# 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 three different 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 one basic ODT pass-thru display transform called ODT_monitor.ctl. This transform should be suitable for displaying images on a monitor device, as the only transformation performed on the image is to apply the luminance and color primaries of your monitor.

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 takes scene-linear RGB images to CIE XYZ by converting through HDTV Rec709. Depending on the project, the rendering transform will vary. In the future, AMPAS will provide a Reference Rendering Transform for Film which may become the standard for most film productions. If your project requires some special film effect (like bleach film), a special render transform should 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.