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<!DOCTYPE html>
<!--
SPDX-FileCopyrightText: Chris Pressey, the original author of this work, has dedicated it to the public domain.
For more information, please refer to <https://unlicense.org/>
SPDX-License-Identifier: Unlicense
-->
<head>
  <meta charset="utf-8">
  <title>Hanging a Canvas</title>
  <style>
header {
    background: goldenrod;
}
article {
    width: 100%;
    text-align: center;
}
  </style>
</head>
<body>
<header>
  <h1>Hanging a Canvas</h1>
  <i></i>
</header>
<article>
  <canvas id="canvas">
  Your browser doesn't support displaying an HTML5 canvas.
  </canvas>
</article>
</body>
<script>
var resizeCanvas = function() {
    var desiredWidth = 640;
    var desiredHeight = 400;
    var rect = canvas.parentElement.getBoundingClientRect();
    var absTop = Math.round(rect.top + window.pageYOffset);
    var absLeft = Math.round(rect.left + window.pageXOffset);
    var html = document.documentElement;
    var availWidth = html.clientWidth - absLeft * 2;
    var availHeight = html.clientHeight - (absTop + absLeft * 2);
    var widthFactor = desiredWidth / availWidth;
    var heightFactor = desiredHeight / availHeight;
    var scale = 1 / Math.max(widthFactor, heightFactor);

    // If you don't mind expanding the canvas, you can leave out
    // this scale-clipping guard.  The aspect ratio will still be preserved.
    scale = scale > 1 ? 1 : scale;
    
    var newWidth = Math.trunc(desiredWidth * scale);
    var newHeight = Math.trunc(desiredHeight * scale);
    if (canvas.width !== newWidth || canvas.height !== newHeight) {
        canvas.width = newWidth;
        canvas.height = newHeight;
        // and redraw
        var ctx = canvas.getContext('2d');
        ctx.fillStyle = 'red';
        ctx.fillRect(0, 0, canvas.width / 2, canvas.height / 2);
        ctx.fillStyle = 'blue';
        ctx.fillRect(canvas.width / 2, canvas.height / 2, canvas.width, canvas.height);
    }

    // If you want to center the canvas vertically, here's one way:
    // figure out how much space you have available below it, and give
    // the canvas a top-margin of 1/2 that.
    if (availHeight > canvas.height) {
        canvas.style.marginTop = Math.round((availHeight - canvas.height) / 2) + "px";
    }

    document.getElementsByTagName('i')[0].innerHTML = (
        '(available: ' + availWidth + 'x' + availHeight + ', ' +
        'scale: ' + scale + ', ' +
        'result: ' + canvas.width + 'x' + canvas.height + ')'
    );
};

window.addEventListener("load", resizeCanvas);
window.addEventListener("resize", resizeCanvas);
</script>