HTML Canvas: Scaling image to fit without stretching?

Question

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]);
}

Answer 1

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