What is difference between b3dm and gltf ?
When to use b3dm and when to prefer gltf over b3dm ?
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Update, 2021-11-10: b3dm is being retired. 3D Tiles is upgrading to a new version dubbed "3D Tiles Next," and the b3dm wrapper is being replaced with raw glTF 2.0 (GLB) files plus an extension, according to the blog.
A single glTF file delivers a (relatively) small amount of localized 3D geometry, along with materials, textures, and other metadata needed to render it on a client. The 3D Tiles standard provides a mechanism to specify a hierarchy of geo-located glTFs at different resolutions. For example, a vehicle may be easily portrayed in a single glTF file, but a whole city with all of its buildings and trees would need to be broken up into 3D Tiles, using multiple glTF files internally.
(Disclaimer: I work for the company that was originally responsible for b3dm and 3D Tiles, but I'm not on that particular team myself and I don't speak for them).