What is a USDZ file?
A file with .usdz is a ZIP archive for the USD (Universal Scene Description) file format that contains and proxies for files of other formats embedded within the archive. USDZ files are packages whose design is based on the new Ar-level abstraction of a package. Usdz was registered with IANA and has media type name of model and a subtype name of vnd.usd+zip and its details can be found as on IANA registration page.
USDZ File Format
USDZ files are based on the ZIP file format that archive individual files in its container. This enables the receiver end to just unzip the contents and use the normal USD scene description files to work with and inspect. Almost all operating systems provide builtin support for the ZIP file formats and choosing this archiving format over any custom method makes it easy for USDZ files to be useful as a simple transport protocol.
ZIP Constraints
USDZ file format uses the ZIP file format without any compression and encryption. This was aimed by design to meet two requirements:
- For a package already loaded into memory or as a single file on disk, the API’s available in USD for accessing the data contained within the image
- There should be no need to extract files to disc or allocate more heap storage
With USDZ, both these requirements are met as most of the image formats themselves allow internal compression schemes, resulting in compact file size.
Layout Requirements
USDZ packages require the layout of files within the package is that the data for each file should begin at a multiple of 64 bytes from the beginning of the package. However, the package should start with a native USD file in case of targeting the package with a simple reference. In such a case, this first USD file is referred as the Default Layer. Clients wishing to deliver “streamable content” may wish to consider other layout constraints, as well.