Common LUT Format (CLF)
Look-Up Tables (LUTs) are a common implementation for transformations from one set of color values to another. With a large number of product developers providing software and hardware solutions for LUTs, there is an explosion of unique vendor-specific LUT file formats, which are often only trivially different from each other. This can create workflow problems when a LUT being used on a production is not supported by one or more of the applications being used. Furthermore, many LUT formats are designed for a particular use case only and lack the quality, flexibility, and metadata needed to meet modern requirements.
The Common LUT Format (CLF) is an XML based format that can communicate an arbitrary chain of color operators (also called processing nodes) which are sequentially processed to achieve an end result. The set of available operator types includes matrices, 1D LUTs, 3D LUTs, ASC CDL, and log and exponential shaper functions.
Although the name implies LUT, a CLF transform does not need to contain a 1D or 3D LUT in the traditional sense. A CLF can be used to encapsulate any supported color transforms – and can be as simple as a 3×3 matrix. Because of support for log and exponential functions, it is possible to recreate actual camera log functions without resorting to a sampled 1D LUT, which can improve precision, provide conciseness, and better communicate a specific transform’s intent.
Even though work on CLF has been done under the umbrella of the ACES project, the CLF is not ACES specific. The CLF does, however, provide important features (e.g. floating point support) that make it an ideal and convenient carrier for many ACES transforms. As such, CLFs can be used to encapsulate many ACES transforms – including most Input Transforms (IDTs) and Color Space Conversion Transforms (CSCs). CLF transforms are expected to be used in association with an ACES Metadata File to either carry the Input Transforms used or Look Transforms associated with a given shot or sequence.
Tools for implementers to validate their read/write support of CLF are being produced by the CLF Implementation Virtual Working Group. Increased use of CLF is anticipated as more implementers add support to their tools.