OpenEXR/Imath 2.x to 3.x Porting Guide

This porting guide outlines the several areas where switching from OpenEXR 2.x to OpenEXR 3.x + Imath 3.x will require source code or build changes of downstream software.

In each case, we will often explain both how to change if you are expecting 3.x only hereafter, or usually a more complex accommodation if you want to keep compatibility with both 2.x and 3.x.

OpenEXR and Imath Are Different Packages

If your use of OpenEXR was only for the sake of using the math classes and utilities, maybe you were unhappy that you needed to download and build the full OpenEXR dependency. You are in luck – now Imath is a separate, very lightweight open source package. You can use Imath functionality without needing any of OpenEXR, which as of 3.x only includes the parts you need to read and write OpenEXR image files.

The parts of “IlmBase” that were Imath and half are now repackaged as the Imath library. The IlmThread and Iex libraries have been folded into the OpenEXR package, since they were were not necessary to the rest of Imath.

When building OpenEXR 3.x, note that if Imath 3.x library is not found already installed at build time, it will be automatically downloaded and built as part of the OpenEXR build.


Why is this happening? Here is the relevant history.

The OpenEXR project has historically consisted of four separate subprojects:

  • OpenEXR - the Imf image format

  • IlmBase - supporting utilities (Imath, Half, Iex, IlmThread)

  • PyIlmBase - python bindings for the IlmBase libraries

  • OpenEXR_Viewers - code for an example EXR image viewer

Prior to the 2.4 release in 2019, OpenEXR relied primarily on the Gnu autotools build system and was released as four separate tarballs (ilmbase, pyilmbase, openexr, openexr_viewers) that were constructed via the Gnu tools. This gave direct access to the “IlmBase” libraries independent of the OpenEXR format library. The project also included CMake files but CMake support was incomplete.

With the adoption of OpenEXR by the Academy Software Foundation in 2019, the technical steering committee made several key changes:

  1. Drop support for autotools in favor of CMake. A significant portion of the OpenEXR user base uses Windows, which the Gnu autotools does not support. Supporting two build systems is a maintenance burden that the TSC opted to avoid. We now assume that all modern users of OpenEXR can reasonably be expected to rely on CMake.

  2. Rely on GitHub’s automatic release packaging mechanism. This packages the entire contents of package in a single tarball. Separate tarballs are no longer generated by the Gnu autotools setup.

  3. Deprecate the OpenEXR_Viewers code. It was impossibly out of date and of little modern value.

Thus, with the 2.4 release, the “IlmBase” libraries are no longer distributed in a form that is readily separable from the rest of OpenEXR. The build and installation process for the overall OpenEXR project is complicated by the fact it consists of four separate projects, which added significant complexity to the CMake setup.

Because Imath is generally useful to the community, the TSC decided to simplify the configuration by separating Imath into its own independent project, maintained and released independently of OpenEXR, and introducing it as a new external dependency of OpenEXR.

To further simplify matters, the new Imath library includes the half data type directly, rather than maintaining it in a separate library. Also, the community at large has a strong desire for simple vector/matrix utilities that are unencumbered by Iex, the IlmBase library that provides higher-level exception classes, and even further, a clear delineation between functionality that (1) relies on exception handlings and (2) is free from exceptions. As a result, support for Iex has been removed from Imath, and the Iex library is now packaged as a component of OpenEXR.

The Imath python bindings are a part of Imath as a configuration option, although support is off by default to simplify the build process for most users.

New Library Names and Repository Structures

The new repositories place all source code under the src top-level subdirectory.


├── Imath
├── ImathTest
└── python
    ├── config
    ├── PyImath
    ├── PyImathNumpy
    ├── PyImathTest
    ├── PyImathNumpyTest
    └── PyImathSpeedTest


The IlmImf library has been renamed OpenEXR. No header files have changed names, only their locations in the repo have changes.

