Transforming HDR images to ACEScg for Solaris

I've been experimenting with Academy Colour Encoding System (ACES) colour management, and recently hit a small workflow issue with Houdini. Out of the box, without using any specific colour solution, the Solaris dome light node will expect a HDR sky dome image in a linear sRGB/Rec.709 colour space. As soon as ACES is enabled, the dome light node assumes that same image to be in linear ACEScg colour space.

Frustratingly, Houdini offers no user interface to transform the dome texture into another colour space. Most 3D tools offer a means to switch input space transforms. I have no clue if this is something that has been overlooked by developers, or if it is tied to Solaris' underlying reliance on Open USD that has yet to be considered by the working group.

The solution however is straightforward, if a little unintuitive. Images can be manually converted using OpenImageIO's oiiotool. Houdini ships with a version, but it is also simple to install directly.

Lemon squeezy

If you're using macOS and the Homebrew package manager, run the following terminal command.

brew install openimageio

Or for Ubuntu and Windows Subsystem for Linux (WSL), the following installs the app.

sudo apt-get install -y openimageio-tools

The oiiotool automatically picks up a predefined ACES configuration. If you're using WSL, you will need to make sure to pass that information over to Linux by enabling an extra environment variable named WSLENV with a value of OCIO/p.

After installation, run the following with an existing linear sRGB/Rec.709 image to convert it into the ACEScg colour space.

oiiotool hdri.exr --colorconvert "Utility - Linear - Rec.709" "ACES - ACEScg" -o aces.exr

Note that the two named transforms Utility - Linear - Rec.709 and ACES - ACEScg will vary depending on the specific version of ACES used. Here I am referencing ACES 1.2.

It is worth pointing out that post-conversion, the image might look slightly darker rendered within Karma. Bumping the light intensity up can help bring the image closer to the original non-colour managed texture.

A lemon splash created using Vellum fluids, rendered with Karma XPU in an ACES colour space.

This lemon splash was rendered using Karma XPU backed by ACES and lit by a single dome light. Without manually transforming the input space, sky blue colours visible in the water droplets would have looked oversaturated and unnatural.