Maintain image aspect ratio when changing height

Question

To keep images from stretching in either axis inside a flex parent I have found a couple of solutions.

You can try using object-fit on the image which, e.g.

object-fit: contain;

http://jsfiddle.net/ykw3sfjd/

Or you can add flex-specfic rules which may work better in some cases.

align-self: center;
flex: 0 0 auto;

For img tags if you define one side then other side is resized to keep aspect ratio and by default images expand to their original size.

Using this fact if you wrap each img tag into div tag and set its width to 100% of parent div then height will be according to aspect ratio as you wanted.

http://jsfiddle.net/0o5tpbxg/

* {
    margin: 0;
    padding: 0;
}
.slider {
    display: flex;
}
.slider .slide img {
    width: 100%;
}

No need to add a containing div.

The default for the css "align-items" property is "stretch" which is what is causing your images to be stretched to its full original height. Setting the css "align-items" property to "flex-start" fixes your issue.

.slider {
    display: flex;
    align-items: flex-start;  /* ADD THIS! */
}

Most of images with intrinsic dimensions, that is a natural size, like a jpeg image. If the specified size defines one of both the width and the height, the missing value is determined using the intrinsic ratio... - see MDN.

But that doesn't work as expected if the images that are being set as direct flex items with the current Flexible Box Layout Module Level 1, as far as I know.

See these discussions and bug reports might be related:

• Flexbugs #14 - Chrome/Flexbox Intrinsic Sizing not implemented correctly.
• Firefox Bug 972595 - Flex containers should use "flex-basis" instead of "width" for computing intrinsic widths of flex items
• Chromium Issue 249112 - In Flexbox, allow intrinsic aspect ratios to inform the main-size calculation.

---

As a workaround, you could wrap each <img> with a <div> or a <span>, or so.

jsFiddle

.slider {
  display: flex;
}

.slider>div {
  min-width: 0; /* why? see below. */
}

.slider>div>img {
  max-width: 100%;
  height: auto;
}

<div class="slider">
  <div><img src="https://picsum.photos/400/300?image=0" /></div>
  <div><img src="https://picsum.photos/400/300?image=1" /></div>
  <div><img src="https://picsum.photos/400/300?image=2" /></div>
  <div><img src="https://picsum.photos/400/300?image=3" /></div>
</div>

4.5 Implied Minimum Size of Flex Items

To provide a more reasonable default minimum size for flex items, this specification introduces a new auto value as the initial value of the min-width and min-height properties defined in CSS 2.1.

---

Alternatively, you can use CSS table layout instead, which you'll get similar results as flexbox, it will work on more browsers, even for IE8.

jsFiddle

.slider {
  display: table;
  width: 100%;
  table-layout: fixed;
  border-collapse: collapse;
}

.slider>div {
  display: table-cell;
  vertical-align: top;
}

.slider>div>img {
  max-width: 100%;
  height: auto;
}

<div class="slider">
  <div><img src="https://picsum.photos/400/300?image=0" /></div>
  <div><img src="https://picsum.photos/400/300?image=1" /></div>
  <div><img src="https://picsum.photos/400/300?image=2" /></div>
  <div><img src="https://picsum.photos/400/300?image=3" /></div>
</div>

I have been playing around flexbox lately and i came to solution for this through experimentation and the following reasoning. However, in reality I'm not sure if this is exactly what happens.

If real width is affected by flex system. So after width of elements hit max width of parent they extra width set in css is ignored. Then it's safe to set width to 100%.

Since height of img tag is derived from image itself then setting height to 0% could do something. (this is where i am unclear as to what...but it made sense to me that it should fix it)

DEMO

(remember saw it here first!)

.slider {
    display: flex;
}
.slider img {
    height: 0%;
    width: 100%;
    margin: 0 5px;
}

Works only in chrome yet

This behavior is expected. flex container will stretch all its children by default. Image have no exception. (ie, parent will have align-items: stretch property )

To keep the aspect ratio we've two solutions:

1. Either replace default align-items: stretch property to align-items: flex-start or align-self: center etc, http://jsfiddle.net/e394Lqnt/3/

or

2. Set align property exclusively to the child itself: like, align-self: center or align-self: flex-start etc. http://jsfiddle.net/e394Lqnt/2/