Twig macro for creating a srcset string in Craft

macro.twig

{% macro getSrcSet(image, sizes, upscale) %}
	{%- spaceless %}
		{% set sizesOut = [] %}
		{% set allowScaleLarger = upscale | default(false) %}
		
		{# loop over sizes parameter and generate transforms #}
		{% for size in sizes %}
			{%- spaceless %}
				{% set currentTransform = {} %}
				
				{% if size.quality is defined %}
					{% set currentTransform = currentTransform | merge({ quality: size.quality }) %}
				{% endif %}
				
				{% if size.format is defined %}
					{% set currentTransform = currentTransform | merge({ format: size.format }) %}
				{% endif %}
				
				{% if size.width is defined %}
					{% set currentTransform = currentTransform | merge({ width: size.width }) %}
				{% endif %}
				
				{% if size.height is defined %}
					{% set currentTransform = currentTransform | merge({ height: size.height }) %}
				{% endif %}
				
				{% if size.mode is defined %}
					{% set currentTransform = currentTransform | merge({ mode: size.mode }) %}
					
					{# assumes that you have a asset field named focalPoint that lets the user choose crop position #}
					{% if (size.mode=='crop') and (image.focalPoint is defined) and (image.focalPoint!='') %}
						{% set currentTransform = currentTransform | merge({ position: image.focalPoint }) %}
					{% endif %}
				{% endif %}
				
				{# only add transform if original image is larger, or upscale was set to true #}
				{% if allowScaleLarger or (image.getWidth() >= image.getWidth(currentTransform)) %}
					{% set sizesOut = sizesOut | merge([currentTransform]) %}
				{% endif %}
			{% endspaceless -%}
		{% endfor %}

		{# if no transform was made, but transform enforces a specific aspect, create it #}
		{% if sizesOut | length == 0 %}
			{% set size = sizes[0] %}
			{% if (size.width is defined) and (size.height is defined) %}
				
				{% set newHeight = (image.getWidth() * (size.height/size.width)) | round %}
				{% set currentTransform = { width: image.getWidth(), height: newHeight } %}
				
				{% if size.mode is defined %}
					{% set currentTransform = currentTransform | merge({ mode: size.mode }) %}
					
					{# assumes that you have a asset field named focalPoint that lets the user choose crop position #}
					{% if (size.mode=='crop') and (image.focalPoint is defined) and (image.focalPoint!='') %}
						{% set currentTransform = currentTransform | merge({ position: image.focalPoint }) %}
					{% endif %}
				{% endif %}
				
				{% set sizesOut = sizesOut | merge([currentTransform]) %}
				
			{% endif %}
		{% endif %}

		{# if any transforms were made, loop over them and output srcset. if not, use original file (means it's smaller than the smallest size) #}
		{% if sizesOut | length %}
			{% for size in sizesOut %}
				{{ image.getUrl(size) }} {{ image.getWidth(size) }}w{% if not loop.last %},{% endif %}
			{% endfor %}
		{% else %}
			{{ image.getUrl() }} {{ image.getWidth() }}w
		{% endif %}
	{% endspaceless -%}
{% endmacro %}

template.twig

<img sizes="100vw" src="{{ getBase64Pixel() }}" srcset="{{ getSrcSet( topImage, [{ width: 640, height: 800, mode: 'crop' }, { width: 960, height: 1200, mode: 'crop' }, { width: 1280, height: 1600, mode: 'crop' }] ) }}">