Heheh, I literally have my led Dreamcatcher sitting here 75% done (I posted a photo of it finished, but then I took it apart cause I’d screwed up the rim, and in redoing, improved on the technique. And then I had a pixel go dead as I was wrapping, and put it aside to remove the dead one (which with these preaddressed pixels is actually a small problem, I’ll have a missing pixel I need to account for in the map count.)
Yes, the 2d photo mapper is a good idea for an XY mapping
For Polar, I’d just take a ruler and an angle measure, and measure the pixels, for maximum accuracy. Distance is relative and the angle is too, so long as you measure the right direction (counter clockwise from 3o’clock)
However, you also need to decide in where the center is. True circle center, or wrapped center (which is what is visible in the video).
You could actually measure both ways (2D and true center will be/should be analogous, and math verified by the other), and have 3 mappings all of which would be useful.