drawImage - Maintain scale while resizing

Question

drawImage - Maintain scale while resizing

I am currently working on resizing an image using drawImage while maintaining the scale. Here is what I have so far...

window.onload = function() {
            var c = document.getElementById("myCanvas");
            var ctx = c.getContext("2d");
            var img = document.getElementById("scream");
            ctx.drawImage(img, 0, 0, 600, 428);
        }

<p>Image to use:</p>
        <img id="scream" width="400" height="300" src="http://lorempixel.com/1280/960/sports/" alt="The Scream">

        <p>Canvas:</p>
        <canvas id="myCanvas" width="428" height="600" style="border:2px solid black;">
        Your browser does not support the HTML5 canvas tag.
        </canvas>

I am attempting to make the image fill the container in order to eliminate the white space at the bottom. However, if I input exact dimensions, the image gets stretched.

Can someone point me towards an example or provide some guidance?

javascript html css drawimage

Answer 1

Answer №1

To begin with, you must calculate the aspect ratios of both the image and the canvas. This can be achieved by:

var canvasRatio = canvas.width / canvas.height,
    imgRatio = img.width / img.height,
    s = 1;

If a ratio is less than 1, it indicates a portrait orientation, whereas a ratio equal to or greater than 1 suggests a landscape orientation.

Next, you should compare these ratios as follows:

// If the image is more portrait
// Scale by width to fully cover the canvas
if (imgRatio < canvasRatio) {
  s = canvas.width / img.width;

// If the image has the same or more landscape ratio than the canvas
// Scale by height to cover
} else {
  s = canvas.height / img.height;
}

// Scale the context accordingly
ctx.scale(s, s);
cts.drawImage(…);

UPDATE

An even shorter method (without using ratios or conditionals):

var s = Math.max(canvas.width/img.width, canvas.height/img.height);

To ensure the image fits appropriately, utilize Math.min.

Answer 2

To begin with, you must calculate the aspect ratios of both the image and the canvas. This can be achieved by:

var canvasRatio = canvas.width / canvas.height,
    imgRatio = img.width / img.height,
    s = 1;

If a ratio is less than 1, it indicates a portrait orientation, whereas a ratio equal to or greater than 1 suggests a landscape orientation.

Next, you should compare these ratios as follows:

// If the image is more portrait
// Scale by width to fully cover the canvas
if (imgRatio < canvasRatio) {
  s = canvas.width / img.width;

// If the image has the same or more landscape ratio than the canvas
// Scale by height to cover
} else {
  s = canvas.height / img.height;
}

// Scale the context accordingly
ctx.scale(s, s);
cts.drawImage(…);

UPDATE

An even shorter method (without using ratios or conditionals):

var s = Math.max(canvas.width/img.width, canvas.height/img.height);

To ensure the image fits appropriately, utilize Math.min.

drawImage - Maintain scale while resizing

Answer №1

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