Color Transformation Language (CTL)

What is CTL?

The Color Transformation Language, or CTL, is a small programming language designed to serve as a building block for digital color management systems. It allows users to describe color transforms in a concise and unambiguous way by expressing them as programs.

How it Works

A color management system that supports CTL includes an "interpreter", a software program that performs the operations described in a CTL program on the pixels of an image. To apply a transform, the system instructs the interpreter to load and run the specific CTL program. The original and the transformed images constitute the program's input and output.

Interchange: Color transforms can be easily shared by distributing CTL programs. Two parties using the same CTL program can apply the identical transform to an image.

Parameters: CTL programs can include input parameters (such as "exposure") that adjust the transformation. To guarantee identical results, parties must agree on both the CTL program and the settings for the transform's parameters.

Why A Domain-Specific Language?

While general-purpose programming languages like C, C++ or Python can describe color transforms, they are often unsuitable for transform interchange due to several factors:

• Some languages require the recipient to explicitly compile and link source code before it can be executed.
• Code must be carefully written to ensure it behaves identially and remains portable across different operating systems.
• If general purpose code is executed inside a larger application, bugs can cause the entire application to malfunction or crash.
• Providing reliable protection against viruses and Trojan horses is difficult with most general-purpose programming languages.

By contrast, CTL is designed to allow only the kinds of operations that are needed to describe color transforms. This focused approach improves program portability, protects users against application software crashes and malicious code, and allows for efficient interpreter implementations.

Scope and Constraints

CTL is a specialized tool and not intended for general-purpose image processing.

• CTL is restricted to color space transforms and other single-pixel operations.
  • CTL's syntax, concepts, and functions are documented in SMPTE RDD 15 and the documentation in this repo
• CTL cannot express operations that require surrounding pixel data, such as convolving an image with a filter kernel (blurs/sharpens) or calculatating global image statistics (like a sum of all pixels in an image).

Important

While the source code for CTL is maintained within the ACES project ecosystem, CTL is very much its own standalone project. It is important to distinguish the language from its usage:

• CTL is a color transformation language, and can be used to express any color transform.
• ACES makes use of CTL as language to provide its reference transforms.
• CTL ≠ ACES - CTL's utility extends far beyond its usage in the ACES framework.

Respository Contents

• .github/ - Github CI workflow files
• cmake/ - cmake support files
• ctlrender/ - a command-line tool to apply CTL transforms to images
• doc/ - CTL documentation
• docker/ - dockerfiles that compile CTL on various platforms
• lib/ - CTL libraries and the CTL interpreter
• OpenEXR_CTL/ - sample CTL applications utilizing IlmImfCtl
• resources/ - scripts and support files for unit tests
• unittest/ - unit test files

Installation

Quick Start (macOS)

For most users on macOS, the fastest way to install CTL is with Homebrew:

brew install ctl

This will install CTL along with all required dependencies.

Build from Source

CTL can also be installed from source (Linux, macOS or Windows) or be run in a Docker container.

See INSTALL.md for details.

License

CTL is licensed under the Apache 2.0 license.