In my quest for the best responsive image approach ever, I have tried lots of things. However, every solution has its cons.
I found statamic plugins that upload to multiple sizes S3 - but I would like to transform the images in my templates and not when uploading them. I can offcourse save multiple files in different dimensions with the transform tag, but if I am right I can only save those images on the same server – and disk space on hosting accounts often is rather limited or expensive.
I also used resrc.it which was by far the easiest to implement. However, images showed up very slowly instead of much faster. And it seems that resrc.it shows an image resolution based on the initial viewport. So if you open a website at 800px wide and resize your browser to be 1200 wide, it still showed the image for the lower resolution. You don't have this problem with the srcset technique.
So, best for now would be uploading the file to S3, in the original size. Then in the templates use the transform tag but save it to S3 instead of the local server. Is something like this possible?