HLG Normalized Transform Manual

Preamble

The HLG Normalized Transform was originally developed to support Hybrid Log Gamma (HLG) source footage within an ACES workflow as existing solutions remain inadequate or incorrect. The transform was developed using the specifications outlined in ITU-R BT.2408 with a recommended 18% Middle Gray exposed at 38% IRE and the BT.2100 specification with a Middle Gray exposed at 21% IRE. The transform intersperses the normalization with the Hybrid Log Gamma inverse Opto-Electronic Transfer Function (OETF) as per the ITU-R BT.2100 specification. As of release 2.0, the Sony specific HLG implementations have now also been included and map the 18% Middle Gray at 19%, 20% and 21% IRE for HLG1, HLG2 and HLG3 respectively.

When the Normalized HLG equations are plotted on a 10-bit characteristic curve, the BT.2408 curve provides just under 4.5 stops of exposure above Middle Gray, whilst the Camera Native BT.2100 reference provides roughly 6 stops of exposure latitude above 18% Middle Gray at 21.2% IRE. A graph of the BT.2100, BT.2408, Sony HLG1, HLG2 and HLG3 characteristic curves is shown below for reference.

The following table documents the appropriate IRE values when over-exposing or under-exposing footage with Hybrid Log Gamma profiles aligned to BT.2408, relative to 18% Middle Gray at 38% IRE (37.8% IRE to be precise).

The following table documents the Camera Native IRE values when over-exposing or under-exposing footage with Hybrid Log Gamma profiles aligned to BT.2100, relative to 18% Middle Gray at 21.2% IRE.

Prerequisites

The DaVinci Color Transform Language (DCTL) files are compatible with DaVinci Resolve 16 and above. They are meant to be used as an Input Transform (IDT) for content recorded using HLG and are especially targeted for ACES workflows, though they can also be used in DaVinci YRGB Color Managed and Non-Color-Managed workflows. The DCTLs will work in DaVinci Resolve Studio and DaVinci Resolve Free. Note that the Free version of Resolve may display a watermark for anything outside of the ACES IDTs due to DCTL feature limitations.

Installation

The HLG Normalized Transform pack includes the following transforms:

The transforms are included in a LUT and IDT folder. The files must be copied to the designated folder hierarchy as described below. The ACES custom IDT & ODT feature was introduced in Resolve 17.

Windows

On Windows, the files within the LUT folder must be copied to the following directory:

C:\ProgramData\Blackmagic Design\DaVinci Resolve\Support\LUT\

If using Resolve 17 and above, the files within the IDT folder must be copied to the following directory:

%APPDATA%\Blackmagic Design\DaVinci Resolve\Support\ACES Transforms\IDT\

MacOS

On MacOS, the files within the LUT folder must be copied to the following directory:

/Library/Application Support/Blackmagic Design/DaVinci Resolve/LUT/

If using Resolve 17 and above, the files within the IDT folder must be copied to the following directory:

~/Library/Application Support/Blackmagic Design/DaVinci Resolve/ACES Transforms/IDT/

Note: the tilde (~) symbol expands to your home directory. For example: /Users/YourUsername/Library/Application Support/Blackmagic Design/DaVinci Resolve/ACES Transforms/IDT/

iPadOS

On iPadOS, the files within the LUT folder must be copied to the LUT directory under the DaVinci Resolve App folder hierarchy.

For Resolve 18.6.2 and higher, the files within the IDT folder must be copied to the following folder under the DaVinci Resolve App folder hierarchy:

ACES Transforms/IDT/

If these folders do not exist, they must be created manually.

Linux

On Linux, the files within the LUT folder must be copied to the following directory:

/home/resolve/LUT/

If using Resolve 17 and above, the files within the IDT folder must be copied to the following directory:

~/.local/share/DaVinciResolve/ACES Transforms/IDT/

Usage

As of Resolve 17, there are three main ways to use the HLG Normalized Transform:

1. As an ACES Input Transform (IDT)
2. As an Input DCTL
3. Through Resolve’s Node structure
   • Using the DCTL ResolveFX Plugin
   • Via the LUT Menu

ACES Input Transform Method

This feature set is only available in Resolve 17 and above. The HLG Transforms can be selected at the Project Level or via the ACES Transforms plugin. As of Resolve 17.2, the HLG ACES IDTs can also be selected at the individual clip level via the ACES Input Transform selection. This is now the preferred way of selecting the HLG IDTs for ACES.

