Using object-fit on a <div> with child elements, including a <canvas>

Question

I have an outer container that is variable in size and width. Suppose that inside this container, I have a canvas that I want to have grow as much as it can while maintaining proportion and not cropping. For this, I would normally use object-fit: contain.

Now, suppose instead of just a canvas, I have a canvas with another element placed next to it.

HTML:

<div class="outerContainer">
  <canvas width="640" height="360"></canvas>
  <div class="beside">
  </div>
</div>

CSS:

.outerContainer {
  display: flex;
  border: 0.5em solid #444;
  margin-bottom: 2em;
  object-fit: contain;
}
.outerContainer canvas {
  flex-grow: 1;
  background: #77a;
}
/* This element has a fixed width, and should be whatever height the <canvas> is */
.outerContainer .beside {
  flex-basis: 3em;
  flex-grow: 0;
  flex-shrink: 0;
  background: #7a7;
}

In this case, I want to scale the whole outerContainer size, just like I did with canvas. The problem is that object-fit doesn't actually scale the element... it scales its contents. This doesn't seem to apply to normal block elements, resulting in a case where the canvas inside is potentially skewed if there is enough width.

If I add object-fit: contain to the canvas element, it maintains proportion but still uses the full width, meaning the .beside element is all the way to the right. This is visualized with the purple background on the canvas.

What I would like is the outerContainer to be scaled with the canvas contents, so that .beside always has the height of the canvas contents. The .outerContainer should be centered in the parent element, taking up as much space as it can without distorting the canvas. Like this, at any proportional scale:

Is this doable with modern CSS? Or must I use a scripted solution?

Fiddle with examples: https://jsfiddle.net/nufx10zc/

Solution 1

I don't know if it's ok for you, but I think it can be done mostly in css if you allow a little bit more HTML.

Consider the following html

<div class="outerContainer">
  <div class="canvasContainer">  
    <canvas width="640" height="360"></canvas>
  </div>
  <div class="beside">
  </div>
</div>

I just added a little div around the canvas. This way we let the canvas handle it's things and we use the div for the flex things.

Using the following css:

* {
  box-sizing: border-box;
}
.outerContainer {
  display: flex;
  border: 0.5em solid #444;
  margin-bottom: 2em;
}
.canvasContainer canvas {
  width: 100%;
  background: #777;
  margin-bottom: -4px
}
.canvasContainer {
  flex-grow: 1;
  background: #77a;
}
/* This element has a fixed width, and should be whatever height the <canvas> is */
.outerContainer .beside {
  flex-basis: 3em;
  flex-grow: 0;
  flex-shrink: 0;
  background: #7a7;
}

We have canvas container taking all the available space. And then the canvas adapt accordingly with the image scaling in it. However I don't know why, there was a bit of a margin on the bottom of the canvas, hence the negative margin on it.

fiddle: https://jsfiddle.net/L8p6xghb/3/

As a side note, if you're looking to use that for captions, there are html5 element for it, like <figure> and <figcaption>!

Solution 2

I think a pure CSS solution is a bit far-fetched considering the solution you require - and here is a proposal using script that involves the following steps:

1. Find the aspect ratio for the image that best fits the available space for canvas

2. Update width for canvas and the flexbox's height.

3. Draw the image now in the canvas.

Note that for illustration, on window resize I have reloaded the frame - see demo below for details (more explanations are given inline):

// Document.ready
$(() => {
  putImageOnCanvas();
});
// Window resize event
((() => {
  window.addEventListener("resize", resizeThrottler, false);
  var resizeTimeout;
  function resizeThrottler() {
    if (!resizeTimeout) {
      resizeTimeout = setTimeout(function() {
        resizeTimeout = null;
        actualResizeHandler();
      }, 66);
    }
  }
  function actualResizeHandler() {
    // handle the resize event - reloading page for illustration
    window.location.reload();
  }
})());
function putImageOnCanvas() {
  $('.outerContainer canvas').each((index, canvas) => {
    const ctx = canvas.getContext('2d');
    canvas.width = $(canvas).innerWidth();
    canvas.height = $(canvas).innerHeight();
    const img = new Image;
    img.src = 'https://static1.squarespace.com/static/56a1d17905caa7ee9f27e273/t/56a1d56617e4f1177a27178d/1453446712144/Picture7.png';
    img.onload = (() => {
      // find the aspect ratio that fits the container
      let ratio = Math.min(canvas.width / img.width, canvas.height / img.height);
      let centerShift_x = (canvas.width - img.width * ratio) / 2;
      let centerShift_y = (canvas.height - img.height * ratio) / 2;
      canvas.width -= 2 * centerShift_x;
      canvas.height -= 2 * centerShift_y;
      // reset the flexbox height and canvas flex-basis (adjusting for the 0.5em border too)
      $('.outerContainer').css({
        'height': 'calc(' + canvas.height + 'px + 1em)'
      });
      $('.outerContainer canvas').css({
        'width': 'calc(' + canvas.width + 'px)'
      });
      // draw the image in the canvas now
      ctx.clearRect(0, 0, canvas.width, canvas.height);
      ctx.drawImage(img, 0, 0, img.width, img.height, 0, 0, img.width * ratio, img.height * ratio);
    });
  });
}

* {
  box-sizing: border-box;
}
body {
  margin: 0;
}
.outerContainer {
  display: flex;
  border: 0.5em solid #444;
  margin-bottom: 2em;
  /*max available flexbox height*/
  height: calc(100vh - 2em);
}
.outerContainer canvas {
  background: #77a;
  /*max available canvas width*/
  width: calc(100vw - 4em);
}
.outerContainer .beside {
  flex-basis: 3em;
  flex-grow: 0;
  flex-shrink: 0;
  background: #7a7;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div class="outerContainer">
  <canvas></canvas>
  <div class="beside">
  </div>
</div>

EDIT:

1. I was setting flex-basis for canvas initially - but Firefox was not behaving well - so now I'm using width instead.

2. Snippet has some issue on screen resize (page reload) again in Firefox - so have included a fiddle too.

UPDATED FIDDLE

Comments

[Py.]

What browser are you targeting? For me it works differently in firefox and chrome. Firefox doesn't seem to stretch the image, just scale it for both image in jsfiddle. While chrome does the behavior you describe in your post.

[Brad]

@Py. Chrome first, Firefox second, but the solution needs to work in both in the end.

[Christos Lytras]

Nice solution, but why do you use a direct call to a direct unnamed function for the Window resize event ((() => {...})());? Couldn't you just have the code outside an unnamed function like you have the function putImageOnCanvas()?

[kukkuz]

:) well, no particular reason... was learning about them and their effect on this, arguments etc, and just tried it out...