The specs for the implicit tiling part of 3D Tiles 1.1 say that in some cases the contentAvailability shall be omitted.
Specifically it says
If the implicit root tile does not have content then contentAvailability shall be omitted.
(See here: 3d-tiles/specification/ImplicitTiling at main · CesiumGS/3d-tiles · GitHub)
Can some explain to me what this means?
The way I understood the concept is, that the contentAvailability tells the reader of the subtree if there is content available somewhere in the subtree so obviously it doesn’t make sense to omit the contentAvailability bitstream when the root tile has no content but some child has?
I guess I am reading the specs wrong? 
the contentAvailability tells the reader of the subtree if there is content available somewhere in the subtree
The contentAvailability is supposed to tell the reader of the subtree if there is content available in a certain tile, so I think that you are right: It does not make sense to omit this only because of no content being available in the root tile.
One could, in theory, require it to not be present when the whole subtree does not contain content. But even that would be overly constraining. So I think that this requirement should not exist at all (or at the very least, it should not refer to ‘the root tile’).
I opened Clarification for content availability of implicit tileset roots · Issue #769 · CesiumGS/3d-tiles · GitHub to keep track of this.
Oh okay. I was just too harsh with my own reading comprehension then! 
Thanks for opening the issue!
You probably saw the clarification that Sean posted in the issue: Clarification for content availability of implicit tileset roots · Issue #769 · CesiumGS/3d-tiles · GitHub
So omitting the contentAvailability does make sense when the implicit root does not define a content (or contents), meaning that in fact the whole implicit tileset does not have content. The specification text should be clarified accordingly.
1 Like