The OpenEXR API#

There are two separate, independent APIs for reading and writing EXR image files: the traditional C++ API, and the newer C API, referred to as OpenEXRCore. The C++ API is the original, widely-used interface first released in 2003. It consists of the OpenEXR, Iex, and IlmThread libraries, with functions and classes in the Imf:: namespace (or “image format”, the original the name of the library at ILM prior to its public release).

The OpenEXRCore library, written primarily by Kimball Thurston at Weta Digital, was introduced in July, 2021. This C-language implementation of the file format is the result of a significant re-thinking of image file I/O and access to image data. The OpenEXRCore library provides thread-safe, non-blocking access to files, which was not possible with the older C++ API, where the framebuffer management is separate from read requests. This new low-level API allows applications to do custom unpacking of EXR data, such as on the GPU, while still benefiting from efficient I/O, file validation, and other semantics. It provides efficient direct access to EXR files in texturing applications. This C library also introduces an easier path to implementing OpenEXR bindings in other languages, such as Rust.

Currently, the two libraries and APIs sit alongside each other, although in future OpenEXR releases, the C++ API will migrate to use the new core in stages. It is not the intention to entirely deprecate the C++ API, nor must all applications re-implement EXR I/O in terms of the C library. The C API does not, and will not, provide the rich set of utility classes that exist in the C++ layer. The OpenEXRCore library simply offers new functionality for specialty applications seeking the highest possible performance.

API