UHD / HDR / WCG Calibration

The calibration of UHD displays, with HDR and WCG.

The calibration of UHDTVs, specifically those with HDR and WCG (High Dynamic Range and Wide Colour Gamut) capability, is presently what is know as a 'Crap Shoot'... For the real videophile there is presently no truly accurate way to calibrate a UHD/HDR/WCG TV, although we are working with companies such as Lumagen to change this situation.

Most 'professional' UHD/HDR/WCG grading displays can be calibrated, as they have internal 3D LUT calibration, and are integrated with LightSpace. Such displays include the Dolby PRM series, and the Canon HDR displays.

The following outlines the present calibration possibilities for both consumer UHD/HDR/WCG TVs, as well as professional monitors.


UHDTV

Guesswork Calibration

UHDTV (Ultra High Definition TV) has had something of a difficult birth, with different display manufacturers effectively defining their own 'Ultra HD' specifications.

In response, the UHD Alliance has released a definitive (for now) Ultra HD specification, linking together all the display parameters required to be accepted as UHDTV, although the individual aspects of the UHDTV specification can, and often are, used in isolation.

There is nothing to stop a standard gamut display (Rec709), with standard HD or even SD resolution, working with HDR for example.

For the sake of simplicity within this guide, we will assume that UHDTV means UHD 4K resolution, ST2084 HDR gamma (EOTF), and WCG Rec2020 colour gamut. HLG and Philips HDR will be added at a later date, when relevant (when TVs with HLG/Philips HDR are available, and sources mastered in those standards are available).


HDR UHDTV (Home TV) User Calibration

User calibration of home UHDTVs (with PQ based HDR and WCG as defined above) is actually something of an oxymoron...

For most videophiles, home cinema calibration means taking control of all aspects of the display's image path, enabling precise control to be attained over all the display's colourimetry. Often, this means using 3D LUT based calibration, as this is the pinnacle of all calibration techniques.

With non-HDR UHDTVs (SDR Rec709 TVs), external LUT boxes, such as the eeColor, the Lumagen Radiance, and Murideo Prisma, as well as the software based madVR playback system, can be use to very accurately calibrate connected displays.

Direct internal 3D LUT based calibration of home TVs is not an option as few TV manufacturers offer in-built 3D LUT capability... This lack of in-built 3D LUT capability also extends to new HDR UHDTVs.

However, the issue with HDR UHDTVs at the moment is there are few viable ways to calibrate them with 3D LUTs, as even though LUT boxes, such as the Lumagen Radiance Pro, can work with UHD/HDR/WCG images, most HDR UHDTVs have their factory presets fixed, and cannot have them disabled, while still maintaining HDR compatibility. This makes secondary calibration impossible at the moment.

We are working on this situation with various TV manufacturers, and Lumagen, but this is a work-in-progress, and as yet any results are a long way off.

COMPLAIN TO YOUR HDR TV SET MANUFACTURER AND DEMAND THEY ADDRESS THESE ISSUES!
Light Illusion will be delighted to assist any TV manufacturers that wish to address correct UHD/HDR/WCG calibration.


Manual HDR UHDTV Calibration - Overview

For home HDR UHDTVs, the lack of viable 3D LUT calibration capability leaves us with Manual Calibration...

The problem with manual display calibration is that few displays (including traditional SDR Rec709 displays) are designed to calibrate accurately through the use of the provided manual controls. This can be due to poor internal image processing electronics, with poor colour decoupling (RGB cross-colour contamination) for example, which causes the controls to not work as expected, or due to simple poor implementation of the display's CMS (Colour Management System).

This is why true videophiles use 3D LUT boxes for SDR Rec709 display calibration accuracy, as this negates all the display's internal design issues, and produced the best possible final calibration.

And HDR UHDTVs are no exception. They are manufactured by the same manufacturers after all...

So, as there are no viable ways to calibrate home HDR UHDTVs via 3D LUTs, we are limited by the level of accuracy in-built into the display by the manufacturer. But, what exactly does this mean for HDR UHDTV calibration?
(The issue is that at the present time there are no home HDR TVs with 3D LUT calibration capabilities, and most do not allow for their in-built EOTF to be disabled, making external 3D LUT Box calibration unusable...)

