How to convert X3D
to FBX?

X3D is ISO standard XML-based file format for representing 3D computer graphics. X3D features extensions to VRML (e.g. CAD, geospatial, humanoid animation, NURBS etc.), the ability to encode the scene using an XML syntax as well as the Open Inventor-like syntax of VRML97, or binary formatting, and enhanced application programming interfaces (APIs).

The X3D extension supports multi-stage and multi-texture rendering; it also supports shading with lightmap and normalmap. Starting in 2010, X3D has supported deferred rendering architecture. Now X3D can import SSAO, CSM and Realtime Environment Reflection/Lighting. The user can also use optimizations including BSP/QuadTree/OctTree or culling in the X3D scene.

FBX (Filmbox) is a proprietary file format (.fbx extension) developed by Kaydara and owned by Autodesk since 2006. It is used to provide interoperability between digital content creation applications.

The format saw wider support from other 3D software packages such as Cinema 4D, SoftImage 3D, PowerAnimator, LightWave 3D and 3D Studio MAX. Today, FBX is also widely adopted among VR applications, which are typically built on top of major game engines.