About OpenEXR

OpenEXR is a project of the Academy Software Foundation. The format and library were originally developed at Industrial Light & Magic and first released as open source in 2003. Weta Digital, Walt Disney Animation Studios, Sony Pictures Imageworks, Pixar Animation Studios, DreamWorks, and other studios, companies, and individuals have made contributions to the code base.

Read the origin story of OpenEXR on the ASWF Blog.

OpenEXR is included in the VFX Reference Platform.

OpenEXR Features

  • High dynamic range and color precision.

  • Support for 16-bit floating-point, 32-bit floating-point, and 32-bit integer pixels.

  • Multiple image compression algorithms, both lossless and lossy. Some of the included codecs can achieve 2:1 lossless compression ratios on images with film grain. The lossy codecs have been tuned for visual quality and decoding performance.

  • Extensibility. New compression codecs and image types can easily be added by extending the C++ classes included in the OpenEXR software distribution. New image attributes (strings, vectors, integers, etc.) can be added to OpenEXR image headers without affecting backward compatibility with existing OpenEXR applications.

  • Support for stereoscopic image workflows and a generalization to multi-views.

  • Flexible support for deep data: pixels can store a variable-length list of samples and, thus, it is possible to store multiple values at different depths for each pixel. Hard surfaces and volumetric data representations are accommodated.

  • Multipart: ability to encode separate, but related, images in one file. This allows for access to individual parts without the need to read other parts in the file.

  • Versioning: OpenEXR source allows for user configurable C++ namespaces to provide protection when using multiple versions of the library in the same process space.

OpenEXR and Imath Version 3

With the release of OpenEXR 3, the Imath library formerly distributed via the IlmBase component of OpenEXR is now an independent library dependency, available for download from https://github.com/AcademySoftwareFoundation/Imath. You can choose to build OpenEXR against an external installation of Imath, or the default CMake configuration will download and build it automatically during the OpenEXR build process. Note that the half 16-bit floating point data type is included in Imath.

See OpenEXR/Imath 2.x to 3.x Porting Guide for details about differences from previous releases and how to address them. Also refer to the porting guide for details about changes to Imath.

New Features in OpenEXR v3.1

The 3.1 release of OpenEXR introduces a new library, OpenEXRCore, which is the result of a significant re-thinking of how OpenEXR manages file I/O and provides access to image data. It begins to address long-standing scalability issues with multithreaded image reading and writing.

The OpenEXRCore library provides thread-safe, non-blocking access to files, which was not possible with the current API, where the framebuffer management is separate from read requests. It is written entirely in C and provides a new C-language API alongside the existing C++ API. 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.

The 3.1 release represents a technology preview for upcoming releases. The initial release is incremental; the existing API and underlying behavior has not changed. The new API is available now for performance validation testing, and then 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 3.1 release of the OpenEXRCore library simply offers new functionality for specialty applications seeking the highest possible performance. In the future, the ABI will evolve, but the API will remain consistent, or only have additions.

See Reading and Writing Image Files with the OpenEXR Library for more information.


The ILM OpenEXR file format was originally designed and implemented at Industrial Light & Magic by Florian Kainz, Wojciech Jarosz, and Rod Bogart. The PIZ compression scheme is based on an algorithm by Christian Rouet. Josh Pines helped extend the PIZ algorithm for 16-bit and found optimizations for the float-to-half conversions. Drew Hess packaged and adapted ILM’s internal source code for public release and maintains the OpenEXR software distribution. The PXR24 compression method is based on an algorithm written by Loren Carpenter at Pixar Animation Studios.

For a complete list of contributors see the CONTRIBUTORS.md file.