How to scale (resize) image keep aspect ratio in javascript

Question

I have some images with random dimension and the the question is how can I scale (resize) it to exactly 960×1280 for example in JavaScript, but keep image origin aspect ratio:

• If the image is bigger than the expected size, it is scaled down (keeping aspect ratio) and the empty areas are filled with transparent color.
• If the image is smaller than the expected size, it is not scaled but centered and the empty areas are filled with transparent color.

I had read some on this topic but still could not resolve the problem.

This is not working for me: How to resize images proportionally / keeping the aspect ratio?

UPDATED: Working solution here, many thank to @Mr. Polywhirl Update solution

Answer 1

In order to figure out the aspect ratio to use for scaling, you need to figure out which dimension is larger. One you do that, you just divide the image width/height by the viewer's width/height and that should determine the scaling factor.

Centering can be achieved by finding the offset of the scaled image within the viewer.

var ctx = document.getElementById('image-viewer').getContext('2d');
var images = [
  'http://i.imgur.com/5PF0Xdi.jpg',
  'http://i.imgur.com/po0fJFT.png',
  'http://i.imgur.com/Ijzdt0o.png'
];
var counter = 0;

// Load a new image after 2 seconds.
setInterval(function() {
  loadScaleAndCenterImage(ctx, images[counter++ % images.length]);
}, 2000);

function loadScaleAndCenterImage(ctx, url) {
  var imageObj = new Image();
  imageObj.onload   = function(e) {
    var ctxWidth    = ctx.canvas.width,
        ctxHeight   = ctx.canvas.height;
    var imgWidth    = imageObj.width,
        imgHeight   = imageObj.height;
    var ratioWidth  = imgWidth  / ctxWidth,
        ratioHeight = imgHeight / ctxHeight,
        ratioAspect = ratioWidth > 1 ? ratioWidth : ratioHeight > 1 ? ratioHeight : 1;
    var newWidth    = imgWidth / ratioAspect,
        newHeight   = imgHeight / ratioAspect;
    var offsetX     = (ctxWidth  / 2) - (newWidth  / 2),
        offsetY     = (ctxHeight / 2) - (newHeight / 2);
    ctx.clearRect(0, 0, ctxWidth, ctxHeight);
    ctx.drawImage(this, offsetX, offsetY, newWidth, newHeight);
  };

  imageObj.src = url;
}

#image-viewer {
  background-color: #E4E4E4;
  background-image:
    linear-gradient(45deg, #7F7F7F 25%, transparent 25%, transparent 75%, #7F7F7F 75%, #7F7F7F), 
    linear-gradient(45deg, #7F7F7F 25%, transparent 25%, transparent 75%, #7F7F7F 75%, #7F7F7F);
  background-size: 20px 20px;
  background-position: 0 0, 30px 30px
}

<canvas id="image-viewer" width="1280" height="960"></canvas>