Display Brightness Variations

One of the biggest issues, specifically with PQ based HDR although HLG can be affected too, is screen brightness/luminance variations, due to a range of associated problems with the way HDR works on most displays.

The basic issue is HDR can 'change' the screen/image brightness/luminance in ways that cause fundamental variations in the viewed image, potentially in ways that distort the original artistic intent of the graded footage, as defined by the film's director and colourist.

Such issues can be defined as part of the 'expected' HDR workflow, such as Dynamic metadata, technical limitations with the display technology used such as ABL (Auto Brightness Limiting) and Local Dimming, or unexpected brightness/luminance changes due to incorrect implementation within the display, specifically home TVs, where the display deviates from the expected HDR standard, due to the manufacturers believing they are generating a 'better' final image.

Dynamic Metadata

The use of metadata to dynamically define the brightness of the displayed image is something that is marketed by many HDR aficionados as a real benefit of PQ based HD, enabling dark scenes to be 'brightened', and bright scenes to be darkened to preserve hight detail.

But, is that really good?

When a film is graded, the 'look' is used to help define emotion, setting the viewer's expectation as to what the director is attempting to portray. Messing with the brightness dynamically risks upsetting the planned look, and hence destroying the original artistic intent of the film/program.

While Dynamic metadata is intended to be defined by the colourist/director via a secondary grading pass, inherent limitations in the visual perception of the process mean it is unlikely that the same visual intent will be maintained.

In reality, as nominal diffuse white is defined to be approximately 100 nits, with just spectral highlight information beyond that level, the inherent visual intent of the image will be contained below the 100 nits level, meaning that for correctly graded HDR, the theoretical 'best' approach to displaying the image on a lower peak luminance display would be to simply clip at the display's maximum luminance, potentially with the use of roll-off to prevent highlight 'blocking', with no use of dynamic metadata at all.

And as mentioned previously, the increase in achievable peak brightness levels for many displays and TVs also negates the need for any metadata, dynamic or static.

ABL

Another of the often overlooked potential issues with HDR has to do with the (legal) need to limit the power requirement of the display, as obviously extreme brightness causes excessive power consumption. That in itself is a cause for concern, based both on the power costs, and potential environmental issues. Hopefully, both those can be overcome with more efficient display back-lighting technologies.

However, in an attempt to overcome extreme power requirements, just about all HDR displays use one form or another of ABL (Auto Brightness Limiting - often called Power Limiting in HDR terminology). In very simple terms ABL reduces the power to the screen dependant on the percentage screen area that goes over a predetermined brightness level, so reducing the overall brightness of the scene. The PQ HDR specification defines what is known as MaxCLL (Maximum Content Light Level) and MaxFALL (Maximum Frame-Average Light Level) which are intended to be part of the HDR mastering metadata, from which the viewing display will calculate how to show the image, limiting potentially high power requirements.

Obviously, this causes the same image to be viewed differently on different displays, with different shots of the same scene, with different framing, to also be seen differently on the same display as the average picture brightness level will be different depending on the shot framing, potentially causing different power limiting to be applied by the display in an almost perceptually random way.

Such variations cause serious issues with accurate display calibration and image playback.

Local Dimming

Local Dimming is used in LCD based HDR displays, and consists of an array of back-lights to provide localised 'bright' image areas, without having to have a single backlight that is always 'bright', as that would greatly lift the black level, so greatly compromising the display.

A 'partial' solution is to divide the backlight into multiple zones, that can be independently controlled based on the image content, so 'dimming' the backlight areas/zones with dark content, compared to areas with bright content.

The obvious issue with this approach is that the backlight areas/zones will have a defined size/position, so will cause light bleed, or 'clouding', around objects that demand a bright backlight area/zone.

The greater the number of backlight areas/zones, the less visible the clouding issue.

Some newer LCD displays have what is effectively a backlight per pixel, such as the new Eizo Prominence CG3145, and FSI's XM310K, totally overcoming the Local Dimming issue.

OLED displays inherently have a backlight per pixel, as each pixel is self illuminating, but cannot reach the high peak luminance levels of LCD displays.

Deviation from the HDR Specification

A final issue with a lot of displays, specifically home TVs, is the manufacturers deliberately deviating from the HDR specification, in an attempt to generate what they view as 'better' images.

This obviously means the same source footage will be seen very differently on different displays, even if the displays are defined as being 'calibrated'.

However, this issue is actually something we have sympathy for, because as mentioned previously above, the PQ HDR specification is flawed, as the standard is 'absolute', and includes no option to increase the display's light output to overcome surrounding room light levels. The result is that in less than ideal viewing environments, where the surrounding room brightness level is relatively high, the bulk of the HDR image will appear very dark, with shadow detail potentially becoming very difficult to see.

