Ultra Engine plans to use glTF for their main 3D model file format, so that final game models can be easily loaded back into a modeling program for editing whenever needed. glTF supports a lot of useful features and is widely supported, but there are a few missing pieces of information Ultra Engine need to add into it. Fortunately, this JSON-based file format has a mechanism for extensions that adds new features and data to the format. In this blog, the custom extensions added for Ultra Engine are explained.