How to make traces appear on scroll?

Question

css

Greetings, I’ve already searched everything, but I don’t have any ideas how to implement it, we want to make animal tracks slowly appear throughout the site along vertical scrolling, there is a picture traced in the layout with a layer on the entire site, we thought to make it a background, but somehow it doesn’t work out that way , I would like some beautiful animation of gradual appearance when scrolling, so that traces appear, can you tell me where to dig or maybe there are ready-made solutions?