For our app we were looking at minimising texture space, and I was leering at some really large shapes that I needed big textures for.

TL😃R: Trace your shape and swap out the texture for a way smaller swatch of just the color.

This is the hand, pointing at things, I made:

Look at that huge png!

Here it is, traced, thank you esoteric, that's good stuff:

Now to replace the attachment:

Spine will ask you if you want to scale the texture coordinates, but DO NOT LET IT. I did this first, and it ended in tears. (*1)

"NO"... the answer is "NO" here 😢

To verify the UV mapping at this point, go into edit mesh mode, and toggle "deformed"

Here it is, that big attachment, mapped onto a little square, that's all the texture space it's using now!

Make sure the edge vertices are not on the exact edge, move them in a little, otherwise they'll be on the edge of the alpha sampling, and you'll likely not want that. Like this:

Here's one of the end results:

Enjoy your smaller texture atlas!

(*1) When I first clicked YES on scaling the texture coordinates, the UV map ended up self intersecting, and since Spine renders one sided it was a strange look, and it took us some time figuring it out what was wrong.