LMTs

Look Modification Transforms (LMTs) are a very powerful component of the Academy Color Encoding System and offer extraordinary flexibility in ACES-based workflows. This knowledge base article will explain what LMTs are, why they are so important, and how they can be constructed and applied. It is structured from general to specific, basic to technical.

What are LMTs?

Academy Documentation of LMTs Available for Download

LMT README on GitHub

Look Modification Transforms (LMTs): ACES-to-ACES transforms that systematically change the appearance of ACES-encoded data viewed through an ACES Output Transform.

LMTs are mechanisms to apply an infinite variety of “looks” to images in ACES-based workflows. Any adjustment away from the starting default reference rendering is considered a “look” within an ACES framework. By this definition, a “look” can be as simple as ASC CDL values preserved from set to define a starting grade. They exist because some color manipulations can be complex, and having a pre-set for a complex look makes a colorist’s work more efficient.

A key distinction is interactive image modification, which can either be across the entire frame or isolated regions of interest is termed ‘grading.’ The ACES term for preset systematic, full-frame image modification is ‘look modification’.

The LMT precedes the sequential application of the ACES Reference Rendering Transform (RRT) and a particular Output Device Transform (ODT), and allows custom and systematic color changes to a set
of clips or entire timeline to realize a chosen creative intent.

As seen in the diagram above, LMTs are always ACES-to-ACES transforms. In other words, ACES2065-1 data directly translated from camera-native image data via an Input Transform is manipulated by the LMT to output new ACES2065-1 data (designated in diagrams as ACES’) that is then viewed through an ACES Output Transform.

Applications of LMTs

There are Input Transforms for all major camera and film types, that convert camera-native (log) image encodings into ACES2065-1. This means that any camera-native image can be viewed through the ACES processing path. Once the image is in ACES, LMTs can be applied in a variety of ways.

Creating a Starting Point for Grading

LMTs supplement traditional colorists’ tools for grading and manipulating images. They can be used project-wide to reduce contrast and saturation across all shots, providing what might be a more preferred starting point for colorists accustomed to flatter and more muted starting images.

Matching or Transferring to an Existing Look

LMTs can also encapsulate a preset creative look, while still remaining within the context of an ACES-based workflow and viewing through the ACES Output Transform. Very complex creative looks, such as film print emulation, are more easily modeled in a systematically derived transform than by asking a colorist to manually match using only the controls in a color corrector. Having a preset for a complex creative look can make colorists’ work more efficient by allowing them to quickly get the creative look they (and the filmmaker) want, and to spend more time on shot- and/or region-specific creative color requests from clients.

The following graphic represents the workflow of emulating a desired print-film emulation of a post production facility in an ACES environment.

In ACES, well-designed LMTs are interchangeable. This means, for example, that LMTs designed to match a camera-specific rendering can be used for other cameras, or print film emulation LMTs can be used with digital cameras.

Compiling Resources

Once constructed LMTs can be a valuable tool for those using ACES for their color pipelines. In ACES-based workflows, disparate camera-specific encodings are translated into a common encoding space: ACES2065-1. That enables looks to be reusable and applicable within all contexts, regardless of image source. LMTs are basically “preset” looks, and one may find it desirable to build up a “library of looks” made up of pre-built filters that can be used to quickly decide an initial grade or a “show LUT” look on any given project. Further grading adjustments can be made on top of the new starting point.

Experiment on your own and try using LMTs to implement your own looks. Your ACESCentral friends would love to see what you come up with. Feel free to extend the provided functions or write additional ones.

Implementation

The transforms used in all examples are implemented using CTL code. LMTs do not need to be implemented in CTL. CTL was the easiest way to unambiguously express the operations used. If you are a “tinkerer” feel free to take a closer look at the formulas inside the CTL provided later in the article for the order of operations and other details on how these LMTs work.

The Common LUT Format (CLF), a core component of ACES 1.x, is designated as the official carrier for certain kinds of LMTs. CLF’s can encapsulate any number or ordering of process nodes that include 1D LUTs, 3D LUTS, matrices, and ASC CDL. However, CLF does not support math formulas, so CTL used for even simple shaper functions would need to be sampled to LUTs for implementation in CLF. This is potentially limiting, but extending CLF, or adding support for algorithmic description of LMTs, is under consideration for upcoming ACES enhancements and extensions.

Please note that CLF is a required item for LMT-supporting production and postproduction tools that wish to achieve the ACES Logo. We encourage all end-users to work with ACES Product Partners to provide you with this powerful capability.

Regarding LUTs, keep in mind that the quality of 3D-LUTs requires knowledge of input and output ranges as well as grid point spacing. ACES values are radiometrically linear and have a very wide floating point range. Spacing nodes directly in linear space does not always provide a good sampling of the useable range. CLF provides for forward and inverse ‘shaper LUT’ operations that (when wrapped around an appropriately constructed 3D-LUT) can effectively produce well-spaced LUTs with minimal interpolation errors. Implementers should review the ‘1D LUT’ section of the Common LUT Format specification [S-2014-006 14] for more information.

A Deeper Look

LMTs can be characterized as either “empirical” or “analytic”. Empirical LMTs are derived from sampling another color reproduction process and are typically implemented as 3D LUTs that encapsulate a subsampling of such processes. Analytic LMTs are defined mathematically and are typically expressed as a set of ordered mathematical operations to adjust ACES values. These specific LMTs and a deeper look at the processes are contained in the other Knowledge Base articles on LMTs.

This knowledge base article was created by Simon Yahn largely with the help of Scott Dyer’s previous forum posts on LMTs and the Academy TB-2014-010 Document linked at the beginning of the article.