Half is a class that encapsulates our 16-bit floating-point format.

IlmThread is a thread abstraction library for use with OpenEXR
and other software packages.  It currently supports pthreads and
Windows threads.

Imath implements 2D and 3D vectors, 3x3 and 4x4 matrices, quaternions
and other useful 2D and 3D math functions.

Iex is an exception-handling library.