├── bin
│   ├── exr2aces
│   ├── exrbuild
│   ├── exrcheck
│   ├── exrenvmap
│   ├── exrheader
│   ├── exrmakepreview
│   ├── exrmaketiled
│   ├── exrmultipart
│   ├── exrmultiview
│   └── exrstdattr
├── lib
│   ├── Iex
│   ├── IexMath
│   ├── IlmThread
│   ├── OpenEXR
│   └── OpenEXRUtil
├── examples
└── test
    ├── IexTest
    ├── OpenEXRTest
    ├── OpenEXRUtilTest
    └── OpenEXRFuzzTest

Finding and Using OpenEXR and Imath CMake Configs

OpenEXR/Imath 3.x Only

If you are only concerned with OpenEXR/Imath 3.x going forward, this is the recommended way to find the libraries in a downstream project that uses the CMake build system:

find_package(Imath CONFIG)
find_package(OpenEXR CONFIG)

Note that the second line may be omitted if you only need the Imath portions.

And then your project can reference the imported targets like this:

target_link_libraries (my_target

You only need the parts you use, so for example, if you only need Half and Imath, you can omit the OpenEXR target. Also note that in our example above, we have used the PRIVATE label, but you should specify them as PUBLIC if you are exposing those classes in your own package’s public interface.

Accommodating OpenEXR/Imath 3.x or OpenEXR 2.x

On the other hand, to accommodate both 2.x and 3.x, it’s admittedly inconvenient because the packages and the import targets have changed their names. We have found the following idioms to work:

Finding either/both packages:

# First, try to find just the right config files
find_package(Imath CONFIG)
if (NOT TARGET Imath::Imath)
    # Couldn't find Imath::Imath, maybe it's older and has IlmBase?
    find_package(IlmBase CONFIG)
endif ()
find_package(OpenEXR CONFIG)

To link against them, we use CMake generator expressions so that we can reference both sets of targets, but it will only use the ones corresponding to the package version that was found.

target_link_libraries (my_target
        # For OpenEXR/Imath 3.x:
        # For OpenEXR 2.4/2.5:

Again, you can eliminate the references to any of the individual libaries that you don’t actually need for your application.

Simultaneous Static/Shared Build

The OpenEXR 2.x CMake configuration had options to simultaneously build both shared and statically linked libraries. This has been deprecated. A CMake configuration setting specifies whether to build static or shared, but if you want both, you will need to run cmake and build twice.

Simultaneous Python 2/3 Build

The PyIlmBase 2.x CMake configuration had options to simultaneously build both python2 and python3 bindings. This has been deprecated. A CMake configuration setting specifies whether to build for python 2 or python 3, but if you want both, you will need to run cmake and build twice.

Imath Include Files Are in a Different Subdirectory

Imath 3.0 will copy its headers to some include/Imath subdirectory instead of the old include/OpenEXR.

OpenEXR/Imath 3.x Only

If you know that you are only using Imath 3.x, then just change any include directions, like this:

#include <OpenEXR/ImathVec.h>
#include <OpenEXR/half.h>

to the new locations:

#include <Imath/ImathVec.h>
#include <Imath/half.h>

Accommodating OpenEXR/Imath 3.x or OpenEXR 2.x

If you want your software to be able to build against either OpenEXR 2.x or 3.x (depending on which dependency is available at build time), we recommend using a more complicated idiom:

// The version can reliably be found in this header file from OpenEXR,
// for both 2.x and 3.x:
#include <OpenEXR/OpenEXRConfig.h>
                                  (100*OPENEXR_VERSION_MINOR) + \

// There's just no easy way to have an ``#include`` that works in both
// cases, so we use the version to switch which set of include files we
// use.
#if COMBINED_OPENEXR_VERSION >= 20599 /* 2.5.99: pre-3.0 */
#   include <Imath/ImathVec.h>
#   include <Imath/half.h>
    // OpenEXR 2.x, use the old locations
#   include <OpenEXR/ImathVec.h>
#   include <OpenEXR/half.h>

Include Files Include Fewer Other Headers

Extraneous #include statements have been removed from some header files, which can lead to compile failures in application code that previously included certain headers indirectly.