With non-HDR/UHD TVs (SDR Rec709 TVs) calibration is based on matching the TV as closely as possible to a given standard, such as Rec709 with a power law or BT1886 gamma. With HDR UHDTVs the issue is that the target WCG (Wide Colour Gamut) colour space is Rec2020, which no TV can attain. This means the TV cannot be calibrated directly to the standard, as only a small part of the Rec2020 gamut will be covered by the display. Additionally, the ST2084 HDR Gamma (EOTF - Electro Optical Transfer Function) is not 'relative', as it is a nits based 'absolute' standard, based on a 10000 nit maximum luminance standard. This means different displays with different peak luminance levels will use a different 'portion' of the full ST2084 EOTF.

The result is this is less about calibration standards, and more about the process the display manufacturer enables any user to adopt for manual calibration. There is really no 'calibration' being performed, as the process used just verifies display setting, such as maximum luminance, black/white points of the display, and the peak RGB gamut values, and then hopes the display's own processing will correctly place the volumetric colours where they should be within the target colour space.

This is the basic reality of HDR UHDTV calibration. NON-EXISTENT!

HDR-10, Dolby Vision, and HLG
At the moment there are three competing HDR standards - HDR-10, Dolby Vision, and HLG (Hybrid Log-Gamma). As Dolby Vision is a proprietary format, and requires all elements in the image chain to be Dolby licensed, we will focus on the open (and presently far more popular) HDR-10 standard. HLG is as yet not widely deployed, but tis will probably change in the near future.

LG TV HDR Setup

As an example of the extremely limited HDR calibration offered by TV manufacturers for UHD/HDR/WCG, here is LG's documentation for UHD Alliance 'Greyscale Tracking'.

As can be seen this guide has no 'colour' calibration component at all - it is just setting the greyscale (as the document title states), and setting the white point to D65.

The TV makes the assumption that all UHD material is graded to P3 primaries (which is presently true, although there is no guarantee to that fact, and in reality is an incorrect step, as all UHD consumer sources have the material mapped into Rec2020 gamut as the delivery container!), and then assumes that the internal colour management of the TV is good enough to correctly map all volumetric colour space accurately.

And as all Home Cinema enthusiasts know, no TV manufacturers ever gets that right with SDR TVs. So why should UHD/HDR/WCG be any different?


LG HDR Calibration

Vizio Dolby Vision Setup

Another example of the extremely limited HDR calibration offered by TV manufacturers can be seen here with Vizio's Dolby Vision Calibration (setup!) Guide.

Click the image to the right to download the PDF guide.

The guide describes the use of Calman to again perform nothing more that a 'setup' of the display through greyscale measurements, and a validation that the display's peak colour values matched against the 'Golden Reference' file for the Vizio display.

The 'Golden Reference' file simply contains the expected maximum primary and secondary colour gamut plots, and the 'calibration' process is simply to match the displays (if possible) to these targets, and then assumes that the internal colour management of the TV is good enough to correctly map all volumetric colour space accurately.

Again, as all Home Cinema enthusiasts know, no TV manufacturers ever gets that right with SDR TVs. So why should UHD/HDR/WCG be any different?


This is why this sub-section is called HDR & Dolby Vision 'Setup', as it really can't be called 'Calibration'...

HDR-10 Profiling & Calibration
HDR-10 Calibration

Understanding the above, the first stage for HDR-10 calibration is about profiling the display to assess its present level of accuracy, and involves using an HDR-10 pattern generator connected to the LightSpace CMS laptop to inject the correct meta-data into the signal chain, which triggers the display's HDR and WCG UHDTV capabilities. Without the correct meta-data in the HDMI signal path most home HDR UHDTVs will default to non-HDR/WCG operation, so will not profile as expected.

If the TV/display can be manually switched into HDR mode there will be no need for a special HDR signal generator - any patch generator will work as required.

Ideal pattern generators for this include the Murideo SIX-G and Radiance Pro as they will both act as a patch generator, and insert the required HDR-10 meta-data into the signal path.

HDR-10 Calibration

HDfury

An alternative HDR metadata insertion is the HDfury Integral4K60, which can be used within the HDMI signal path from the LightSpace CMS laptop to the HDR UHDTV, injecting the required meta-data into the signal path.

Using the HDfury Integral4K60 makes use of the free, in-built Light Space CMS Patch Generator for direct display profiling, using the HDfury's own control program to pre-set the required HDR settings.

With this configuration any HDR-10 display can be profiled with LightSpace CMS.

