Dolby Vision let creatives use dynamic metadata to tone-map their HDR grade to other targets like SDR. This automated process can be manually adjusted and advanced by the use of Dolby vision Trims. In this article we want to explore the Dolby Vision Trim controls.

We will cover the following topics:

• CMv2.9 L2 trim controls
• CMv4.0 L8 Trim controls

 

 

CMv2.9 L2 trim controls

For CMv2.9, there are six controls that can be used to modify the mapped image. These values are stored as Level 2 metadata.

Lift, Gamma and Gain are the most common controls used to modify the shadows, mid tones and highlights of the image. In CMv2.9, 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.

 

Note:

Tone Detail Trim is disabled for the Rec.709, 100-nit trim.

 

CMv4.0 L8 Trim controls

For CMv4.0, there are a total of 21 trim controls that can be used to modify the mapped image. The trim controls are in the table below and stored as Level 8 metadata. Lift, Gamma and Gain trim controls have been improved in CMv4.0 and are very similar to traditional Lift, Gamma, Gain controls that colorists are used to.

The extended toolset of trim controls in Cmv4 lets colorists refine and modify the metadata with increased precision and flexibility.

 

CMv4.0 Trim Controls Improvements

CMv4.0 introduces a new and improved way to create Dolby Vision content using color correction systems that have been updated to support CMv4.0.

For additional information about Dolby Vision solutions from our partners, select the Solutions tab at https://www.dolby.com/us/en/technologies/dolby-vision/dolby-vision-for-creative-professionals.html

This section describes the new tools and improvements of CMv4.0 using the implementation in Blackmagic Resolve v15.x as a reference.

 

CMv4.0 Global Mid Tone Offset Trim Control Settings

This control is useful for matching the overall exposure of the mapped SDR signal to the HDR master or to an SDR reference. It acts as 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.

The following figure shows the global Mid Tone Offset controls in Resolve.


 

CMv4.0 Primary trim controls

The Primary trim controls are recorded as L8 metadata for each shot or frame of the project. 
The following figure shows primary trim controls in Resolve.


 

Lift/Gamma/Gain

In CMv4.0, the feel and response of these controls have been greatly improved over the previous version. These trims are designed to work like traditional lift, gamma and gain controls in the main color correction panel. These three controls modify the shadows, mid tones, and highlight areas of the mapped image.
 

Note:

The same positive lift guidelines described above also applies to Dolby Vision version 4.0.

 

Saturation Gain

Saturation Gain trim enables colorists to adjust the overall saturation of the mapped image.

 

Note:

This 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

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.

 

Note:

If large Chroma Weight adjustments are made at 100 nits, we recommend checking the mapping at 600 nit and adjust Chroma Weight as required.

 

Tone Detail

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

New to CMv4.0, the Mid Contrast Bias trim compresses and stretches the image around the mid tone region and can increase or decrease contrast in the mid tones of the mapped image. For more precise results, use Mid Contrast Bias adjustments with Lift or Gain.

 

Highlight Clipping

New to CMv4.0, the Highlight Clipping trim reduces or limits details in the highlights by clipping them as required, especially 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.
 

Note: 

This trim can be very useful when trying to match the mapped SDR to an existing SDR Reference.

 

CMv4.0 Secondary Trim Controls

Secondary trim controls are recorded as L8 metadata for each shot or frame of the project and enables per-color channel saturation and hue adjustments per target.
The following figure shows secondary trim controls in Resolve.

 

Secondary 6-Vector Saturation Trim Controls

The six 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.

 

Secondary 6-Vector Hue Trim Controls

The six 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 especially useful when trying to fit/shift the WCG (Wide Color Gamut) of P3 or Rec 2020 into the limited color gamut of Rec 709/SDR.

 

Note:

Secondary Trims are only used to generate the SDR version and are not backwards compatible with version 2.9.