Convert archive image formats

[neodescis]

I have a rather large pmtiles archive that has PNG tiles inside (digital terrain tiles). I'd like to convert it to WebP format. It'd be great if pmtiles convert could do this for me. Alternatively, if there is any other (relatively) expedient way to make such a conversion, I'm all ears!

[bdon]

This won't be part of the default functionality because there is no optimized WebP writer, as far as I know, that uses Go only. Depending on a native library would make it impossible to easily cross-compile the single binary for this program.

We could instead expose an iterator convenience function over an archive that writes a new archive, performing an operation on each unique tile, and then you could write a short program that reads in PNG and writes WebP.

[neodescis]

That sounds like a good idea to me, as it would open the door to many other processing abilities as well. It would need to allow for modification of the metadata as well somehow to support a format change.

[neodescis]

For my particular use case, I think I've got a workaround. It's slow-going (I'm still on step 3), but it appears that this will work:

1. Use the python pmtiles-convert to convert back to .mbtiles
2. Use mb-util to export tiles to a directory as WebP
3. Use pmtiles-convert to take the WebP tiles into another .pmtiles archive