You’ll need to run an edge detection algorithm on the image. The Sobel operator is the most common one used although it can be hard to get clean enough lines to use for something like 3D printing. However, if you start with an illustration with clean lines (like the examples in drawsth3d.com) it would probably work.
Thanks Looeee for your prompt reply! to be honest I am not a progammer so it’s hard to for me get
through the alogrithm logic… Meanwhile, I found another way that is to change the image into coloring page for kids (like this: ScrapColoring - Convert your Drawings and Photos to Coloring Pages ), it is feasible! however, I am not sure how the scrapcoloring site works.