1) The Protomaps schema is mostly a re-implementation of the Tilezen project https://tilezen.readthedocs.io/en/latest/ which is a linux foundation project. OpenMapTiles, which OpenFreeMap uses, while open source, does not have a license that encourages derivative works or enables distributing styles under a standard FOSS software license (https://www.npmjs.com/package/protomaps-themes-base). That's also one motivation for developing Shortbread which is at this point less developed than Tilezen.
2) The file format (PMTiles) addresses a different audience than either MBTiles or Btrfs images. Both of those require administering a server for tiles, while PMTiles requires static blob storage and nothing else. You do have the option of using a server like MBTiles/btrfs which ought to be comparable in latency, and that's documented here: https://docs.protomaps.com/deploy/ as well as Lambda, Cloudflare Workers, Google Cloud Run and Azure Serverless functions.
3) There are no existing styles for MapLibre GL that work off the Tilezen layer, generalization and tagging scheme, so we need to develop one style, with multiple themes.
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.
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.
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.
> That is exactly the opposite of OMT's copyright interpretation.
They are entitled to their opinion, but I don't consider a database to be a derivative work of a schema (this should be clear, since databases of facts aren't copyrightable anyway), same for software (it's at least fair use in the US, see Google v. Oracle). This goes back to the debate on copyrightability of APIs, where a decision for copyrightability would ruin swathes of the software industry and much of free software.
Though I understand if you'd rather avoid using projects by people with weird legal opinions.
> That is the definition of license incompatibility as described in the Creative Commons documentation above.
Where do you see that? All I found was a statement that the CC share-alike licences can be converted to GPL, but no word on non-SA licences:
> Version 4.0 of CC’s Attribution-ShareAlike (BY-SA) license is one-way compatible with the GNU General Public License version 3.0 (GPLv3).
It's not uncommon for a piece of code to have multiple licences in a big project. One for licencing to that project, and another for sublicencing to the end user. Proprietary apps that include FLOSS code do this all the time.
"Additionally, our licenses are currently not compatible with the major software licenses, so it would be difficult to integrate CC-licensed work with other free software. Existing software licenses were designed specifically for use with software and offer a similar set of rights to the Creative Commons licenses." https://creativecommons.org/faq/
> Though I understand if you'd rather avoid using projects by people with weird legal opinions.
The scale of the Protomaps project is small (I'm the only full-time developer) and I don't have the resources to interpret novel copyright situations. I think it's best for the ecosystem to abide by the license terms stated by the open source developer, instead of challenging their validity.
2) The file format (PMTiles) addresses a different audience than either MBTiles or Btrfs images. Both of those require administering a server for tiles, while PMTiles requires static blob storage and nothing else. You do have the option of using a server like MBTiles/btrfs which ought to be comparable in latency, and that's documented here: https://docs.protomaps.com/deploy/ as well as Lambda, Cloudflare Workers, Google Cloud Run and Azure Serverless functions.
3) There are no existing styles for MapLibre GL that work off the Tilezen layer, generalization and tagging scheme, so we need to develop one style, with multiple themes.