{% macro picture(st, basename, widths, formats, sizes=None, alt='') -%}
{% if not sizes %}
{% set sizes = '{}px'.format(widths[0]) %}
{% endif %}


<picture>
    {%- for format in formats %}
    <source type='image/{{format}}' sizes='{{sizes}}' srcset="
    {%- for width in widths -%}
        {{ st('{}-{}.{}'.format(basename, width, format)) }} {{width}}w
        {%- if not loop.last %},{% endif %}
    {%- endfor -%}
    "/>
    {% endfor %}
    <img src="{{ st('{}-{}.{}'.format(basename, widths[0], formats[-1])) }}" alt="{{alt}}" />
</picture>

{%- endmacro %}