Srcset Image Tags for Middleman

Install libvips 8.6 or higher. Unfortunately on Debian that means compiling it from source.

Add the gem to your site's Gemfile.

gem 'middleman-srcset_images', github: 'jkraemer/middleman-srcset_images'

Create a configuration file as outlined below.

In your Markdown files, use this syntax for images:

![Alt text](/path/to/image.jpg!half)

where half is one of the configured image sizes (see below). The result will be an image tag with srcset and sizes attributes. To create a linked image, add another exclamation mark followed by the destination URL:

![Linked Image](/path/to/image.jpg!half!/path/or/url)

Relative image paths are assumed to be local to the article, and the file extension is assumed to be jpg if not present. So when using the 'one directory per page' approach where you have a file layout like this:

source/
  some-article.html.md
  some-article/
    image.jpg
    another_image.jpg

you can significantly shorten your markup like this:

![Lorem Ipsum](image!full)

Configuration takes place in data/srcset_images.yml.

The keys in this hash are the sizes that can be used in Markdown / with the image_tag helper. The value is put into the sizes attribute of the resulting img tag. The purpose of this attribute is to give the browser a hint about how big this image will actually be rendered (relative to the screen size) due to your CSS rules.

The sample below is for a site which can display content images in 3 different sizes, and that has a separate config for teaser images. On small devices, all images are rendered at full width, while on larger devices, they only take a fraction of the actual screen width. The separate teaser config is only there to allow for different cropping rules in the image versions config.

Configure scaling options for landscape and portrait images. These rules determine which sizes of images will be created when building your site, and also what goes into the srcset attribute of the img tag.

Besides the landscape and portrait keys, which act as fallbacks for images of landscape or portrait dimensions, you can add any other keys here for different use cases (i.e. cropping to a fixed xy ratio for teaser images as is done in the sample below).

When rendering an img tag, the image version config to be used is picked as follows:

• if there is a key matching the size parameter, use this config. This would be the case for teaser images in the sample below.
• otherwise, check the layout of the image and pick the portrait config if the image is higher than wide, and the landscape otherwise.

---

sizes:
  full: "(min-width: 768px) 90vw, 100vw"
  half: "(min-width: 768px) 45vw, 100vw"
  third: "(min-width: 768px) 30vw, 100vw"
  teaser: "(min-width: 768px) 30vw, 100vw"

images: posts/**/*.jpg
# in case you have symlinked directories to your actual photos, use something
# like this:
# images: posts/**{,/*/**}/*.jpg

image_versions:
  # configuration for landscape and square images
  landscape:
    quality: 80
    srcset:
      -
        width: 2000
      -
        width: 1400
        is_default: true
      -
        width: 800
      -
        width: 400

  # portrait content images, cropped to 3:4
  portrait:
    quality: 80
    crop: true
    srcset:
      -
        height: 1800
        width: 1350
      -
        height: 1200
        width: 900
        is_default: true
      -
        height: 800
        width: 600

  # teaser image, cropped to landscape 3:2
  teaser:
    crop: true
    quality: 80
    srcset:
      -
        width: 1800
        height: 1200
      -
        width: 1200
        height: 800
        is_default: true
      -
        width: 600
        height: 400

MIT, see LICENSE