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Binary JData Format Specification Development Guide

We use this repository to gather feedback from the community regarding the "Binary JData Format Specification", or Binary JData (BJData) format. Such feedback is crucial to finalize this file specification and help improve it in the future once disseminated.

The latest version of the BJData specification can be found in the file named The specification is written in the Markdown format for convenient editing and version control.

This specification was derived from the Universal Binary JSON (UBJSON) Specification Draft 12 developed by Riyad Kalla and other UBJSON contributors. The MarkDown version of this specification was derived from the documentation included in the Py-ubjson repository (Commit 5ce1fe7). The BJData format is no longer backward compatible with UBJSON.

Libraries that support this specification include

Acknowledgement: This specification was developed as part of the NeuroJSON project (, with funding support from the US National Institute of Health (NIH) under grant U24-NS124027 (PI: Qianqian Fang).

What is BJData

BJData is a binary JSON format. It is similar to JSON but allows to store strongly-typed binary data. The BJData format improves upon the widely supported UBJSON ( format by adding the below key features

  • added 4 new data types previously missing from UBJSON, including [u] - uint16, [m] - uint32, [M] - uint64 and [h] - half/float16,
  • first among all binary JSON formats to support packed N-dimensional arrays (ndarray) - a data type that is of great importance to the scientific community,
  • addopted Little-Endian as the default byte-order, as opposed to Big-Endian for UBJSON/MessagePack/CBOR,
  • only non-zero-fixed-length data types are allowed in optimized container types ($), which means [{SHTFTN can not follow $, but UiuImLMLhdDC are allowed

How to participate

You can use a number of methods to provide your feedback to the working draft of this file specification, including

  • Create an "Issue"
    • This is the most recommended method to provide detailed feedback or discussion. An "Issue" in github is highly versatile. One can ask a question, report a bug, provide a feature request, or simply propose general discussions. Please use URLs or keywords to link your discussion to a specific line/section/topic in the document.
  • Write short comments on Request for Comments (RFC) commits
    • A milestone version of the specification will be associated with an RFC (Request for comments) commit (where the entire file is removed and re-added so that every line appears in such commit). One can write short comments as well as post replies on this RFC page.
    • The latest stable release is Version 1 Draft 2. Please use this link to comment.
    • To add a comment, you need to first register a github account, and then browse the above RFC page. When hovering your cursor over each line, a "plus" icon is displayed, clicking it will allow one to comment on a specific line (or reply to other's comments).
    • The RFC page can get busy if too many comments appear. Please consider using the Issues section if this happens.
    • One can browse the commit history of the specification document. If anyone is interested in commenting on a particular updated, you can also comment on any of the commit page using the same method.
  • Use NeuroJSON mailing list
    • You may send your comments to the neurojson mailing list (neurojson at Subscribers will discuss by emails, and if a motion is reached, proposals will be resubmitted as an Issue, and changes to the specification will be associated with this issue page.

For anyone who wants to contribute to the writing or revision of this document, please follow the below steps

  • Fork this repository and make updates, then create a pull-request
    • Please first register an account on github, then, browse the BJData Spec repository; on the top-right of this page, find and click the "Fork" button.
    • once you fork the JData project to your own repository, you may edit the files in your browser directly, or download to your local folder, and edit the files using a text editor;
    • once your revision is complete, please "commit" and "push" it to your forked git repository. Then you should create a pull-request (PR) against the upstream repository (i.e., NeuroJSON/bjdata). Please select "Compare cross forks" and select "NeuroJSON/bjdata" as "base fork". Please write a descriptive title for your pull-request. The project maintainer will review your updates and choose to merge to the upstream files or request revision from you.


Binary JData Specification - a portable interchange format for complex binary data






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