Remove CESIUM_z_up extension and add a detailed implementation note

[lilleyse]

Specifically check out Coordinate Reference System and contained glTF section.

Thoughts @ggetz @pjcozzi @emackey?

[ggetz]

Overall, I prefer the extension to having to specify these implementation notes, as one of the most common use cases, geospatial data which in inherently z-up, has to go through several additional steps. But I'm willing to here other opinions.

[ggetz]

I think it's important to state geographic coordinate system (latitude, longitude, height) because everything else is in a right handed (x,y,z) coordinate system.

[ggetz]

This formatting makes the matrix hard to see at a glance, and is inconsistant with how we format them elsewhere in the spec.

Both here and throughout the remaining changes.

[lilleyse]

Yeah I think I prefer a row-major, line separated matrix too.

The reason I changed it here was to be in the same form as the matrix in the gltf snippet so it was obvious how the two compare. But I'll change it back.

[ggetz]

Specify the language for syntax highlighting like json

[emackey]

I think letting go of the extension is the right move for both the glTF and the 3D Tiles ecosystems.

Such an extension can't be vendor-specific to Cesium if we expect other vendors to implement 3D Tiles as an open standard and interoperate with Cesium and ion-generated tiles. For example, desktop applications that want to render ion-served tiles will need a desktop implementation of the single-vendor extension.

The idea was poorly received by several participants in the main glTF repo, as it runs counter to the spirit there: glTF asks everyone to agree to use the same conventions (axes selection, texture channel selection, etc) specifically to avoid the pain felt in older formats that allowed exporters to write whatever was convenient for the exporter, introducing too much variance into the resulting library of assets for any one reader to fully cope with.

Also, the extension was marked optional. Sorry, my mistake, it's marked required. But by adding a fallback, it could be converted to optional. Authors of optional extensions are asked to include fallbacks for core readers to do the best they can do with the model. The fallback here is incredibly trivial: Just add the above matrix to the root node, at which point the whole extension becomes moot.

I think letting go of this extension is well worth the effort, particularly to maintain the message that 3D Tiles uses core glTF without any vendor extensions, and the ecosystem is stable.

[lilleyse]

@ggetz updated based on your comments.

[ggetz]

OK I'm good with this solution. Should we go ahead and close the z-up extension?

@lilleyse I made some tweaks to formatting and wording just for readability, feel free to switch it back don't like any of those changes.

[lilleyse]

The formatting tweaks are good.

Yeah let's close the z-up extension.

[ggetz]

OK, thanks @lilleyse !