Javascript – HTML Canvas: Scaling image to fit without stretching

canvashtmljavascript

Im using the code below to draw and scale an image inside a canvas. The issue being that the image rendered inside is stretched to fit.

If possible, I would like it to be scaled based on width but to maintain its aspect ratio.

Any ideas?

//IMAGE LOADER
var canvas = document.getElementById('image-canvas');
var canvas2 = document.getElementById('image-canvas2');
ctx = canvas.getContext('2d');
ctx2 = canvas2.getContext('2d');

// Trigger the imageLoader function when a file has been selected
var fileInput = document.getElementById('fileInput');
fileInput.addEventListener('change', imageLoader, false);

function imageLoader() {
    var reader = new FileReader();
    reader.onload = function(event) {
        img = new Image();
        img.onload = function(){
            ctx.drawImage(img,0,0,canvas.width, canvas.height);
            ctx2.drawImage(img,0,0,canvas.width, canvas.height);
        }
        img.src = reader.result;
    }
    reader.readAsDataURL(fileInput.files[0]);
}

Best Solution

You can figure out dimensions of the image by adding it to the DOM. Once you know the ratio, you can scale your image to fit within your canvas.

ratio = width / height;
width = height * ratio;
height = width / ratio;

The following should work whether an image is landscape or portrait.

var canvas = document.getElementById('image-canvas');
var canvas2 = document.getElementById('image-canvas2');
var ctx = canvas.getContext('2d');
document.getElementById('fileInput').addEventListener('change', imageLoader, false);

function imageLoader() {
    var reader = new FileReader();
    reader.onload = function(event) {
        var img = new Image();
        img.onload = function(){
            var ct = document.getElementById('measure'); 
            ct.appendChild(img);
            var wrh = img.width / img.height;
            var newWidth = canvas.width;
            var newHeight = newWidth / wrh;
            if (newHeight > canvas.height) {
                newHeight = canvas.height;
                newWidth = newHeight * wrh;
            }
            ct.removeChild(img);
            ctx.drawImage(img,0,0, newWidth , newHeight);
        }
        img.src = reader.result;
    }
    reader.readAsDataURL(fileInput.files[0]);
}
/* Add image to the DOM here so user can't see it */
#measure { position: absolute; left: -10000px; top: -100000px;}
canvas { border: 1px solid red; }
<canvas id="image-canvas" width="300" height="300"></canvas>
<input type="file" id="fileInput" />
<div id="measure"></div>

You can play with it here