Input DCTL Method

Prior to Resolve 17, this was the preferred method of loading an ACES IDT. This required bypassing any input IDT at the clip level and selecting the Resolve DCTL for each clip under the DaVinci CTL right-click sub-menu.

As of Resolve 17, the DaVinci CTL drop-down is no longer available and has instead been amalgamated into the LUT sub-menu. To facilitate using this method and to avoid confusion between the IDT and LUT/DCTL transforms, all DCTL files now end with a “DCTL” suffix.

Resolve’s DCTL Plugin Method

In order to use this method, the relevant DCTL will need to be selected through the DCTL OpenFX Plugin. Similar to the ACES DCTL method, any Input Color Space or ACES Input Transform will need to be explicitly disabled at the clip level. It’s also recommended to have this as the first node in the structure.

Via the LUT Menu

To use via the LUT drop-down, use the first node in Color page and either right click to select the DCTL thru the LUT drop down, or via the LUTs tab.

Usage Parameters

The default parameters allow the transforms to be used within an ACES workflow. Some use cases may require operation from within the DCTL OpenFX plugin in order to manipulate the available parameters/modifiers.

Note: Only the HLG Normalized DCTL has parameters that can be manipulated. All other included DCTL files are streamlined HLG transforms for use directly as an ACES Input Transform (IDT) through the ACEScc and ACEScct Color Science options.

The sections below will outline the parameters available and sample workflows.

Exposure Correction

The Exposure Correction parameter allows for exposure adjustment in Exposure Values (EV) from -8 to +8. The values can be manipulated using the slider or by inputting the EV directly in the designated field. Source footage that is either over-exposed or under-exposed can be corrected this way.

Custom IRE

The Custom IRE parameter allows the selection of Custom IRE values for footage that is exposed to non-standard IRE values. The values can be manipulated using the slider or by inputting the IRE value directly in the designated field. The default value is set to the HLG.BT2408 recommendation of 38% IRE (37.83% to be exact).

Highlight Rolloff

Tone mapping adjustment. Set to zero for no tone mapping or set to highest level for significant tone mapping.

Gamut Mapping

Gamut compression mapping is now enabled by default.

Desaturate

Toggle desaturation of colors. If enabled, saturation is reduced down to 85%.

Color Science

The selection of Color Science is important as it dictates Color Space and Gamma behavior within the transform. The default Color Science is DaVinci YRGB. When selecting ACES AP0 or ACES AP1 Color Space with ACEScc / ACEScct Color Science, a CAT02 Chromatic Adaptation is automatically applied to move from a D65 white point to a ACES ~D60 white point.

HLG Type

The HLG Type setting allows for the desired HLG implementation to be selected. The options are as follows:

• HLG BT.2100
• HLG BT.2408
• Sony HLG1
• Sony HLG2
• Sony HLG3
• iPhone HLG

The default selection is HLG BT.2408.

HLG Color Gamut

This setting allows for the desired Color Gamut to be selected based on different HLG settings. This has been specifically added for Sony’s HLG1 – HLG3 implementations. The options are Rec.709 and Rec.2020. The default option is Rec.2020 as per the HLG BT.2100 specification and should not be changed unless the HLG color gamut settings have been altered on camera.

Timeline Color Space

This setting will convert the source to the desired color space, normally aligning with the Timeline Color Space in Resolve. The default Color Space is Rec.709

The Color Space Options are:

• ACES AP0
• ACES AP1
• P3 (D65)
• Rec. 709 (D65)
• BT. 2020 (D65)
• DaVinci Wide Gamut

ACES AP1 is the color space associated with ACEScc, ACEScct and ACEScg. ACES AP0 consists of the ACES 2065-1 color primaries and is the default color space to allow for direct usage as an IDT within ACES workflows.

