I've been searching for weeks and can't find a clear explanation of how
.cube files are written. Please help me understand them.

I have a .cube file that was generated with 3D Lut Creator, with an
image source of a HALD color image (64x64x64). No adjustments were
applied to the image, so the .cube file should show input=output. But
the decimal values in the lookup table mystify me. How were they
generated? The decimal values, when multiplied by 64, do round to a
number close to list position in the (64 64 64) list, but surely there's
a precise algorithm to output those decimal values.

I've tried reading the OCIO C++ code to get the answer, but I don't
really know the language. I want to write luts in Java. Can someone
there help me?

Thanks.

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I've been searching for weeks and can't find a clear explanation of how
.cube files are written. Please help me understand them.

I have a .cube file that was generated with 3D Lut Creator, with an
image source of a HALD color image (64x64x64). No adjustments were
applied to the image, so the .cube file should show input=output. But
the decimal values in the lookup table mystify me. How were they
generated? The decimal values, when multiplied by 64, do round to a
number close to list position in the (64 64 64) list, but surely there's
a precise algorithm to output those decimal values.

I've tried reading the OCIO C++ code to get the answer, but I don't
really know the language. I want to write luts in Java. Can someone
there help me?

Thanks.

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Just had a look in the Python API and wondered how I could get the referenced colorspace of a named role. Some of the roles are predfined as constants (OCIO.Constants.ROLE_...), but what's with user defined roles? So for example the compositing_linear one in the ACES config, or a self defined one?

Shouldn't there be just an easy function like 'getRole(rolename)' that just gives back the name of the colorspace that the role is linking to!?

Really confused at the moment on how to use roles via API.

Thanks!

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I've read the source. I didn't see anything special about the "first" colorspace. Unless someone can show me (either in the code or by an API call example) where it is somehow special, I won't believe it.

///d@

On Fri, Apr 20, 2018 at 5:58 PM, <chris....@...> wrote:

Hi folks-

H.P., I also agree with that definition. But - it's also true that if you are converting to or from the 1st colorspace in the config list (aka "Reference" colorspace) we should see the to_reference or from_reference transform (or inverse as-needed) applied alone. More specifically, we don't need to explicitly convert from colorspace A to Reference colorspace, then from Reference colorspace to colorspace B because the conversion from Reference to Reference is a non-op. So the OCIO ColorSpace isn't ignoring the reference colorspace in its conversion (as we can explicitly use it if desired), it's simply skipping the conversion from Reference<>Reference that is implied which seems to be standard OCIO behavior. Nevertheless I agree - it's up to the config author to accuratly define the to_reference and from_reference transforms. 

From my view, the relevant issues that crop up are symptoms of inconsistent implementations, esp. where Roles are used for implied conversions in the host application.

For example, I can see that Nuke OCIO CDLTransform uses the "reference" ROLE assignment for converting to/from the "working space" assigned in the OCIO CDLTransform node. This is inconsistent, because for other standard operations Nuke honors the reference colorspace that is implied by the first colorspace in the config file. A Read node transforms colors from the Read node colorspace to the Project Settings working space, as do other standard Nuke nodes with colorspace assignments. Of course, we can set the Project Settings working space to the reference/1st colorspace if we want to, and the conversions still happen as expected.

As for the Roles in the config - some of them are used to populate the color Project Settings assignments for "working space," "log files," etc. when a given config file is first loaded. But when some plugins like OCIO CDLTransform use OCIO Roles in its own way, ignoring the Nuke color Project Settings, things are getting overly complicated.

From the source code comments: 

https://github.com/imageworks/OpenColorIO/blob/a557a85454ee1ffa8cb66f8a96238e079c452f08/export/OpenColorIO/OpenColorTypes.h#L341

    ColorSpace Roles are used so that plugins, in addition to this API can have

    abstract ways of asking for common colorspaces, without referring to them

    by hardcoded names.

