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The Colour Transform File Format

There are a wide variety of colour transform (also known as "LUT") formats used in the media and entertainment industry. This creates workflow problems since you may receive a LUT which is not supported by one or more of the applications you use. Furthermore, most of these LUT formats are based on out-dated technology and lack the quality, flexibility, and metadata needed to meet the requirements of floating-point, high-dynamic range imaging systems such as ACES.

To address these problems, the technology committee of the American Society of Cinematographers convened a group of experts to design a common transform format that could become an industry standard. The first version was published in 2008 and was soon adopted and further developed by the ACES project. This format became known as the Academy/ASC Common LUT Format or CLF.

Despite the name, it is actually much more than just a "LUT format". It supports stacking a variety of floating-point colour processing operators into flexible processing pipelines.

Autodesk was an early adoptor of this format as the native format for our colour management technology and we have provided a large collection of transforms in this format with various applications.

Autodesk defined several extensions to CLF (ASC CDL, Exponent, Log, and the half-domain Lut1D) which the ACES project added as part of the CLF 2.0 and 3.0 specifications. The CLF 3.0 specification may be found in the ACES Technical Documentation.

Further extensions have been developed and the resulting format is called the Colour Transform Format or CTF. Autodesk contributed the CTF technology to OpenColorIO as part of the OCIO v2 initiative. CTF is now the native tranform format of OCIO. Every mathematical operator in OCIO may be losslessly serialized to this format. OCIO fully supports CLF as well, since it is a subset of CTF.

The extensions on top of CLF version 3.0 that are available in CTF include:

  • ExposureContrast: Exposure, gamma, and contrast with an adjustable pivot to be used specifically for dynamic diagnostic adjustments in viewports.
  • FixedFunction: A set of fixed functions that implement specific algorithms, including the custom operators needed to support the ACES Output Transforms.
  • GradingPrimary: A set of primary grading controls.
  • GradingTone: A set of grading controls for specific tonal ranges.
  • GradingRGBCurve: A set of spline curves for grading.
  • Reference: The ability to import another CTF file, by reference.

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