gltf extension handler release notes

release-content/release-notes/gltf_extension_handling.md

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+---
+title: "Userspace glTF Extension Handling"
+authors: ["@christopherbiscardi"]
+pull_requests: [22106]
+---
+
+glTF has two mechanisms for extending glTF files with additional user data: Extras and Extensions.
+
+**Extras** are meant to be arbitrary application-specific data, often authored by users directly in tools like Blender's custom properties.
+Extras are historically well supported by Bevy; If you add a custom property in Blender that data will end up in one of the `GltfExtras` components on the relevant `Entity`.
+
+**Extensions** are meant for data that can be shared across applications.
+They are more flexible, allowing for new data in more places inside a glTF file, and more powerful as a result.
+For example, there are a few object types in a glTF file including meshes, nodes, scenes, and more.
+Extensions can add new object types, such as `lights` from the `KHR_lights_punctual` extension, as well as arbitrary buffers, data that is at the root of the glTF file, and more.
+
+More examples of extensions can be found in the [KhronosGroup git repo](https://github.com/KhronosGroup/glTF/blob/7bbd90978cad06389eee3a36882c5ef2f2039faf/extensions/README.md)
+
+## GltfExtensionHandler
+
+Having extension data in a glTF file isn't enough; There also needs to be an application _producing_ the data as well as _consuming_ the data.
+Prior to 0.18, extension handling for extensions like `KHR_lights_punctual` was hardcoded into the glTF loader.
+In 0.18, users implement the `GltfExtensionHandler` trait to do stateful processing of glTF data.
+Hooks are called during the loading process, from the glTF loader, which allows for modifying Scene entities, creating additional new assets, and more.
+
+[Skein](https://github.com/rust-adventure/skein), which is most well known for its Blender/Bevy integration addon, defines a glTF extension that allows adding Bevy Components to glTF objects.
+These components are then inserted on entities in a scene at the same time built-in components like `Transform` and `Mesh3d` are.
+
+Using glTF Extensions for this data means that other level editors like Trenchbroom can also write the same format.
+Any third party software that writes component data into a glTF file can use Skein's `GltfExtensionHandler`, resulting in components being "ready-to-go" when spawning `Scene`s.
+
+Processing glTF Extension data is only half the story because to process Extension data you also have to be able to process the non-extension data.
+Two new examples show off use cases:
+
+- The first builds an `AnimationGraph` and inserts it onto the animation root in a Scene, which means it is now accessible to play animations using the `AnimationPlayer` on the same `Entity` later when that Scene is spawned.
+- The second uses a `GltfExtensionHandler` to switch the 3d Mesh and Material components for their 2d counterparts. This is useful if you're using software like Blender to build 2d worlds.
+
+```shell
+cargo run --example gltf_extension_animation_graph
+cargo run --example gltf_extension_mesh_2d
+```