It’s been about 20 years since I learned PoT in math class… some available reading material or walkthrough of how it works would’ve been nice to jog the memory. Google was of no help.
Animation of 13 frames
a grid of 4x4 of 128px frames, results in 512x512, with a loss of three slots.
If I known this earlier, I made the animation 16 frames, full sprite sheet and smoother animation.
There is a useful script-fu for flattening layer groups (http://registry.gimp.org/node/26479).
It can flatten all layer groups in file and can be reverted back to with one undo step…
Here’s my portcullis as a sprite sheet. Snap-to-grid was really useful!