[SOLVED] Responsive Image full screen and centered - maintain aspect ratio, not exceed window

Responsive Image full screen and centered - maintain aspect ratio, not exceed window

So I want an img to be displayed

as big as possible (filling the width when it is landscape / height when it is portrait)
no crop
no skew or stretch (original aspect ratio)
centred both vertically and horizontally

Also, the image's original size is not known.

I've tried quite a few different options for this, including a flexbox one (to get the vertical center), but nothing seems to tick all the boxes.

Ideally I'd like this to be an all CSS solution of course, but I have been looking into some JS as well.

Solution

To center it, you can use the technique shown here: Absolute centering.

To make it as big as possible, give it max-width and max-height of 100%.

To maintain the aspect ratio (even when the width is specifically set like in the snippet below), use object-fit as explained here.

.className {
    max-width: 100%;
    max-height: 100%;
    bottom: 0;
    left: 0;
    margin: auto;
    overflow: auto;
    position: fixed;
    right: 0;
    top: 0;
    -o-object-fit: contain;
    object-fit: contain;
}

<img src="https://i.imgur.com/HmezgW6.png" class="className" />

<!-- Slider to control the image width, only to make demo clearer !-->
<input type="range" min="10" max="2000" value="276" step="10" oninput="document.querySelector('img').style.width = (this.value +'px')" style="width: 90%; position: absolute; z-index: 2;" >