|Internet media type||image/x-exr|
|Developed by||Industrial Light & Magic|
12 April 2022
|Type of format||High-dynamic-range imaging|
|Open format?||Yes, Modified BSD License|
OpenEXR is a high-dynamic range, multi-channel raster file format, released as an open standard along with a set of software tools created by Industrial Light & Magic (ILM), under a free software license similar to the BSD license.
It is notable for supporting multiple channels of potentially different pixel sizes, including 32-bit unsigned integer, 32-bit and 16-bit floating point values, as well as various compression techniques which include lossless and lossy compression algorithms. It also has arbitrary channels and encodes multiple points of view such as left- and right-camera images.
A full technical introduction of the format is available on the OpenEXR website.
OpenEXR, or EXR for short, is a deep raster format developed by ILM and broadly used in the computer-graphics industry, both visual effects and animation.
OpenEXR's multi-resolution and arbitrary channel format makes it appealing for compositing, as it alleviates several painful elements of the process. Since it can store arbitrary channels—specular, diffuse, alpha, RGB, normals, and various other types—in one file, it takes away the need to store this information in separate files. The multi-channel concept also reduces the necessity to "bake" in the aforementioned data to the final image. If a compositer is not happy with the current level of specularity, they can adjust that specific channel.
OpenEXR's API makes tools development a relative ease for developers. Since there are almost never two identical production pipelines, custom tools always need to be developed to address problems (e.g. image-manipulation issue). OpenEXR's library allows quick and easy access to the image's attributes such as tiles and channels.
The OpenEXR library is developed in C++ and is available in source format as well as compiled format for Microsoft Windows, macOS and Linux. Python bindings for the library are also available for version 2.x.
OpenEXR was created by ILM in 1999 and released to the public in 2003 along with an open source software library. It soon received wide adoption by software used in computer graphics, particularly for film and television production. The format has been updated several times, adding support for tiles, mipmaps, new compression methods, and other features. In 2007, OpenEXR was honored with an Academy Award for Technical Achievement.
OpenEXR 2.0 was released in April 2013, extending the format with support for deep image buffers and multiple images embedded in a single file. Version 2.2, released August 2014, added the lossy DWA compression format.
The OpenEXR software distribution includes:
OpenEXR has support for color depth using:
There are three general types of lossless compression built into OpenEXR, with two different methods of Zip compressing. For most images without a lot of grain, the two Zip compression methods seem to work best, while the PIZ compression algorithm is better suited to grainy images. The following options are available:
From OpenEXR.org's Technical Introduction:
The ILM OpenEXR file format was designed and implemented 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.