Interpreting "should" around extensions definition in spec

[weegeekps]

This morning a discussion arose around what is the correct interpretation of "should" in this clause of the 3D Tiles spec:

All extensions used in a tileset or any descendant external tilesets shall be listed in the entry tileset JSON in the top-level extensionsUsed array property...

My interpretation of this clause is that the "shall" indicates that the specification is only satisfied if all criteria is true. That is that:

• An extension is being used (included) in a extensions field for the tileset JSON of the tileset or a descendant external tileset.
• The extension is specified in the extensionsUsed for the tileset.

So, in the event that only one of these components is true, then the implementation is incorrect. E.g. An extension is specified in extensionsUsed but not used anywhere in any extensions field for the tileset or its descendants.

As stated in the thread my interpretation is based on my understanding of the ISO directives that define "shall" and related terms.

I'm not entirely sure what the correct answer is here, but I think it's important for us to answer this question. If my interpretation is incorrect, I absolutely want to know. Conversely, if my interpretation is in fact correct, there's potentially a fix we need to do for Reality Analysis.

[javagl]

Although I try to be precise when it comes to language and specs, I might be limited or mislead, by not knowing how certain phrases are commonly interpreted, and the subtleties of translating the highly specific vocabulary that I was taught in logic lectures into English.

Based on my understanding, the statement

All extensions used in a tileset ... shall be listed in the ... extensionsUsed array ...

could (a bit more formally, but without changing the meaning(!)) be written as an ~"implication" (The right term might be "material conditional"...)

If an extension is used in a tileset, then it shall be listed in the extensionsUsed array...

So I'd intepret this as one-directional, from the antecedent to the consequent.

I would not think that this also means

If an extension is listed in the extensionsUsed array, then it shall be used in a tileset

Maybe there is an intuitive meaning of the word "all" that I do not properly grasp here. Maybe the word "all" can be understood to mean that "the set of used extensions" has to be exactly the same as "the set of extensions lised in 'extensionsUsed'".

If there is this potential for misunderstanding, then the suggestion would be to rephrase that into an "if-then" statement.

---

From a more practical perspective: Requiring all extensions that are listed in extensionsUsed also appear in the tileset would raise a few questions. These would mostly be related to the part about external tilesets (which I conveniently omitted in the considerations above 😬 ). I think that the main point of the extensionsUsed array is to make clear which extensions the client might have to expect in the tileset or one of its external tilesets. That's particularly important for the extensionsRequired case, where the client should have an option to bail out early and avoid crashes, and instead, can just say log("Nope, I don't support that").