Force responsive images to be generated.

[gtlitc]

Hi There,
I have a conversion with responsive images set up on my model.
But I am just wondering if there is a way to generate the responsive images straight away from within my controller so that I can use getResponsiveImageUrls() or getSrcset() to return URLs for the responsive images. The use case is that I have a WYSIWIG editor and at the moment I am storing regular img tags with single src urls. Id like to be embedding srcsets instead. At the moment getResponsiveImageUrls() or getSrcset() are returning empty. I guess because the responsive images havent been created yet...
Any guidance much appreciated. Awesome package BTW.
Thanks,