For example, the Imath header files no longer include float.h, so application code that references symbols such as FLT_MAX may need to add an explicit #include <float.h> or equivalent.

If your application code reports compile errors due to undefined or incompletely-defined Imath or OpenEXR data types, locate the Imath or OpenEXR header file that defines the type and include it explicitly.

Symbols Are Hidden by Default

To reduce library size and make linkage behavior similar across platforms, Imath and OpenEXR now build with directives that make symbol visibility hidden by default, with specific externally-visible symbols explicitly marked for export. See the Symbol Visibility in Imath/OpenEXR and the appropriate *Export.h header file for more details.

Imath Now Uses Standard C++ Exceptions and noexcept

In OpenEXR 2.x, the Imath functions that threw exceptions used to throw various Iex varieties.

In Imath 3.x, these functions just throw std::exception varieties that correspond to the failure (e.g., std::invalid_argument, std::domain_error, etc.). For that reason, all of the Iex exceptions are now only part of the OpenEXR library (where they are still used in the same manner they were for OpenEXR 2.x).

Imath 3.x has very few functions that throw exceptions. Each is clearly marked as such, and each has a version that does not throw exceptions (so that it may be used from code where exceptions are avoided). The functions that do not throw exceptions are now marked noexcept.

Some Headers and Classes Have Been Removed from Imath 3.x

  • The Math<T> class (and ImathMath.h header file) are deprecated. All of the Math<T> functionality is subsumed by C++11 std:: math functions. For example, calls to Imath::Math<T>::abs(x) should be replaced with std::abs(x).

  • The Limits<T> class (and the ImathLimits.h and ImathHalfLimits.h headers) have been removed entirely. All uses of Limits<> should be replaced with the appropriate std::numeric_limits<> method call. The Imath-specific versions predated C++11, and were not only redundant in a C++11 world, but also potentially confusing because some of their functions behaved quite differently than the std::numeric_limits method with the same name. We are following the precept that if C++11 does something in a standard way, we should not define our own equivalent function (and especially not define it in a way that doesn’t match the standard behavior).

  • Vec<T>::normalize() and length() methods, for integer T types, have been removed. Also the standalone project() and orthogonal() functions are no longer defined for vectors made of integer elements. These all had behavior that was hard to understand and probably useless. They still work as expected for vectors of floating-point types.

  • The Int64 and SInt64 types are deprecated in favor of the now-standard int64_t and uint64_t.

File/Class-specific Changes

half in half.h

  • The half type is now in the Imath namespace, but a compile-time option puts it in the global namespace, except when compiling for CUDA, in which case the ‘half’ type refers to the CUDA type:

    #ifndef __CUDACC__
    using half = IMATH_INTERNAL_NAMESPACE::half;
    #include <cuda_fp16.h>

If you desire to use Imath::half inside a CUDA kernal, you can refer
to it via the namespace, or define ``CUDA_NO_HALF`` to avoid the CUDA
type altogether.
  • HALF_MIN has changed value. It is now the smallest normalized

    positive value, returned by std::numeric_limits<half>::min().

  • New constructor from a bit pattern:

enum FromBitsTag

constexpr half(FromBitsTag, unsigned short bits) noexcept;

Imath::Box<T> in ImathBox.h

  • baseTypeMin() is replaced with baseTypeLowest()

Color3<T>, Color4<T> in ImathColor.h

  • baseTypeMin() is replaced with baseTypeLowest()

Imath::Frustum<T> in ImathFrustum.h

Akin to the Vec classes, there are now seperate API calls for throwing and non-throwing functions:

These functions previously threw exceptions but now do not throw and are marked noexcept:

