And for most render engines, calculations based on light spectra are still done with sRGB/Rec.709 primaries, such as : But as surprising as it may sound, there are different levels of integration for OCIO/ACES. Autodesk has even come up with a CTL integration of ACES in Maya. Most render engines have integrated OCIO which give us access to ACES. The release notes for the ACES OCIO configs are available here. Or even using FilmLight’s Baselight… With OCIOv2, the ACES OCIO config should not present any discrepancies anymore. I don’t know if these images were generated in CTL or OCIO though. You can also check different examples from Alex Fry’s presentation. Since we render in ACEScg, you will need to convert your footage with an “ OCIOColorSpace” node before plugging your footage. Please note that these nodes work with ACES2065-1 footage. These Nuke nodes have a 100% match to a pure CTL implementation but are way faster. This is the last thread about CTL that I know of. It has not been maintained nor updated properly, as it is quite a rabbit hole. Please be aware that CTL is pretty much a dead language. Jed Smith has also done a Nuke implementation of the ACES Output Transforms. ![]() So it looks like CTL would be worth using especially if you are working on saturated animated feature films ! Alex Fry was kind enough to share with us this Pure Nuke ACES RRT & ODT. December 2016 and Alex Fry had already understood so much stuff… With normally exposed images, especially from live action cameras, you’re unlikely to see any values that render differently to the OCIO config, but with CG images containing extreme intensity and saturation you will see more pleasing roll off and desaturation. How important is this ? Should we stick to OCIO or look into this CTL implementation ? I guess the best answer I have read on this topic comes (once again !) from Alex Fry : How does this translate visually ? Let’s have a look at some renders with extreme saturation to compare different solutions.īecause of Arnold default settings, the base color weight is at 0.8 in these renders. This results in errors of chromaticites in highlights due to the discretisation.In very large areas these interpolations lack accuracy.Between each slice we have linear interpolations.Not every mathematical formula is available in OCIO 1.1.1, only OCIO v2 will allow to represent accurately the necessary calculation of ACES. Sometimes it would be better to manage color interpolation in the Lab colorspace. Even if we change gamut, we still work in RGB and linear interpolations are done on a line going from A to B. Therefore the display tone mapping (ODT) makes these false chromaticities really visible by increasing the exposure.Īlso even if the transformation is mathematically defined in OCIO, the fact that it runs on GPU rather than on CPU leads to a discretization of the formula : the graphic card actually creates a LUT ! Really these issues are endless… Color interpolation gapsįurthermore, these gaps (from the discretization) are filled linearly which is not necessarily the most natural way. We do not split the dynamic range into equal zones as we prefer to split in detail most current values at the expense of highlights. The discrete transform actually covers huge zones. When we go from Scene Referred to Display Referred, it implies to cover a high dynamic range (ACES deals with something like 15 stops). It is actually pretty easy to understand. He has showed me the concept of Continuous and Discrete that is happening with the baked LUTS from OCIO. What is happening here ? The answer has been given to me by my colleague, Christophe Verspieren. ![]() The reason the CTF works better is because it is more of an “exact math” implementation of the ACES CTL code rather than baking down into a LUT-based representation. It is really well explained in this post and also here. Most color pipelines nowadays are set through OCIO which is great because of its compatibility with many softwares : Maya, Guerilla, Nuke, Mari, Rv… But there is one downside using OCIOv1 and LUTs : you loose precision. Thanks ! ACES implementation ACES, OCIO and CTL Please take with a pinch of salt since it has not been updates with my latest ACES findings. ![]() This page used to be part of the ACES chapter but it is so specific that I thought it would make more sense to have it a separated article. Reasons for modifying the ACES OCIO config.
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