CSS: Make Canvas as big as possible while keeping aspect ratio

Question

I have a Canvas element that is inside a container div. When the user selects an image from his machine, this image should be displayed on the canvas. I want the canvas to be big as possible but at the same time keep the aspect ratio of the image. I know neither the proportions of the image nor the size of the container div, as this is relative to the screen/window size of the user.

If I set max-width and max-height to e.g 100% the canvas will not fill the container if the selected image is smaller then the container. If I set width and height instead of max-width and max-height the canvas doesn't keep the aspect ratio.

Does anyone have an idea how to solve this?

Answer 1

If you're willing to use JQuery (or regular JavaScript), then a solution like this might work:

<script>

    // Note: this uses jQuery.
    // It makes getting/setting the dimensions easier,
    // but you can do this with normal JavaScript

    var img = $("#img");
    var container = $("#container");

    var width = img.width();
    var height = img.height();
    var maxWidth = container.width();
    var maxHeight = container.height();

    var ratio = maxWidth / width;
    if(height * ratio > maxHeight) {
        ratio = maxHeight / height;
    }
    img.width(width * ratio);
    img.height(height * ratio);

</script>

What this does is that it finds the ratio to multiply the width and the height by, whichever one is smaller (so that it will always fit in the window).

Update: Tested on JSFiddle.net. See it here.

I hope this helps you!