Drop-in replacement for Hugo figure shortcode with img srcset support

figure.html

{{/*

figure with auto-resizing and srcset v2020-05-10

Drop-in replacement for Hugo's figure shortcode as of 2020-05-02 that uses img srcset
to enable browsers to download only the resolution that they need.

The resizing and srcset magic only works for images that are part of the page
bundle. It will fall back to stock Hugo figure behaviour otherwise.

Improvements that were initially out of reach of my Hugo template programming "skills"
but have now been taken care of:
- [x] gracefully handle images that are not in page bundle, i.e. no image processing available
- [x] use a single configurable sizes array, and derive everything from there

See https://cpbotha.net/2020/05/02/drop-in-replacement-for-hugo-figure-shortcode-with-img-srcset-support/

- original srcset img shortcode from: https://laurakalbag.com/processing-responsive-images-with-hugo/
- original hugo figure shortcode from: https://github.com/gohugoio/hugo/blob/master/tpl/tplimpl/embedded/templates/shortcodes/figure.html
- no unnecessary resizes and more nudges by Stéfan van der Walt https://mentat.za.net/
- mashing together and srcset logic fixes by Charl P. Botha https://cpbotha.net/

Changes:
- 2020-05-10 fall back to stock Hugo behaviour when no page bundle found
- 2020-05-04 no unnecessary resizes, sizes in array
- 2020-05-02 initial release

*/}}

{{/* hugo will resize to all of these sizes that are smaller than your original. configure if you like! */}}
{{ $sizes := (slice "480" "800" "1200" "1500") }}

{{/* get file that matches the filename as specified as src="" in shortcode */}}
{{ $src := .Page.Resources.GetMatch (printf "*%s*" (.Get "src")) }}

<figure{{ with .Get "class" }} class="{{ . }}"{{ end }}>
    {{- if .Get "link" -}}
        <a href="{{ .Get "link" }}"{{ with .Get "target" }} target="{{ . }}"{{ end }}{{ with .Get "rel" }} rel="{{ . }}"{{ end }}>
    {{- end }}
    <img
        {{ if $src }}
            sizes="(min-width: 35em) 1200px, 100vw"
            {{/* only srcset images smaller than or equal to the src (original) image size, as Hugo will upscale small images */}}  
            srcset='
            {{ range $sizes }}
                   {{ if ge $src.Width . }}{{ ($src.Resize (printf "%sx" .)).Permalink }} {{ (printf "%sw" .) }},{{ end }}
            {{ end }}'

            {{/* when no support for srcset (old browsers, RSS), we load small (800px) */}}
            {{/* if image smaller than 800, then load the image itself */}}
            {{ if ge $src.Width "800" }}src="{{ ($src.Resize "800x").Permalink }}"
            {{ else }}src="{{ $src.Permalink }}"
            {{ end }}

        {{ else }}
            {{/* fall back to stock hugo behaviour when image is not available in bundle */}}
            src="{{ .Get "src" }}"
        {{ end }}

        {{- if or (.Get "alt") (.Get "caption") }}
            alt="{{ with .Get "alt" }}{{ . }}{{ else }}{{ .Get "caption" | markdownify| plainify }}{{ end }}"
        {{- end -}}
        {{- with .Get "width" }} width="{{ . }}"{{ end -}}
        {{- with .Get "height" }} height="{{ . }}"{{ end -}}
    /> <!-- Closing img tag -->
    {{- if .Get "link" }}</a>{{ end -}}
    {{- if or (or (.Get "title") (.Get "caption")) (.Get "attr") -}}
        <figcaption>
            {{ with (.Get "title") -}}
                <h4>{{ . }}</h4>
            {{- end -}}
            {{- if or (.Get "caption") (.Get "attr") -}}<p>
                {{- .Get "caption" | markdownify -}}
                {{- with .Get "attrlink" }}
                    <a href="{{ . }}">
                {{- end -}}
                {{- .Get "attr" | markdownify -}}
                {{- if .Get "attrlink" }}</a>{{ end }}</p>
            {{- end }}
        </figcaption>
    {{- end }}
</figure>

Hey, this is very nice, thank you.

Could it be that there is a minor error in row 51.

{{ if ge $src.Width "800" }}src="{{ ($src.Resize "800x").Permalink }}"

when it should be

{{ if ge "800" $src.Width }}src="{{ ($src.Resize "800x").Permalink }}"

---

Never mind, I was wrong. Just didn't understand what the src-part did

Thanks for the note, and the thanks!

I did just double-check: Fortunately the comment right above that line fortunately describes the intent, and the code does follow that.

Very cool.
I've made some changes to handle the situation where:

1. The image is an SVG
2. The image needs to be rotated

I'm not great with git/github so see https://petersmith.org/blog/2023/01/19/figure-shortcode-again-/ for details

If you add the below on line #59 you will get a better pagespeed score with your images. Surprisingly, it only cares if the value is defined. Not that it matches the dimensions of the specific image being served.

       width="{{ $src.Width }}"
       height="{{ $src.Height }}"