Topics covered in this article:
- Trim pass general guidelines
- CM 2.9 trim pass
- CM 4.0 trim pass
- Creating time metadata for 100 / 600-nit targets
To deliver a Dolby Vision Master, the following steps will have to be completed:
- Preparation for Dolby Vision Analysis
- Create L1 metadata using Dolby Vision Analysis
- Check the L1 metadata using Dolby Content Mapping (CMU)
- Performing the necessary modifications to the metadata through the Trim Pass thereby creating L2/L3/L8 metadata (optional)
If the colorist is not satisfied with the mapping after checking it through the CMU (iCMU or eCMU), they can use the Dolby Vision trim controls to adjust and modify the mapping to match their creative intent.
The Dolby Vision Metadata Trim Pass – General Guidelines
- The Dolby Vision metadata trim pass offers the colorist the option to check the mapping resulting from the L1 metadata (generated from the analysis) and make any required changes or adjustments to obtain a final result that matches their creative intent. The changes to the metadata can be made using a set of trim controls that are provided on the color correction or mastering system. The trim controls produce new offset metadata that will modify the resulting mapping, and the colorist can use any combination of the available controls to produce the desired result.
- While it is highly recommended to check the mapping affected by the L1 metadata for all shots on the timeline, it is not mandatory to trim (or adjust the trim controls) for every shot. If the L1 mapping is acceptable to the colorist, they may skip the trims on that shot and move on to subsequent shots on the timeline.
The trims performed on a shot may be copied to other shots on the timeline especially when they are similar in terms of light, color, and composition.
- While the trim controls are designed to mimic the look and feel of color correction tools/controls that colorists are familiar with, it is important to remember that they are essentially metadata modifier controls that do not perform any color correction or alter the HDR Master grade in any way.
- Modifying the trim controls produces new metadata that is sent to the CMU (eCMU or iCMU), resulting in a change in the mapping that is observed on the output (target display). Since the update happens in real time (as soon as an adjustment is made to the trim controls), it is often mistaken to be color correction. Modifying the trim controls only changes the output of the CMU and not the original HDR grade.
- Modifying the trim controls produces new Dolby Vision metadata, which can then be exported as an XML file. This offset metadata will differ based on the version of Dolby Vision used:
Dolby Vision v. 2.9 – L2
Dolby Vision v. 4.0 – L3/L8
The Dolby Vision Metadata Trim Pass – v. 2.9
Dolby Vision v. 2.9 provides a set of trim controls with which the colorist can modify the mapping resulting from the L1 metadata. Dolby Vision v. 2.9 offers a set of 6 controls which are designed to look and feel like color correction that a colorist is familiar with. The controls are:
- Lift, Gamma, and Gain are the most common controls used to modify the shadows, mid-tones, and highlights of the image. In CM v. 2.9, these controls do not function exactly like the traditional lift, gamma, and gain controls. Instead, these three controls are essentially adjusting the tone-mapping curve while mimicking the response of traditional lift, gamma, and gain controls.
- Tone Detail restores sharpness in the highlight areas of the mapped image. It is disabled for the Rec.709, 100-nit target trim and not very effective on other PQ targets in v. 2.9. This feature has been considerably improved and is a useful feature in Dolby Vision v. 4.0.
- Chroma Weight trim helps preserve color saturation in the upper mid-tones and highlight areas, especially when mapping down from HDR to SDR. This reduces luminance in highly saturated colors, thereby adding detail in those areas. Chroma Weight ranges from minimum luminance with maximum saturation on one end to maximum luminance with minimum saturation on the other end. If large Chroma Weight adjustments are made at 100 nits, it is recommended to check the mapping at 600 nits as well and adjusting Chroma Weight as required.
- Saturation Gain trim enables colorists to adjust the overall saturation of the mapped image. Saturation Gain affects all colors in the image.
Adjustments made to the mapping using these controls will be recorded as L2 metadata in the XML.
The Dolby Vision Metadata Trim Pass – v. 4.0
CM v. 4 introduces a new and improved way to create Dolby Vision content using color correction and mastering systems that have been updated to support CM v. 4.
- Mid-Tone Offset is useful for matching the overall exposure of the mapped SDR signal to the HDR master or to an SDR reference. It acts an offset to the L1 mid values and adjusts the image’s mid-tones without affecting the blacks and highlights. This global offset is per-shot and applies to all targets for the shot. The changes made to this modifier are recorded as part of L3 metadata for each shot or frame of the project.
- Lift, Gamma, and Gain response has been greatly improved over v. 2.9 and they feel a lot more like traditional lift, gamma, and gain controls . These three controls modify the shadows, mid-tones, and highlight areas of the mapped image.
