To keep the library as simple and minimal as possible, it does not support common animation file formats out of the box. There are too many out there and many of them would require linking 3rd party libraries, adding dependencies in the process. It is assumed that most games already have a pipeline to load animation files and perhaps even pre-process them in some way before handing the data over to the compression algorithm. As such, for our internal testing purposes and for debugging, a Simplified JSON file is used to represent an animation clip. These files can easily be compressed with gzip and be read by a human.
Small auxiliary tools exist to convert to this format such as the fbx2acl python script. The library also supports writing ACL files from a populated skeleton and clip.
It is important to note that this file format is not meant to be used in production (although it could). Its primary purpose is to help us debug and reproduce issues by attaching an ACL file to bug reports. For this purpose they can be generated with a binary exact representation where each floating point value is stored as a hexadecimal string of the underlying 64 bit value.
A reference ACL file can be found here.
Note that in order to use the ACL clip reader and writer, you will need to include the sjson-cpp headers prior to the ACL headers. This is done to decouple the dependency should you be using your own version.