All this considered, I conclude that the "reference" role should always be the "reference" (1st) colorspace in the config list. A few suggestions for improved behavior in the wild:

1. As users, get in the habit of setting the reference role to the 1st colorspace in the list. 

2. OCIO sanity checks and ociocheck app should return an error if "reference" role is anything other the 1st colorspace. 

3. Maybe - hardcode the "reference" role to the first colorspace in the list when configs are read in. I.e.: don't allow custom assignments. I expect some backlash for this suggestion - please don't rake me over the coals :) This at least enforces the intended usage. 

-Chris

On Thursday, April 19, 2018 at 8:52:43 AM UTC-7, Dennis Adams wrote:

Yes, I completely agree with that definition. My assertion that 'reference' was related turned out to be something in our own code, not OCIO.

///d@

On Wed, Apr 18, 2018 at 11:25 PM, Haarm-Pieter Duiker <li...@...> wrote:

Cross posting for completeness

The document detailing ACES support in various applications, is here (read only):

https://docs.google.com/document/d/1_GFplxDBvG-8vxHkhe6RV-ZGmkPKnhkUGC8_rzltcyI/edit?usp=sharing

If anyone is interested in updating the document for more current versions, let me know. I'd be happy to provide an editable link. Just message me.

For those interested in implementations, I think the Nuke nodes are supposed to be a reference. The OCIO Colorspace node creates the path from one colorspace to the next with this call

https://github.com/imageworks/OpenColorIO/blob/master/src/nuke/OCIOColorSpace/OCIOColorSpace.cpp#L207

m_processor = config->getProcessor(context, inputName, outputName);

which leads to this call to create a color space conversion

https://github.com/imageworks/OpenColorIO/blob/454f5fef6e991e645401341cd6fab41c2451b8cd/src/core/Config.cpp#L1357

processor->getImpl()->addColorSpaceConversion(*this, context, src, dst);

which eventually leads to this function

https://github.com/imageworks/OpenColorIO/blob/454f5fef6e991e645401341cd6fab41c2451b8cd/src/core/ColorSpaceTransform.cpp#L180

BuildColorSpaceOps

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 has it. 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!

On Mon, Apr 16, 2018 at 12:38 AM, Abraham Schneider <abrah...@...> wrote:

Hi there!

I just posted this question on the ACES website: http://acescentral.com/t/why-utility-raw-as-reference-for-aces1-0-3-and-not-aces2065-1/1378

It seems like it's not just me that is unsure, how OCIO decides, which colorspace in a config is used as the reference space, that all other colorspaces are related to. And what excact purpose the 'reference' role has.

Could you maybe have a look at this and shine some light on this topic? 

Thanks, Abraham

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Hi folks-

H.P., I also agree with that definition. But - it's also true that if you are converting to or from the 1st colorspace in the config list (aka "Reference" colorspace) we should see the to_reference or from_reference transform (or inverse as-needed) applied alone. More specifically, we don't need to explicitly convert from colorspace A to Reference colorspace, then from Reference colorspace to colorspace B because the conversion from Reference to Reference is a non-op. So the OCIO ColorSpace isn't ignoring the reference colorspace in its conversion (as we can explicitly use it if desired), it's simply skipping the conversion from Reference<>Reference that is implied which seems to be standard OCIO behavior. Nevertheless I agree - it's up to the config author to accuratly define the to_reference and from_reference transforms. 

From my view, the relevant issues that crop up are symptoms of inconsistent implementations, esp. where Roles are used for implied conversions in the host application.

For example, I can see that Nuke OCIO CDLTransform uses the "reference" ROLE assignment for converting to/from the "working space" assigned in the OCIO CDLTransform node. This is inconsistent, because for other standard operations Nuke honors the reference colorspace that is implied by the first colorspace in the config file. A Read node transforms colors from the Read node colorspace to the Project Settings working space, as do other standard Nuke nodes with colorspace assignments. Of course, we can set the Project Settings working space to the reference/1st colorspace if we want to, and the conversions still happen as expected.

As for the Roles in the config - some of them are used to populate the color Project Settings assignments for "working space," "log files," etc. when a given config file is first loaded. But when some plugins like OCIO CDLTransform use OCIO Roles in its own way, ignoring the Nuke color Project Settings, things are getting overly complicated.

