# Using the Color Transform Language (CTL)

mrViewer uses AMPAS' Color Transform Language (CTL) for doing its color corrections and table look ups.

mrViewer currently ships with some basic transforms that will internally operate on the image on CIE 1931 XYZ space. CIE XYZ is chosen as it represents a virtually unlimited gamut and dynamic range.

mrViewer can have multiple CTL transformations active at the same time. One transform is attached to the main view window and represents the monitor, video or film stock. This transform within the CTL naming conventions is often called the Output Device Transform (ODT). This transform is set in the main preferences window, in the CTL section. Most projects will require users to set the device transform only once and not change it.

mrViewer ships with several basic ODTs.

An ODT that tries to emulate film will often require a more complex 2D or 3D table lookup to try to emulate the inverse of the Print Film Emulation Transform (PFET). The PFET is particular to each film stock and film developing laboratory and generating it requires careful evaluation of the film stock and developing conditions. For more information about creating the inverse PFET, you should consult your developing laboratory.

A second transform available is the Rendering Transform or RT for short. This transformation is unique to each image and project and represents the transformation needed to take the RGB pixel values stored in an image to a pleasing internal CIE XYZ representation for the project. mrViewer ships with a basic Rendering Transform that was given by AMPAS to provide a Reference Rendering Transform for Film which should become the standard for most film productions. If your project requires some special film effect (like bleach film), a special render transform might need to be used.

A third transform available is the Look Modification Transform (LMT). This transformation is intended mainly for camera devices to take their input images and color correct them based on certain knobs as done on set and is particular to each camera model. You can have an unlimited number of LMTs.

Finally, there's an Input Device Transform (IDT) which is a transform used to take the camera's raw data into a proper ACES workspace. mrViewer ships with a list of input device transforms for a lot of popular film cameras.