ResourceVersion class final

A resource as it stands between one write of it and the next.

The graph names logical resources — "the lit scene" — but a pass that modifies one almost always writes its result into a different texture, because a texture cannot be sampled and written in the same pass. Reflections read the lit scene and produce a second one; every ping-pong effect does the same. Versioning is how the name keeps meaning "the lit scene" while the texture behind it changes: each write produces a new version, each read is bound to whichever version was current where the reader was registered, and an edge runs from the producer of a version to its consumers rather than from every writer of a name to every reader of it.

Zero is what the engine handed in: an external resource, or nothing at all. The first write produces one.