Back in the September release (CesiumJS 1.97), we replaced our implementation of glTF models with a new design that makes it easier to extend. Already it’s made it possible to implement new functionality such as 3D Tiles Next metadata, and custom GLSL shaders for models and tilesets.
Since this was a large change that mostly affects CesiumJS’s private API, we wrote a technical blog post to explain it in more depth. The post describes our design goals, gives a deep dive into the architecture, and highlights how it simplifies part of our 3D Tiles implementation. You can find the blog post here: