I want to create a "blueprint" aesthetic, so I went online and found a website that does a great job but I would like to know HOW it does it. Of course, I don't mean adding the grid or the background blue color, but specifically extracting the outline from an image. Also, I do not have Photoshop and use Gimp or Krita instead, so a general answer might help more. Here's an example:
I tried a few line detection filters but they do not work quite right. Best I got is Sobel-Feldman which gives me the following:
Which looks great but not so much when the image has a lot of white.