With the Murideo SIX-G or Radiance Pro correctly configured for HDR metadata, and connected both to the LightSpace CMS laptop and the HDR-10 display to be profiled, put up a small size (approx. 10% by area) patch window using the 'Upload' menu, and while displaying 100% white using the Manual Measure mode of LightSpace CMS record the peak white value.

The measured peak white value needs to be used to generate and save a new ST2084_Rec2020 colour space for this specific HDR-10 display, using this maximum peak luminance value as the clip point for the display. Name the new colour space something you will easily remember!

Note: the Nits value in the ST2084 colour space can be set to any value, although matching the peak of the display as measured makes logical sense.

ABL and ASBL

Note: When doing manual HDR measurements do not leave any patch static of the display for any extended period of time, as most HDR TVs have aggressive ABL (Auto Brightness Limiting, or Average Brightness Limiting, depending on who you talk to) and ASBL (Auto Static Brightness Limiting, or Average Static Brightness Limiting).

ABL will immediately reduce the display's peak luminance output when the average brightness of the displayed image is high, which is why small (10% by area) patches are used for profiling.

ASBL reduces the displays luminance output when a static image is seen for more than a few seconds.

Both these effects are required for HDR displays to reduce power consumption (to required legal levels in many countries), and to protect the display from overheating, as well as screen burn-out.

The 'EOTF Nits' option can be used to add a 'Soft Roll Off' (SR) or 'Tone Mapping' to the new ST2084 Colour Space, which should NOT be used for Grading Displays, as when grading it is imperative that the display 'clips' at its max peak value. Soft Roll Off is only viable for end user Home HDR TVs, and when performing on-set viewing of HDR footage, for example.

The 'Soft roll off Limit' (Nits) is based on the source material to be viewed, as the aim is to roll-off to the point at which there is no further detail in the source footage. Using a lower value will clip information unnecessarily, and using a higher value will generate flat white spots in the viewed image. The 'Soft roll off Start' is based on the display's peak luma value (as entered into the 'nits' box), and is defined as a percentage point before the max nits clip point of the TV being calibrated, and should be set to generate the required roll-off curve.

Note: The configuration of 'Soft Roll Off' is something that can drastically alter the perception of any HDR display, as different roll off values are required for the different peak luma values used when grading source material. One size will not fit all, due to the issues outlined above. This is one of the problems with HDR TVs without the ability to alter the EOTF - they are attempting to use one Roll Off setting for all source material.

In the example below the HDR display to be profiled has a max luminance of 1108.4 nits (set to be a 1100 nits target) and a 0.0018 min, with a source material (mastering) peak luminance of 2000 nits, and roll off starting at 90% of 1100 nits, with no multiplier in use.

Note: The 'Load' buttons can be user to load into the ST2084 Parameters widow the Min and Max Luma values from the 'Destination' colour space in use.

HDR-10 Calibration

Note: The 'BT2390-1 'Tone Mapping' option use the BT2390-1 specification for roll-off, and has no user user parameters, other than setting the display's specific max and min luma values.

The 'Multiplier' for projection HDR re-scales the EOTF nits value, based on the Multiplier value used. For example, a value of x10 will effectively change an EOTF peak nits value of 97 nits into a perceived level of 970 nits. A value of x1, as shown above will have no effect on the ST2084 scaling.

As the name suggests, this function is used for projection based HDR, as while there is no projection HDR standard, the 'trick' used (including for Dolby Projection HDR) is to use a x10 multiplier to enable an HDR image to be viewed on a display with a much lower peak luminance output.

When a Multiplier value is used it will 'multiply' the value already in the EOTF nits box by the Multiplier value, enabling a new saved ST2084 Colour space work to correctly when used to compare a 'Multiplier' based display profile to it.

An example application would be to enable a projector that has a peak nits value of only 56 nits to 'mimic' HDR operation through the use of a x10 multiplier, which will re-scale the peak EOTF value to 560 nits, as shown here, this time using the BT2390-1 Tone Mapping option.

HDR Multiplier

When a Multiplier value is used it will 'multiply' the value already in the EOTF nits box by the Multiplier value, to allow the new 'perceived' peak luma value to be used for calibration.

When calibrating an HDR ST2084 display you will need to 'save' a new ST2084 Colour Space with the correct 'Nits' value (and Multiplier if used) for the display being calibrated, otherwise the the Gamma profile will not have the correct target to map to, as the default standard is always referenced to 10000 nits!

Note: When verifying the calibration of a home HDR TV it may be better to not use a Soft Roll Off value when defining the new ST2084 colour space reference, and instead use a hard clip, as this will better define the underlying capabilities of the TV.