- Saturation Gain trim enables colorists to adjust the overall saturation of the mapped image. Saturation Gain affects all colors in the image. To adjust saturation only for certain colors, use the saturation controls in the Secondary Trims panel.
- Chroma Weight trim helps preserve color saturation in the upper mid-tones and highlight areas, especially when mapping down from HDR to SDR. This reduces luminance in highly saturated colors, thereby adding detail in those areas. Chroma Weight ranges from minimum luminance with maximum saturation on one end to maximum luminance with minimum saturation on the other end. If large Chroma Weight adjustments are made for the 100-nit target trim pass, it is recommended to check the mapping at 600 nits as well and adjust Chroma Weight as required.
- Tone Detail trim restores sharpness only in the highlight areas of the mapped image. This works well in SDR by restoring some of the sharpness and details in the highlights that may be lost when mapping down from HDR to SDR.
- Mid Contrast Bias compresses and stretches the image around the mid-tone region and can increase or decrease contrast in the mid-tones of the mapped image. Mid Contrast Bias often has to be used along with Lift and/or Gain to produce the desired results.
- Highlight Clipping is another new trim control that has been introduced in CMv. 4. It allows the colorist to set the level of detail in the highlights by either retaining or clipping them as required. Clipping the highlights may be required when the mapped image displays details that are undesirable. The resulting clipping may extend into the upper mid tones and may require some compensation using Gamma or Gain. This trim can be very useful when trying to match the mapped SDR to an existing SDR reference.
Secondary trim controls are recorded as L8 metadata for each shot or frame of the project and enable per-color channel sat and hue adjustments per target.
- The color Saturation controls allow colorists to adjust the saturation of the mapped image individually across red, yellow, green, cyan, blue, and magenta, or all colors collectively when linked together.
- The color Hue controls allow colorists to offset the hue of the mapped image individually across red, yellow, green, cyan, blue, and magenta. These controls are useful when trying to fit/shift the larger Color Gamut of P3 or Rec2020 into the smaller color gamut of Rec709/SDR.
Secondary Trims are not backwards compatible with version 2.9 and will be ignored for all backwards compatibility calculations. The secondary trims are used to generate the SDR version. Current Dolby Vision TVs and devices do not support secondary trims for mapping.
Adjustments made to the mapping using the controls will be recorded as L8 metadata in the XML.
The Dolby Vision Metadata Trim Pass – Targets
It is highly recommended to check the mapping for a 100-nit, Rec709, Gamma 2.4 target for two reasons:
- The 100-nit target is the lowest that the Dolby Vision mapping will be required to map down to. From a colorist’s perspective, checking the 100-nit target is akin to checking the mapping for the worst-case scenario.
- Most studios only require the Dolby Vision master as the primary deliverable for their content and do not require a separate SDR version. In such cases, the SDR version is derived from the delivered Dolby Vision master. It therefore becomes the facility’s responsibility to ensure that the derived SDR matches the creative intent. A check and trim pass at a 100-nit, Rec709, Gamma 2.4 target ensures that the derived SDR meets the creative intent and expectations.
Studios may also request for an additional trim at a 600-nit, P3 (or Rec2020), PQ target. This is requested because:
- It gives the creative team a preview of the consumer experience, since the 600-nit/PQ target approximately matches the average Dolby Vision device that is available in the market today.
Additionally, checking the mapping at a PQ/HDR target in addition to the 100-nit/SDR target helps verify the playback experience on a wider range of display devices that the content may be potentially viewed on.
When performing target trims for multiple targets, it is always important to start with the lowest target before proceeding to a higher one. Lower nit target trims will interpolate to higher nit trims. Hence, if the request is for 100-nit and 600-nit target trims, the colorist must finish the 100-nit trim completely before starting on the 600-nit trim.
On Resolve when switching from a 100-nit trim to a 600-nit trim pass, the appropriate target must be selected in both the Target Display Output and the Trim Controls for fields. The Target Display Output switches the CMU output to the correct target and the Trim Controls for switches the trim control panel to the corresponding target.
Similarly, the monitor being used as the target display must also be set to match the Target Display Output setting.
You’ll see that shots that have 100-nit trims will have small (non-zero) trim values present in the 600-nit trim fields. These values are interpolated from the 100-nit trims and are created automatically by the system. The 600-nit trims can be further modified from this starting point or reset to zero before any adjustments are made.
Having completed the checks and corresponding trims for the requested targets, the Dolby Vision master and metadata are now ready for export and delivery.