From the source code comments: 

https://github.com/imageworks/OpenColorIO/blob/a557a85454ee1ffa8cb66f8a96238e079c452f08/export/OpenColorIO/OpenColorTypes.h#L341

    ColorSpace Roles are used so that plugins, in addition to this API can have

    abstract ways of asking for common colorspaces, without referring to them

    by hardcoded names.

All this considered, I conclude that the "reference" role should always be the "reference" (1st) colorspace in the config list. A few suggestions for improved behavior in the wild:

1. As users, get in the habit of setting the reference role to the 1st colorspace in the list. 

2. OCIO sanity checks and ociocheck app should return an error if "reference" role is anything other the 1st colorspace. 

3. Maybe - hardcode the "reference" role to the first colorspace in the list when configs are read in. I.e.: don't allow custom assignments. I expect some backlash for this suggestion - please don't rake me over the coals :) This at least enforces the intended usage. 

-Chris

On Thursday, April 19, 2018 at 8:52:43 AM UTC-7, Dennis Adams wrote:

Yes, I completely agree with that definition. My assertion that 'reference' was related turned out to be something in our own code, not OCIO.

///d@

On Wed, Apr 18, 2018 at 11:25 PM, Haarm-Pieter Duiker <li...@...> wrote:

Cross posting for completeness

The document detailing ACES support in various applications, is here (read only):

https://docs.google.com/document/d/1_GFplxDBvG-8vxHkhe6RV-ZGmkPKnhkUGC8_rzltcyI/edit?usp=sharing

If anyone is interested in updating the document for more current versions, let me know. I'd be happy to provide an editable link. Just message me.

For those interested in implementations, I think the Nuke nodes are supposed to be a reference. The OCIO Colorspace node creates the path from one colorspace to the next with this call

https://github.com/imageworks/OpenColorIO/blob/master/src/nuke/OCIOColorSpace/OCIOColorSpace.cpp#L207

m_processor = config->getProcessor(context, inputName, outputName);

which leads to this call to create a color space conversion

https://github.com/imageworks/OpenColorIO/blob/454f5fef6e991e645401341cd6fab41c2451b8cd/src/core/Config.cpp#L1357

processor->getImpl()->addColorSpaceConversion(*this, context, src, dst);

which eventually leads to this function

https://github.com/imageworks/OpenColorIO/blob/454f5fef6e991e645401341cd6fab41c2451b8cd/src/core/ColorSpaceTransform.cpp#L180

BuildColorSpaceOps

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 has it. 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!

On Mon, Apr 16, 2018 at 12:38 AM, Abraham Schneider <abrah...@...> wrote:

Hi there!

I just posted this question on the ACES website: http://acescentral.com/t/why-utility-raw-as-reference-for-aces1-0-3-and-not-aces2065-1/1378

It seems like it's not just me that is unsure, how OCIO decides, which colorspace in a config is used as the reference space, that all other colorspaces are related to. And what excact purpose the 'reference' role has.

Could you maybe have a look at this and shine some light on this topic? 

Thanks, Abraham

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Cross posting for completeness

The document detailing ACES support in various applications, is here (read only):

https://docs.google.com/document/d/1_GFplxDBvG-8vxHkhe6RV-ZGmkPKnhkUGC8_rzltcyI/edit?usp=sharing

If anyone is interested in updating the document for more current versions, let me know. I'd be happy to provide an editable link. Just message me.

For those interested in implementations, I think the Nuke nodes are supposed to be a reference. The OCIO Colorspace node creates the path from one colorspace to the next with this call

https://github.com/imageworks/OpenColorIO/blob/master/src/nuke/OCIOColorSpace/OCIOColorSpace.cpp#L207

m_processor = config->getProcessor(context, inputName, outputName);

which leads to this call to create a color space conversion

https://github.com/imageworks/OpenColorIO/blob/454f5fef6e991e645401341cd6fab41c2451b8cd/src/core/Config.cpp#L1357

processor->getImpl()->addColorSpaceConversion(*this, context, src, dst);

which eventually leads to this function

https://github.com/imageworks/OpenColorIO/blob/454f5fef6e991e645401341cd6fab41c2451b8cd/src/core/ColorSpaceTransform.cpp#L180

BuildColorSpaceOps

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 has it. 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!

