[bdon]

OMT use a CC-BY license: https://creativecommons.org/faq/#can-i-apply-a-creative-comm... (edit: link)

This means that software that implements OMT, even if written from scratch, cannot be re-used by other FOSS projects (Apache, BSD, GPL, AGPL, other software in the OpenStreetMap ecosystem, etc) without affecting the license.

Ideally for Protomaps it should be possible to re-use just one portion - like only the label layer with your own layers from other sources, or even bundle it as a JS dependency in another open source project - without affecting the license of downstream projects.

[dvdkon]

Are you talking about just the OpenMapTiles spec, or some adjacent software? I'm certain that you can build software to some specification without ever agreeing to the spec text's licence, and that a CC-BY licenced spec doesn't limit any implementing system's licence.

Even so, CC-BY is permissive and you could include CC-BY content in a, say, GPL project. You just need to include both licences.

[bdon]

> I'm certain that you can build software to some specification without ever agreeing to the spec text's licence

That is exactly the opposite of OMT's copyright interpretation: https://github.com/openmaptiles/openmaptiles?tab=readme-ov-f...

> You just need to include both licences.

That is the definition of license incompatibility as described in the Creative Commons documentation above. The license is open source and a good fit for if you are running a paid map SaaS or free service as an end product, but is not compatible with the open source ecosystem as a building block.