Consider moving OIIO to use OpenJPH

[jfpanisset]

Now that OpenEXR is using OpenJPH, should OIIO be using it as well?

See:

AcademySoftwareFoundation/OpenImageIO#4950

[lgritz]

OIIO will automatically use it if it's in the container, and it will already have the OpenEXR dependencies as a subset, right?

[jfpanisset]

I just got OpenEXR built against a pre-installed OpenJPH (had to patch OpenEXR's CMake, I'll open an issue against that).

OpenJPH is an internal dependency for OpenEXRCore, everything is built as DSOs so in theory installing OpenEXR for its clients will install the OpenJPH DSO, but I'm not sure it will install its header files, and arguably it shouldn't since it's just a private dependency. Also only OpenEXR 3.4 builds have the OpenJPH dependency.

If the decision is to use OpenJPH for JPEG-2000 support in OIIO, I'd like to make that decision explicit in OIIO's conanfile.py

[lgritz]

We use OpenJPEG to decode jpeg2000 files generally. With the sole exception that we use OpenJPH for the j2c variant.

I'm not aware that OpenJPH can handle everything that we still use OpenJPEG for. Am I wrong?

[lgritz]

To make things really bad, my fingers really really want to type JPG when I try to type JPH, so my writing on this is not always super clear.

To the best of my knowledge, OpenJPH is for reading specifically JPEG2000 Part-15 (HTJ2K). I don't think it supports the rest of the entirety of possible jpeg-2000 files that aren't htj2k codec.

[jfpanisset]

It is terribly confusing.

Here's what I've found so far:

• When building OIIO in the context of Conan without declaring an explicit dependency on openjph, the OIIO config scripts find openjph (from the transitive dependency in OpenEXR) but don't add it to the includes, so the build fails when it tries to include openjph includes. This is likely my fault since I'm not super clear on how to add OpenJPH to the OpenEXR Conan recipe purely as an internal dependency.
• When I add the explicit dependency on openjph in the OIIO Conan recipe, it compiles and links against both OpenJPEG and openjph

#8 12.09 -- Found OpenJPEG 2.4.0
#8 12.09 -- Found openjph 0.24.5

• What I haven't tried yet is building OIIO outside of Conan, will try that next.

src/jpeg2000.imageio/CMakeLists.txt does imply that openjph is treated as an "add on" to openjpeg:

if (OPENJPEG_FOUND)
    set(_jpeg2000_includes ${OPENJPEG_INCLUDES})
    set(_jpeg2000_lib_dirs ${OPENJPEG_LIBRARY_DIRS})
    set(_jpeg2000_libs ${OPENJPEG_LIBRARIES})
    set(_jpeg2000_defs "USE_OPENJPEG")

    if (openjph_FOUND)
        list(APPEND _jpeg2000_defs "USE_OPENJPH")
    endif()

    add_oiio_plugin(jpeg2000input.cpp jpeg2000output.cpp
        INCLUDE_DIRS ${_jpeg2000_includes}
        LINK_DIRECTORIES ${_jpeg2000_lib_dirs}
        LINK_LIBRARIES ${_jpeg2000_libs}
                       $<TARGET_NAME_IF_EXISTS:openjph>
        DEFINITIONS ${_jpeg2000_defs}
    )
else()
    message (WARNING "Jpeg-2000 plugin will not be built")
endif()

and so do the #includes in jpeg2000input.cpp and jpeg2000output.cpp

This issue becomes "add openjph to OIIO" rather than "replace OpenJPEG with openjph", and since that exercises an additional dependency / additional code in OIIO, that seems to be a good thing.

[lgritz]

As far as OIIO is concerned:

• OpenJPEG only = read j2k files (common), but not j2c (rare enough that nobody complained for 10 years)
• OpenJPEG + OpenJPH = read j2k and j2c
• OpenJPH only = read neither, because all jpeg2000 related code is disabled without OpenJPEG

[jfpanisset]

Addressed in 2025.4 / 2026.2 releases, OIIO now builds and links with additional OpenJPH dependency to read .j3c files.