On Mon, Apr 16, 2018 at 12:38 AM, Abraham Schneider <abraham....@...> wrote:

Hi there!

I just posted this question on the ACES website: http://acescentral.com/t/why-utility-raw-as-reference-for-aces1-0-3-and-not-aces2065-1/1378

It seems like it's not just me that is unsure, how OCIO decides, which colorspace in a config is used as the reference space, that all other colorspaces are related to. And what excact purpose the 'reference' role has.

Could you maybe have a look at this and shine some light on this topic? 

Thanks, Abraham

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Hi there!

I just posted this question on the ACES website: http://acescentral.com/t/why-utility-raw-as-reference-for-aces1-0-3-and-not-aces2065-1/1378

It seems like it's not just me that is unsure, how OCIO decides, which colorspace in a config is used as the reference space, that all other colorspaces are related to. And what excact purpose the 'reference' role has.

Could you maybe have a look at this and shine some light on this topic? 

Thanks, Abraham

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The reference role doesn't appear to be used anywhere inside OCIO itself. The choice of a reference space is just an choice when creating all your transforms. That said, it is a good idea to specify this role for people reading the config, and some applications "could" potentially use it (but setting it to something weird like "utility - raw" would work technically fine, it is only confusing)

The suggestions in that thread of "it's the first colorspace" is probably just because the example SPI config a happen to specify their "lnf" space first. However this is mistaken - there is nothing special about the first defined space.

I would suspect whoever created the ACES config misinterpreted the "reference" role as "what space to use for reference footage"

The meaning of the default roles are defined here:

http://opencolorio.org/userguide/config_syntax.html#roles

As you say, the reference role should be set to the main ACES space, as that is what the other spaces are defined relative to

These meanings should be a bit easier to find (e.g in the C++ API docs and Python bindings too) - made ticket for this,

https://github.com/imageworks/OpenColorIO/issues/533

Sent from my phone

On 17 Apr 2018, at 17:50, Abraham Schneider <abraham....@...> wrote:

Hi there!

Thanks for your answer. The reason for a reference colourspace and the 'star topology' of the colorspaces is perspicous for me. I was just wandering, why the ACES 1.0.3 config has at least 2 colourspace that are missing all 'from' and 'to' transforms (and in this definition would both be THE reference), why the 'reference' role points to 'Utility - Raw' and not to 'ACES2065-1' and why these two spaces have different allocation vars.

And in the discussion on the ACES page now there is the uncertainty, if the reference role defines the reference space or some other logic, because there needs to be a logic which space definition will be used when there are more than one that misses all from/to definitions.

Abraham

Am Dienstag, 17. April 2018 06:17:04 UTC+2 schrieb bsloan:

Hi Abraham,

The reference colorspace is the one that every colorspace defined in the config converts from (or to). It may or may not be defined in the config. If it does, the to_reference and from_reference transforms for the reference colorspace are necessarily null.

As for the purpose of having a reference colorspace, one cannot simply take a bunch of pixel values and transform them to colorspace X without knowing the colorspace associated with the incoming pixel values -- a transformation requires well-defined source and destination colorspaces.

Within ocio, a conversion from colorspace X to colorspace Y is built by concatenating colorspace X's from_reference and colorspace Y's to_reference*.

The alternative to having a reference colorspace would be for the config to define transformations from each colorspace in the config to every other colorspace in the config, which I think is N! entities instead of N. 

The ACES 1.0.3 config uses ACES-2065 as its reference colorspace, but a custom config could as easily define all of it's colorspaces in terms of an ACEScg reference or some camera vendor's encoding and gamut.

*most ocio colorspaces are invertible so they need only provide a from_ or a to_.

Hoping this is helpful.

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