What's the math behind CSS's background-size:cover

Question

I\'m creating an \"image generator\" where users can upload an image and add text and/or draw on it. The outputted image is a fixed size (698x450).

On the client side,

Answer 1

I know this is a very old question, but the answer I wrote is actually cleaner by using max and mins on the ratios between the images instead of each image with itself:

var originalRatios = {
  width: containerWidth / imageNaturalWidth,
  height: containerHeight / imageNaturalHeight
};

// formula for cover:
var coverRatio = Math.max(originalRatios.width, originalRatios.height); 

// result:
var newImageWidth = imageNaturalWidth * coverRatio;
var newImageHeight = imageNaturalHeight * coverRatio;

I like this approach because it is very systematic — maybe it's the wrong word —. What I mean is you can get rid of the if statements and make it work in a more "math formula" kind of way (input = output, if that makes sense):

var ratios = {
  cover: function(wRatio, hRatio) {
    return Math.max(wRatio, hRatio);
  },

  contain: function(wRatio, hRatio) {
    return Math.min(wRatio, hRatio);
  },

  // original size
  "auto": function() {
    return 1;
  },

  // stretch
  "100% 100%": function(wRatio, hRatio) {
    return { width:wRatio, height:hRatio };
  }
};

function getImageSize(options) {
  if(!ratios[options.size]) {
    throw new Error(options.size + " not found in ratios");
  }

  var r = ratios[options.size](
    options.container.width / options.image.width,
    options.container.height / options.image.height
  );

  return {
    width: options.image.width * (r.width || r),
    height: options.image.height * (r.height || r)
  };
}

I created a jsbin here if you want to take a look at what I mean with systematic (it also has a scale method that I thought was not needed in this answer but very useful for something other than the usual).