HDR Hard Clip

Gamma

HDR Roll-Off

Gamma

HDR & Soft Roll Off/Tone Mapping

LightSpace CMS provides two options for Soft Roll-Off/Tone Mapping, with the Soft Roll-Off option being a Light Illusion derived process, with extended user controls, while the BT2390-1 Tone Mapping option uses fixed parameters, based on ICtCp colour processing.

With the Light Illusion 'Soft Roll-Off' option the 'Upper Signal Limit' (Nits) value is based on the source material to be viewed (The Mastering peak luma value - 2000 nits in this example), as the aim is to roll-off to the point at which there is no further detail in the source footage. Using a lower value will clip information unnecessarily, and using a higher value will generate flat white spots in the viewed image. The 'Upper Start' (%) is based on the display's peak luma value (as entered into the 'EOTF Nits' box), and is defined as a percentage point before the max nits clip point of the TV being calibrated, and should be set to generate the required roll-off curve. The 'Screen Min Luminance' (Nits) can be used to apply a soft roll-off to the blacks, with the 'Load' button loading the vale from the profile held within the 'Destination' colour space (if set to zero no black roll-off will be applied).

The BT2390-1 Tone Mapping option uses fixed parameters, as defined by BT2390-1, and is far more restrictive than the Light Illusion 'Soft Roll-Off' option as the Max Luma roll-off starts at 50% of the 'Screen Max Luminance' (Nits) value.

Obviously, for aesthetic viewing any desired values can be used, and again show the issue with the HDR concept, as often what are really 'inaccurate' values generate more pleasing results - assuming the EOTF can be user varied!

Note: The configuration of 'Soft Roll Off' is something that can drastically alter the perception of any HDR display, as different roll off values are required for the different peak luma values used when grading source material. One size will not fit all, due to the issues outlined above. This is one of the problems with HDR TVs without the ability to alter the EOTF - they are attempting to use one Roll Off setting for all source material.

Note: If the viewing display matches, or exceeds, the peak luma of the mastering display there is no need for any soft roll-off/tone mapping, and it should not be used.

With the desired new ST2084 Colour Space defined and saved (with a Multiplier value if used), select the desired profiling option from within LightSpace, using the 'Calibration Interface'.

This should be a selection of different Quick Profiles, including Gamut Sweeps and Memory Colours, to enable assessment of different profile reports. While a full Display Characterisation can be used, the fact the HDR-10 displays will not be capable of fully matching Rec2020 means the profile results will not accurately depict the display's 'relative' or 'emulated' gamut calibration.

Run the desired profiles, and then assess the results via the 'Colour Space Library', using the various graphs to compare the measured profile data with the new ST2084_Rec2020 colour space previously made and saved (with Multiplier if used) for this specific HDR-10 display.

In the example below is a HDR projector, with a 79 nit peak output, matched to a new ST2084 colour space standard pre-set, with a 79 x10 Multiplier peak nits Luma value. This shows the projector default HDR EOTF is not as good as it could be, and has no roll-off, which could enhance the final perception of picture quality, if applied to the EOTF.

HDR Gamma


HLG Calibration

HLG RGB Separation

Calibration of HDR displays to the BBC's HLG standard is a far simpler process, as the approach is basically identical to standard SDR calibration, with no metadata requirement, and as it is a relative standard, rather than PQ's absolute standard, it effectively self scales to any display's luminance capabilities.

The one major difference is the standard has built into it a compensation for variable system gamma.

The standard first calculates the luminance of the source (before system gamma) using a weighted sum of the RGB components, as normal. The destination luminance is calculated by applying a pure mathematical gamma function to the source luminance, with the RGB channels scaled by the ratio of the source to destination luminances.

This introduces colour cross coupling, as will be seen via the RGB Separation graph, so don't be surprised when you see such results post-calibration.

Also, as for PQ HDR, the preset colour space must be adjusted to match the peak luma of the display being calibrated, using the 'System Gamma' button, as well as the desired Surround Luminance value set, and the new System Gamma calculated via the 'Calculate' button.

BBC HLG Calibration

When calibrating an HLG display you will need to 'save' a new HLG Colour Space with the correct 'Nits' value and 'Surround Luminance' for the display being calibrated, otherwise the the Gamma profile will not have the correct target to map to as the default setting is referenced to 1000 nits max luminance and 10 nits surround.

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