For ACES color spaces, the White Point is automatically selected based on the chosen Color Science. For ACEScc & ACEScct, the White Point is the ACES default, which is close to D60. For DaVinci YRGB & YRGB Color Managed, the default ACES White Point is D65 based on DaVinci Resolve’s current behavior.

Timeline Gamma

This setting will convert the source to the desired Gamma, normally aligning with the Timeline Gamma settings in Resolve.

The Gamma Options are:

• Linear
• ACEScc
• ACEScct
• ITU-R BT.709 (Scene)
• Gamma 2.2
• Gamma 2.4
• Gamma 2.6
• DaVinci Intermediate
• sRGB

The default Gamma is ITU-R BT.709

Clamping

Some Cameras (such as the Panasonic Lumix series) appear to exhibit negative values when a recording is encoded using HLG. The transform counteracts this by allowing for Clamping of values outside the 0 – 1 nominal range. The options are to Clamp All values outside this range, Clamp Negatives below zero or to disable clamping altogether. The default is to Clamp Negatives.

User Workflows

ACEScc or ACEScct

The Transform can be used directly within the ACEScc / ACEScct Color Science workflow as an IDT or through the Node structure for manipulation of additional parameters.

To use the transforms as an IDT in Resolve 17 and above, simply select the desired HLG variant from the ACES Input Transforms. The following video tutorial provides an overview on how to apply custom ACES Input Transforms:

If parameter manipulation is required in ACES, the HLG Normalized.dctle transform can be used in the Node structure through the DCTL OpenFX plugin. The transform’s Timeline Color Space MUST be set to ACES AP0 and the Timeline Gamma must be either ACEScc or ACEScct.

DaVinci YRGB & YRGB Color Managed

The following workflows only apply to the HLG Normalized DCTL. All other DCTL files are specific to the ACEScc/ACEScct Color Science workflows and should not be used here.

When working in DaVinci YRGB and YRGB Color Managed Color Science modes, it’s recommended to use the DCTL from within the Node Structure through Resolve’s DCTL OpenFX plugin. At the clip level, the Input Color Space should be set to “Same as timeline”. The Color Science settings within the DCTL should be set to DaVinci YRGB and ensure the DCTL Timeline Color Space and Timeline Gamma match the settings of the project appropriately. Where possible, any Input DRT should be disabled to avoid unwarranted Tone and Gamut mapping. In a Color Managed workflow, these parameters will be dependent on the project Timeline color settings. In Non-Color Managed workflow, these will be dependent on the Output color settings.

In Resolve 16 and below, unless otherwise modified, the default option is Rec.709 for Timeline Color Space and Gamma 2.4 for Timeline Gamma.

In Resolve 17 and above, the default option is Rec.709 for Timeline Color Space and either Gamma 2.4 or Scene for Timeline Gamma, depending on project settings.

If working in a Color Managed workflow, the recommendation is to use the Rec.2020 / DaVinci Intermediate (or DaVinci Wide Gamut / DaVinci Intermediate) preset and set Input DRT to None. This will ensure the best possible working space for the HLG HDR content, whether exporting to HDR or Rec.709.

ACES (DaVinci YRGB)

The recommended method to use the transform with ACES inside a YRGB Color Managed workflow is to set the Project Timeline Color Space as ACES and bypass the Input Color Space both in the Project settings and on the individual clips. As of Resolve 17, this is now titled “Same as timeline” which is not to be confused with “Bypass Color Management”. The transform can then be selected through Resolve’s DCTL OpenFX plugin. By default, the Color Space and Gamma are set to ACES and Linear respectively. The Color Science of the transform MUST be modified to DaVinci YRGB as the ACEScc/ACEScct Color Science parameters use the D60 ACES white point by default. Resolve uses a D65 white point when in the DaVinci YRGB & YRGB Color Managed modes – even if ACES is used. If the transform is selected as an IDT using DaVinci CTL (or LUT if using Resolve 17 or above), then a CAT02 Chromatic Adaption must also be applied in the Node structure to move from a D60 white point to a D65 white point.