Clarification for content availability of implicit tileset roots about 3d-tiles

[lilleyse]

I think this is referring to a tile without a content property, e.g.

{
  "root": {
    "boundingVolume": {
      "region": [-1.318, 0.697, -1.319, 0.698, 0, 20]
    },
    "refine": "REPLACE",
    "geometricError": 5000,
    "implicitTiling": {
      "subdivisionScheme": "QUADTREE",
      "availableLevels": 21,
      "subtreeLevels": 7,
      "subtrees": {
        "uri": "subtrees/{level}/{x}/{y}.json"
      }
    }
  }
}

contentAvailability would be omitted since this implicit tileset contains all empty tiles.

I think the line should be reworded to

If the implicit root tile does not have a content property then contentAvailability shall be omitted.

[javagl]

Now this makes sense, on the level of the specification. It may raise questions about use-cases for an implicit tileset that does not have content at all. But one could make a case for such a tileset serving only as a purely spatial datastructure that stores metadata, for example.

To avoid similar confusion, one could probably consider to elaborate that further, with something like

If the implicit root tile does not have a content or contents property, and therefore does not define a content template URI, then this means that none of the implicit tiles has any content, and the contentAvailability in the subtrees shall be omitted.

[lilleyse]

@javagl that sounds good. Could you open a PR with that wording?

[javagl]

Yes, there are a few "smaller" things that I can try to address in a few small PRs during the weekend, like

• this one
• #766
• Maybe #756