You have to use a drawing/photo editor app like photoshop (windows) or procreate (iOS). I know the ‘16 images’ sounds a lil intimidating but I like to think of it more as 16 layers… I would recommend watching some tutorials on how to make animated gifs with layers or even just tutorials about how to make layers in whatever app your using. Once you understand that concept it’s pretty much the same thing. The bottom/first layer would be zero, with the 16th layer being 15.
if you had a full circle and cut it into 15 equal pieces-
(First layer)0= nothing showing at all
(Last layer)15= gauge would be fully maxed out or the circle is fully complete
so in your app, each layer stacks on top of itself if that makes sense?