  • Frustum<T>::projectionMatrix() noexcept

  • Frustum<T>::aspect() noexcept

  • Frustum<T>::set() noexcept

  • Frustum<T>::projectPointToScreen() noexcept

  • Frustum<T>::ZToDepth() noexcept

  • Frustum<T>::DepthToZ() noexcept

  • Frustum<T>::screenRadius() noexcept

  • Frustum<T>::localToScreen() noexcept

These functions throw std::domain_error exceptions when the associated frustum is degenerate:

  • Frustum<T>::projectionMatrixExc()

  • Frustum<T>::aspectExc()

  • Frustum<T>::setExc()

  • Frustum<T>::projectPointToScreenExc()

  • Frustum<T>::ZToDepthExc()

  • Frustum<T>::DepthToZExc()

  • Frustum<T>::screenRadiusExc()

  • Frustum<T>::localToScreenExc()

Imath::Interval<T> in ImathInterval.h

New methods/functions:

  • Interval<T>::operator !=

  • Interval<T>::makeInfinite()

  • Interval<T>isInfinite()

  • operator<< (std::ostream& s, const Interval<T>&)


  • checkForZeroScaleInRow() and extractAndRemoveScalingAndShear()

    throw std::domain_error exceptions instead of Iex::ZeroScale

Matrix22<T>, Matrix33<T>, Matrix44<T> in ImathMatrix.h

  • baseTypeMin() is replaced with baseTypeLowest()

  • invert(bool singExc = false) is replace by:

    • invert() noexcept

    • invert(bool) which optionally throws an std::invalid_argument exception.

  • inverse(bool singExc = false) is replace by:

    • inverse() noexcept

    • inverse(bool) which optionally throws an std::invalid_argument exception.

  • gjInvert(bool singExc = false) is replace by:

    • gjInvert() noexcept

    • gjInvert(bool) which optionally throws an std::invalid_argument exception.

  • gJinverse(bool singExc = false) is replace by:

    • gjInverse() noexcept

    • gjInverse(bool) which optionally throws an std::invalid_argument exception.

New functions:

  • operator<< (std::ostream& s, const Matrix22<T>&)

  • operator<< (std::ostream& s, const Matrix33<T>&)

  • operator<< (std::ostream& s, const Matrix44<T>&)

Other changes:

  • Initialization loops unrolled for efficiency

  • inline added where appropriate


  • When compiling for CUDA, the complex type comes from thrust rather than std

Shear6 in ImathShear.h

  • baseTypeMin() is replaced with baseTypeLowest()


The following functions are no longer defined for integer-based vectors, because such behavior is not clearly defined:

  • project (const Vec& s, const Vec& t)

  • orgthogonal (const Vec& s, const Vec& t)

  • reflect (const Vec& s, const Vec& t)

Vec2<T>, Vec3<T>, Vec4<T> in ImathVec.h

  • baseTypeMin() is replaced with baseTypeLowest()

  • The following methods are removed (via = delete) for integer-based vectors because the behavior is not clearly defined and thus prone to confusion:

    • length() - although the length is indeed defined, its proper value is floating point and can thus not be represented by the ‘T’ return type.

    • normalize()

    • normalizeExc()

    • normalizeNonNull()

    • normalized()

    • normalizedExc()

    • normalizedNonNull()

  • Interoperability Constructors: The Vec and Matrix classes now have constructors that take as an argument any data object of similar size and layout.

Imath Python Changes

In general, the changes in Imath at the C++ level are reflected in the python bindings. In particular:

  • The following methods are removed for integer-based vector and matrix objects and arrays:

    • length()

    • normalize()

    • normalizeExc()

    • normalizeNonNull()

    • normalized()

    • normalizedExc()

    • normalizedNonNull()

  • baseTypeMin() is replaced with baseTypeLowest() for:

    • Vec2, Vec3, Vec4

    • Color3, Color4

    • Matrix22, Matrix33, Matrix44

    • Box

    • Shear6