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The Polar Coordinates Filter sometimes leaves a 1 pixel seam. This often occurs after editing an image which consists of two layers which are merged to one layer. The gap can be avoided by marking the "Map backwards" option. However, this workaround sometimes enforces another correction afterwards. Is there a known approach to handle (i.e. avoid) this 1 pixel seam problem?

Im currently using Gimp 2.10 on macOS Big Sur Version 11.1.

I have seen this before. Does your image have an alpha channel ? Try removing the alpha channel.

Depending on what you are looking for, if for example creating a sphere, you could try the option Filtres-Map-Map object ...

Note that the results are different, but creating a sphere would be an alternative.
[attachment=5621]

Another option:
1- Filters-Distorts- Polar Coordinates - Check only Map from top.
2- Layer-Transform- Flip vertically
3- Filters-Distorts- Polar Coordinates - check only To Polar (or if you like the result check also the option Map Backwards)