How does ReallyColor.com turn photographs into coloring books?

Googling it says it is a patented program they use so we probably won’t know exactly how.

My guess? They built an AI to detect the main focuses and create an outline from there, as I’d imagine a direct translated outline would be too much to color, if that makes sense?

There are algorithms for generating cell shaded images by creating averages of colors and comparing them to nearby pixels to map out color zones of like colors (I actually think it may be the same family of algorithms used in gerrymandering). There are also edge-detection algorithms. Then it’s a matter of drawing boundaries with a solid stroke. Not trivial, but not cutting edge either.

Maybe someone traces the outlines with a stylus. Maybe it’s a filter. Wouldn’t that even be something that a skilled graphic artist could do freehand? It doesn’t seem like it would have to be too high-tech.

When I was a little kid, I had a very realistic, detailed coloring book of Queen Elizabeth’s coronation. I remember coloring those beautiful gowns—and the magnificent jeweled crown! No computers around then.

Here’s an example of how one implementation of edge detection works from Computerphiile.

There are photoshop scripts that can turn an image into all sorts of different art styles. I use those from time to time for work.
Does not even need AI.

There are paint by numbers programs, which not only show edges, but also tell what color a region is,

Old school way for me over many decades is to take my photos to a copy machine and blown them up and copied with a B&W light contrast setting. Then I paint them.