Many home TV manufacturers therefore deliberately 'distort' the PQ HDR EOTF (gamma curve) to attempt to overcome this issue.

WCG - Wide Colour Gamut

As part of the evolving UHDTV standard, WCG is being combined with HDR to add greater differentiation from the existing HDTV standards, using the Rec2020 colour gamut as the target colour space.

The problem is that no (realistically) commercially available display can achieve Rec2020, meaning different UHDTV displays will have to 'adjust' the displayed image gamut based on the actual gamut capabilities of the display. This is provided for by the use of embedded metadata within the UHDTV signal (associated with HDR metadata, mentioned above) defining the source image gamut, aiming to allow the display to 'intelligently' re-map to the available gamut of the display.

The issue is that once again, and as with HDR metadata and peak luma clipping, there is no set gamut re-mapping technique proposed. The result is that different displays will manage the required gamut re-mapping in different ways, generating differing end image results.

The above image shows the issue with attempting to display a wide gamut on a display with a smaller gamut. In this instance the display has a gamut similar to, but not identical to, DCI-P3, which is the stated 'preference' for smallest gamut for UHDTV displays (the smaller internal gamut triangle), while the larger gamut triangle shows Rec2020.

The display has been calibrated to Rec2020, within the constraints of its available gamut, as shown by the gamut sweep plots (the measured crosses match with the target circles). However, the de-saturated area outside the display's available gamut, and within Rec2020, shows colours that will not be displayed correctly, with any colour within this area being effectively pulled-back to the gamut edge of the display.

Obviously, the wider the display's actual gamut capability the less the clipping, and the less the different gamut capability will be visible, especially as within the real world that are few colours that get anywhere near the edges of Rec2020 gamut.

To reduce the harshness of gamut clipping, gamut re-mapping can be used to 'soften' the crossover from in-gamut, to out-of-gamut.

In the above diagram, the area between the new, smaller inner triangle, and the actual display gamut triangle shows an area where the display calibration is 'rolled-off' to better preserve image colour detail, at the cost of colour inaccuracy, effectively compressing all the colours in the de-saturated area into the smaller area between the display's max gamut and the reduced gamut inner triangle.

In reality, gamut re-mapping needs to be far more complex, taking into account the fact that human colour perception reacts differently to different colours, so the re-mapping really needs to take this into account.

The problem is that the UHDTV specifications do not specify the gamut re-mapping to use.

However, from this it can be seen that in the real world no two Ultra HD displays will ever look the same when displaying the same images...

Additionally, the Ultra HD specification, while using Rec2020 as the target (envelope) colours space, actually specifies that any Ultra HD display only has to reach 90% of DCI-P3 to be accepted as a UHDTV display - and a volumetrically, 90% of DCI-P3 is basically Rec709!

The above CIEuv diagram (CIEuv has been used as it is more perceptually uniform than CIExy) shows the gamut difference between 100% DCI-P3 and Rec709, as well as showing Rec2020.

As can be seen, 90% of the DCI-P3 colour space is not much larger than Rec709...

Issues specifying Percentage Gamut Coverage

A real issue with the way the UHDTV specification has been defined for gamut is that it uses a generic percentage value of 90% coverage of the P3 Gamut, but P3 gamut primaries are not aligned with Rec2020 primaries, which can mean a display with a lesser gamut coverage value may actually be better than one with a higher value, if the display with the lower gamut coverage has primaries that are aligned better with Rec2020 promaries.

The above chart shows the problem, with an (approx.) 90% P3 gamut coverage compared to Rec2020 primaries. As can be seen, the peak primaries of the 90% P3 Green and Red are significantly different to Rec2020 primaries, meaning that colours that should be along the Rec2020 primary vectors will be distorted, making the true gamut coverage of the display significantly below the stated 90% coverage.

Colour Perception

And to end, a question regarding colour perception, for those of you Home Cinema enthusiasts...

You watch a new film release in the cinema, in digital projection, using a DCI-XYZ colour space envelope for projection, containing DCI-P3 imagery.

You then purchase the same film on Bluray, and watch it on your Rec709/BT1886 calibrated Home Cinema environment.

Do you perceive any loss in image colour fidelity, assuming the Bluray master has been generated correctly?

The reality is there are few colours in the natural world that exist outside of the Rec709/BT1886 gamut. Colours that do exist outside Rec709/BT1886 gamut tend to be man-made colours, such as neon signs, and the like...

UHD - Resolution

Another component of UHD is the increase in resolution to 4K (3840x2160).

While at first glance such an increase in resolution would appear to be a real benefit of UHDTV, it actually brings with it the question 'can the the benefits really be appreciated?'