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Processing a HDRI in ACES
Please note that the following notes are more of a brain dump and I am still testing our internal CAVE Academy colour pipeline.The aim is to create an off-the-shelf solution for processing HDRIs into an ACES colour space.
I’m working through all the good stuff on ACES Central but I’m sure I’ll miss a couple of things, so if anyone has any additional thoughts, good processes, useful tools, etc. please do send your ideas in this direction (either to: [email protected] or in the comments)
Once I’ve got things fleshed out, I can then do a full step-by-step walkthrough.
When shooting HDRIs, make sure to capture bracketed shots of the0 Macbeth chart, the grey ball and the chrome ball. Non-bracketed exposures for these will cause issues when setting up the look development environments
- Convert CR2 files to linear 16-bit EXR (in ACES directly if possible).
- Deke Kincaid: Don’t go to 16 bit int tiff as this will clamp the image when converting to ACES. There is such thing as a 16 bit float tiff but a majority of all software does not support it and only supports the int flavor. Definitely look at going to EXR.
- How best to do this? Use dcraw. Can I do this in RawTherepee or Nuke? Lightroom is great for batch processing but for colour management – ouch! Affinity Pro. Colour HDRI. ncruces dcraw. Darktable.
- In Nuke, apply the composite matrix to transform from XYZ-to-ACES: http://colour-science.org/cgi-bin/rgb_colourspace_models_transformation_matrices.cgi. If you are not working with film footage, you can use ACEScg as the main rendering, interchange and archiving space.
- Use PTGui (or any other similar app) to merge the 16-bit tiffs to create HDR files, output as scene-linear .exr files
- Deke Kincaid: Most of the CaraVR toolset is now included inside Nuke. So you could do all the stitching in Nuke and cut out ptgui. Either that of use ptgui to do the solve and then read the ptgui file into Nuke to setup the stitcher. https://learn.foundry.com/cara-vr/Content/user_guide/solving_rigs/solver/prepare_camera_rig.html
- In Nuke, load the .exr file. The image will look very underexposed, but this is all good. Use the mmColourTarget gizmo to neutralize the color-casts with your bracketed Macbeth chart and get the base exposure (from the 18% grey patch) with a calculated composite matrix.
- Apply this composite matrix to all your reference images from this shoot, HDRIs, reference photos etc should give you well color managed reference! Note. In, Nuke you probably want to be using the best suited viewing transform, (for example sRGB_for_ACEScg.lut from the OCIO config found on their GitHub) whether it’s an sRGB monitor or P3.
- In Arnold (or your renderer of choice), set your rendering space to be ACEScg, and use the ACES RRT as your viewing transform (i t’s a default Viewing Transform that I believe uses the sRGB ODT). Also any texture maps should be rotated in to ACEScg accordingly.
- Lastly, a calibration device such as a Spyder or X-Rite i1 for your working monitor is worthwhile.
Processing a HDRI in ACES
Match grade a HDRI to a shot