Topics covered in this article:
- The Content Mapping Unit (CMU)
- Content mapping targets
- The Dolby Vision Trim Pass
Once the analysis is done and L1 metadata has been created for the HDR content, it is highly recommended to check the Dolby Vision mapping to a specified target and verify the effectiveness of the L1. Most studios mandate running this check to a Rec709/100 nit/Gamma 2.4 (SDR) target, but additional targets may be requested as well.
Checking the L1 on a color-grading or mastering system requires an application known as the CMU or Content Mapping Unit. In the early days of Dolby Vision, the CMU application would run on an external workstation (eCMU) that was connected to the color correction system over a video and data network. More recently, many of Dolby’s color correction and mastering system partners have integrated the CMU into their systems (iCMU) which has eliminated the need for an external workstation.
The CMU emulates the Dolby Vision mapping process in consumer devices by mapping the HDR image to a specified target, giving the colorist an opportunity to assess whether the mapping is working or whether it needs improvement or adjustment. The color correction system sends the HDR image along with L1 metadata to the CMU, which in turn will map it to the specified target that the user selects from a list of predefined targets. It is recommended to only select CMU targets that are lower than the mastering display that is used. So, if a mastering monitor with a peak luminance of 1000 nits is used for the project, the 100-nit and 600-nit CMU targets can be used to check the mapping.
Dolby, as well as most studios, highly recommend checking the mapping at the Rec709/100 target. The colorist therefore sets the CMU to map to the Rec709/100-nit target and previews the mapping for the entire duration of the content. If the mapping is acceptable, the project is ready for export/output in the required formats. If the colorist feels that the mapping needs modification, they can perform a trim pass and fine tune the mapping to match their creative intent.
The Trim Pass
While checking the mapping resulting from the L1 metadata using the eCMU or iCMU, the colorist may come across shots that require modification or adjustment in order to produce the desired results that conform to their creative intent. Dolby provides a set of trim controls, in the color correction and mastering systems that support Dolby Vision content creation, to make these changes to the metadata.
Trim controls and designed to look and respond like the color correction controls that colorists are familiar with, but it is important to bear in mind that they are only metadata modifiers that allow the colorist to adjust the mapping as required. The colorist can use any combination of adjustments to achieve the result they want and in the process will create additional offset metadata (L2/L3/L8) that together with the L1 will produce the mapping that is desired. There have been significant improvements in CM v4.0 as compared to CM v2.9, and colorists now have a lot more control and flexibility during the trim pass.