#CraftCMS: SRCset Macro

example.twig

{# can define custom sizes, if required #}
{% set outputWidths = [580, 690, 768] %}
{% set aspectRatio = (3/5) %}

{{ srcset(entry.image, outputWidths, aspectRatio) }}

srcset-macro.twig

{% macro srcset(image, outputWidths, aspectRatio) %}
  {# setup #}
  {%- set outputWidths = outputWidths | default([640, 1024, 1600]) -%}
  {%- set srcset = [] -%}

  {# if output width is smaller than or equal to the original image width #}
  {%- for outputWidth in outputWidths -%}
    {%- if outputWidth <= image.width -%}
      {%- if aspectRatio -%}
        {%- set transformOptions = { width: outputWidth, height: outputWidth * aspectRatio } -%}
      {%- else -%}
        {%- set transformOptions = { width: outputWidth } -%}
      {%- endif -%}
      
      {%- set srcset = srcset | merge([image.url(transformOptions) ~ ' ' ~ outputWidth ~ 'w']) -%}
    {%- endif -%}
  {%- endfor -%}

  {# output srcset #}
  {{- srcset | join(', ') -}}
{% endmacro %}