While reading through the official 1.0.3 config file and trying to understand all bits and peaces, I often come across the ‘reference’ role definition. In the official OCIO docs, the ‘reference’ role is said to be the ‘heart’ of the whole config, so all the ‘from_reference’ and ‘to_reference’ transforms are working with the colorspace defined with this role, right?
So why is it not pointing to ‘ACES - ACES2065-1’, which in my opinion is the ‘heart’ of the ACES world. And why has ‘ACES - ACES2065-1’ different definitions than the ‘Raw’ definition (isdata false vs. true; allocation, etc.).
I always find this very confusing, especially if you try to explain the concept of ACES and OCIO.
The first colorspace defined in the config is the ‘reference’ that the other colorspaces’ to_reference and from_reference transform lists go to and from.
The ‘reference’ role is a little bit ill-defined but, I think, is supposed to be used for reference photography. When we were choosing the colorpsaxes foe the roles, choosing Utility - Raw for the reference role was mostly a pass on making a decision.
The two uses of ‘reference’ are an unfortunate coincidence. Note that none of the roles are required to have a valid config.
First the lnf colorspace (short for linear float) is used as our reference colorspace. The name can be anything, but the idea of a reference colorspace is an important convention within OCIO: all other colorspaces are defined as transforms either to or from this colorspace.
The note on the reference role is confusing though. I’ve wished for a while that roles acted more directly like colorspaces. In practice, they are only labels.
Perhaps the next iteration of the config should still make the reference role equal to the reference space.
Now that you’ve got the “reference color space” concept clarified, I wanted to mention one other thing.
Something that can be frustrating when setting OCIO configs is that the implementation (or lack thereof) of the “roles” feature is different from one host application to the next, and those implementation details are often undocumented. Sometimes the role assignments are mapped to user settings or operations in the app (e.g. Nuke).
Other times the role color spaces affect the image processing and the user may not be aware. An example might be rendered image data being converted to the “rendering” role color space when output files are written, even though no explicit GUI setting is given to control that behavior. Anyway, keep your eye on those role assignments if you are troubleshooting the OCIO behavior in a host app.
whenever I am modifying the default ACES OCIO config, I am putting new colorspaces at the top. If the first colorspace of the config would define the reference, these modified configs wouldn’t work anymore. But this is not the case…
My initial guess would be that the reference space that is used for “to_reference” and “from_reference” conversions is the one that is simply missing these parts.
Would be interesting to see what happens if more than one colorspace meets this criterion?
Does this make sense?
In practice, I think it doesn’t really matter, as all colourspaces without from_ and to_ definitions would be ‘the same’, but I’m wondering because they were defined with different allocation params, so there should be differences technically when rendering on the GPU, depending on which one of these colourspaces will be used by OCIO, right?
With all due respect to hpd, I don’t believe that is the case. I’ve looked at the source code, and I don’t see anything special about the first colorspace. I do see that the role called “reference” is used when looking up the to_reference and from_reference transformations. So I believe that the reference role is what determines the reference color space (not the first colorspace listed in the config).
Correction: That’s how our host code calls OCIO. We ask for processors between color spaces and ROLE_REFERENCE (on the way into working color space), or the other way around (on the way out).
OCIO itself does not use ROLE_REFERENCE for anything special, but host apps typically do. You can also ask for processors between any two color spaces and OCIO will implicitly use the “to_reference” and “from_reference” paths to create a processor (including optimizing out redundant steps). It doesn’t use ROLE_REFERENCE to do that (like I thought it did). Still, the “first” color space is not special in any way. All that really matters is the “to_reference” and “from_reference” definitions, and if hosts do use the ROLE_REFERENCE for their working color space, it should ideally be set to the same color space that those transforms use.
Based on the posts on the OCIO list, it looks like neither the reference role nor the first colorspace actually formally specify the ‘reference’ colorspace, despite the web page documentation suggesting that both do so. The reference colorspace is apparently only specified indirectly by the config author, in that the to_reference and from_reference transforms all go to and from a common space. In the case of the ACES config, the colorspaces were all designed to transform to and from ‘ACES - ACES2065-1’, which just happens to be the first colorspace.
Given that some apps depend on the ‘reference’ role also pointing to a colorspace that matches the implied reference colorspace, we should probably change the config to do so.
I may have misinterpreted HP’s comment about the first colorspace but I took it to refer specifically to the aces config Abraham referenced, not to how OCIO configs work in general. As Dithermaster said, all that really matters is the “to_reference” and “from_reference” definitions, since reference is implicit in them. Setting the reference role is a good way to make the intent clear, though.
This conversation gave me an intense sense of deja vu and I just found why:
Alex your read on it probably came from what was originally in the web site text, but the only opinions offered up at the time of that discussion were that it was intended as an identifier for the space that to_reference and from_reference implementations convert to/from. And even the spi-anim and spi-vfx configs had reference set to “lnf”, which wouldn’t likely work for art reference, so it does seem like the intent was always to identify the to/from reference space.
Good to see you Matt
Unfortunately, at the end of the day, it seems inevitably to be a matter of vendor interpretation… it’s really a shame about the ambiguity…
HP, if I recall correctly, you once put together a pretty comprehensive review of applications that supported OCIO at the time… I have no idea where I saw that, and I’m sure that hasn’t been maintained (has it?), but I’d gladly contribute to a group effort to detailing vendor support for OCIO and ACES in a wiki or something… that would be a pretty damn helpful resource,
which takes two colorspaces and concatenates the source colorspace’s to_reference (or inverse from_reference) transforms with the destination colorspace’s from_reference (or inverse to_reference) transforms.
So… the code is pretty clear. The reference colorspace isn’t the first colorspace. It isn’t the space pointed to by the ‘reference’ role. It’s the space that the config colorspace’s various to_reference and from_reference transforms use as a target or as a source. Hopefully all the colorspace’s in the config agree on that definition!
So technically it doesn’t matter, but if you want to explain ACES and/or OCIO to someone, it would really help to have the roles behave ‘logical’, so the reference role points to ACES2065-1 like explained in the OCIO docs.
One thing that I’m still interested in is, why the two ‘identical’ definitions (ACES2065-1 and Raw) have different allocation parameters. Why do you want to use a log2 curve for a linear colorspace?
Allocation variables are primarily used in the GPU path, when needing to construct a 3D LUT to represent a set of transforms. Using a linear encoding for a linear HDR colorspace will get you pretty quickly into quantization land. Using a log encoding avoids that.
As for the value of the reference role, it’s probably worth updating. It would really be best to update the documentation so there aren’t two different places called out as ‘the definition of the reference colorspace’, when neither is, in practice.
Totally agree with the conclusions of @dithermaster and @hpduiker. The OCIO source code doesn’t enforce anything special with regard to the “reference” colorspace. However, host applications may make assumptions with regard to the 1st config colorspace, or the “reference” role colorspace, or both. Keep in mind that this can be an intended application of OCIO, but this behavior may not be universal across host apps.
FYI for anyone using the OCIO nodes in Nuke:
Some operators use the “reference” role colorspace in the OCIO config when converting to/from the node’s “working space.” Even if you set color management to OCIO, and your Project Settings working space to the colorspace of your choice, you may see incorrect or undesired transformations applied in nodes such as OCIO CDLTransform. If you’re using an ACES OCIO config with Nuke, I’d recommend changing the “reference” role colorspace to “ACES - ACES2065-1”, which should keep the OCIO nodes behaving as expected.
I’d love to hear about other people’s experience with this!