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HTML 

              
                <h1>
  Demonstrating <code>fm=json</code> parameter to set aspect ratios based on
  original image sizes using imgix.
</h1>

<br />
<p>
  Originally, we cropped all of our images to fit within the same aspect ratio.
  Originals were set to <code>w=300</code>, <code>h=200</code>, and
  <code>fit=crop</code>.
</p>
<!-- <img> are given a w, h, and palceholder src so that imgix.js can assign placehodlers correctly-->
<image-container>
  <img src="data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==" width="300" height="200" ix-src="https://assets.imgix.net/examples/octopus.jpg?w=300&h=200&fit=crop" alt="An octopus" sizes="300px" class="image-300-200" />
</image-container>
<image-container>
  <img src="data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==" width="300" height="200" ix-src="https://assets.imgix.net/examples/espresso.jpg?w=300&h=200&fit=crop" alt="An espresso" sizes="300px" class="image-300-200" />
</image-container>
<image-container>
  <img src="data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==" width="300" height="200" ix-src="https://miguel.imgix.net/imgix/flower.jpg?w=300&h=200&fit=crop" alt="A flower" sizes="300px" class="image-300-200" />
</image-container>
<image-container>
  <img src="data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==" width="300" height="200" ix-src="https://assets.imgix.net/examples/kayaks.png?w=300&h=200&fit=crop" alt="Some kayaks" sizes="300px" class="image-300-200" />
</image-container>
<p>
  Eventually, we decided that we wanted to maintain the aspect ratios of the
  source images. We can now do that using the
  <a href="https://www.imgix.com/docs/reference/format#param-fm">imgix JSON Output Format</a>. In this pen, we use JavaScript to read metadata about an image then set a
  max dimensions with the longest edge of 300px and a 16:9 ratio.
</p>
<image-container>
  <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" data-src="https://assets.imgix.net/examples/octopus.jpg" class="imgix-item" />
</image-container>
<image-container>
  <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" data-src="https://assets.imgix.net/examples/espresso.jpg" class="imgix-item" />
</image-container>
<image-container>
  <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" data-src="https://miguel.imgix.net/imgix/flower.jpg" class="imgix-item" />
</image-container>
<image-container>
  <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" data-src="https://assets.imgix.net/examples/kayaks.png" class="imgix-item" />
</image-container>
<p>
  Without storing any information server-side, we're able to build rich displays
  of our images based on their natural aspect ratios. This opens up a whole new
  way of working with images on imgix.
</p>

<p>
  imgix is a hosted service that takes your existing static images and turns
  them into dynamic, responsive images that you can deliver to any device, under
  any conditions. It’s free to sign up at
  <a href="https://imgix.com">imgix.com</a>.
</p>
!

CSS 

              
                body {
  text-align: center;
}

image-container {
  display: inline-block;
  vertical-align: top;
  width: auto;
  height: auto;
  max-width: 1280px;
  margin: 20px 0 20px 20px;

  img {
    // img elements need a width in order for placeholders
    // to be sized by imgix.js correctly
    max-width: 100%;
    width: 100%;
  }
}

h1,
p {
  font-family: "Helvetica Neue", Helvetica, Arial, sans;
  max-width: 480px;
  margin: 1em auto 1em auto;
  text-align: left;
  line-height: 1.4;
}

h1 {
  line-height: 1.2;
  margin-top: 2em;
  margin-bottom: 0;
}

a:link {
  color: #0ebeff;
}

a:visited {
  color: #ff853c;
}

code {
  font-family: "Lucida Console", monaco, mono;
  color: #ff853c;
}
!

JS 

              
                // Demonstrate the use of the `fm=json`
// parameter to resize images to a certain
// aspect ratio, using ES6.

const ratio = { h: "16:9", v: "9:16" };
const maxSize = 300;

document.addEventListener("DOMContentLoaded", () => {
  // We pull down the images by the 'data-src'
  // attribute of each .imgix-item.
  const images = document.querySelectorAll(".imgix-item");
  images.forEach((val, i) => {
    const url = val.dataset.src;

    // But, we append the "?fm=json" query
    // string to it. instructs imgix to return
    // the JSON Output Format instead of a
    // manipulated image.
    axios.get(url + "?fm=json").then((response) => {
      // Store the pixel h & w values from the
      // JSON response
      const { PixelHeight, PixelWidth } = { ...response.data };
      let newWidth, newHeight, ar;

      // Next, we compute the new height/width
      // params for each of our images.
      const portrait = PixelHeight > PixelWidth;

      if (portrait) {
        newHeight = maxSize;
        ar = ratio.v;
      } else {
        newWidth = maxSize;
        ar = ratio.h;
      }

      // We set h & w params so the placeholder
      // can be sized correctly as the image
      // loads.
      val.setAttribute("w", portrait ? "9" : "16");
      val.setAttribute("h", portrait ? "16" : "9");

      // Now, we apply these to our actual
      // images, setting the 'src' attribute
      // for the first time.
      const dimensionParams = portrait ? `h=${newHeight}` : `w=${newWidth}`;
      const queryParams = `?${dimensionParams}&fit=crop&ar=${ar}`;
      val.setAttribute("src", url + queryParams);
    });
  });
});
!
999px

Console