Introduction to Dolby Vision Content Mapping
Each Content Mapping (CM) version provides a different algorithm and a different range of tools to control the artistic intent of how content is mapped to targets/devices that do not have the full capability to display the HDR master grade. This can be a range of different Dolby Vision HDR devices, with differing peak brightness and gamut characteristics, all the way down to a non-Dolby Vision device that does Rec. 709 SDR. The two versions are v2.9 and v4.0. For more details about the controls available in each version, see What are Trim Controls and how do I use them?
Should I use CM v2.9 or CM v4.0?
Whether you deliver in v4.0 or v2.9 is really up to the studio you are creating content for. You need to deliver in the format requested by the studio in their delivery specifications, although some studios may let you choose. While it is possible to work in CM v4.0, and then derive a backwards compatible CM v2.9 deliverable that looks very close, it is generally recommended to work natively in the version that you need to deliver.
Important notes when working in v4.0 or v2.9
- v4.0 is a superset of v2.9, and a v4.0 deliverable contains all the metadata that is included in v2.9. If you are creating a v4.0 deliverable, it is important to ensure that the latest backwards compatibility algorithm is used (sometimes referred to as the "L2 from L8 algorithm"), as this gives better quality on a number of legacy consumer devices, and it may also be a requirement from the studio.
- If you are deriving your SDR from your Dolby Vision master, it should be derived from your master format (either v4.0 or v2.9). You should not derive SDR from a v2.9 that has itself been derived from a v4.0. You should go back to the original v4.0. This will ensure the highest quality possible is maintained for each version.
- XML version numbers ≠ CM version numbers. The XML files carry a range of information beyond the content mapping information and so can be updated independently.
When working in v4.0, how should I ensure the latest backwards compatibility algorithm is being used?
The easiest way to know whether the latest algorithm is being used, is to check with your tools provider or their documentation. At the time of writing this article (July 2020), the latest versions of Blackmagic Design DaVinci Resolve, FlimLight BaseLight and Colorfront Transkoder all contain the latest version of the algorithm. However it is also still good to check if you haven't used a particular workflow before. If you are unsure of the tool - you can check the XML output from the tool to see if it is updated.
How to check which algorithm has been used?
You need to have the Dolby Vision XML file to check which version has been used. You can export this file from your color corrector.
You can then use the Dolby Professional Tools Metafier to check which version of the backwards compatible algorithm has been used. To do this, run the validate command.
./metafier --validate <input_cm4.xml>
where <input_cm4.xml> is the filename and location of the XML file containing CM v4 metadata that you would like to check.
- Locate the Level254 section in Metafier onscreen output
- Examine this section and check that CMVersion is "4 1" or later. An example is shown below. If the CMVersion is "4 0", then the backwards compatible metadata must be updated.
It is also possible to check the Level254 value by directly examining the XML. The example below shows the section of the XML file that covers the Level254 information.
For more details on examining XML files, see What's in the Dolby Vision XML file?
What to do if the backwards compatible metadata needs updating?
Firstly it is important to not run the updated algorithm more than once as each time can have slight variations in the metadata values. While this won't have an effect on the perception of the tone mapped versions, it can lead to confusion due these minor differences.
But if the version is "4 0", then you can again use Metafier (v5.0.1 or later) to update the XML. To perform this update, from a command line shell run the following command.
./metafier --update-cm29-metadata <input_cm4.xml>
This will provide an updated XML file and this can be combined with your master image file. If you haven't already done so, export your HDR (PQ) master image as per the delivery spec (ProRes or TIFF).
Assuming the delivery spec requires a Dolby Vision IMP/IMF, combine the updated Dolby Vision XML and Master HDR Image sequence (or Movie) in the appropriate mastering tool.
Following these best practices will ensure that your 4.0 content will look most correct on all Dolby Vision devices in the marketplace, past and present.
Can I automatically convert a CM v2.9 XML to a CMv4.0 XML?
Not at this time. It is recommended to go back to the color grading system and start again on the analysis and trims in version 4.0. As a reminder, SAVE YOUR PROJECT as a new version before erasing the CM 2.9 analysis and trims. You cannot revert back once you switch between CM versions and will lose all your analysis and trim values in that process.
Relationship between CM, XML and Level254 version numbers
The table below shows the relationship between CM, XML and Level254 version numbers.
|Dolby Vision CM Version||XML Version Number||'Level 254' inside the XML||Notes|
|4.0||4.0.2||4 0||Indicates older 2.9 backwards compatible algorithm|
|4.0||4.0.2||4 1||Indicates latest 2.9 backwards compatible algorithm|
For a range of additional information about CM v2.9 and CM v4 see the Dolby Vision Color Grading Best Practices Guide and the Metafier section of the Professional Tools User Manual which can be downloaded from User Manuals.