 col-sm-6 col-xs-12">
    </div>
  </div>
</div>

EDIT:
I saw a typo in the .col-sm-6 CSS class.
height: 200px; did not had a :.

Now the extra div is not needed.

It works, thanks! But you are setting the canvas to a `position:absolute` and adding that extra minimal size div. It would be desirable to find something more elegant — Yeray, Jul 19 '18 at 11:03
You could set the height hard coded in the `resizeCanvas()` but you have to add `canvas.style.height = yourNumber;` in the same function. — TessavWalstijn, Jul 19 '18 at 11:57

Resize Canvas to fit parent boostrap cell in a grid

1 Answers1