The below article contains excerpts from The Display Calibration Guide.
Whether you are setting up HCFR for SDR or HDR calibration, there are three main places we need to do configuration in HCFR:
- In Preferences which can be accessed under the Advanced Menu —> Preferences option
- In Parameters under the Measures Menu —> Parameters option.
- In the Pattern Generator Configuration under the Measures —> Generator —> Configure Menu
Everything else can be accessed using right-click menus and the project screen. Let’s have a look at Preferences in this article.
Configuring Preferences for HDR10
For HDR 10, we will need to select UHDTV – Rec2020 from the dropdown. If you have a display with a limited gamut which can only reach P3, I recommend you select Rec2020/ P3 which will give you the P3 targets to aim for within the Rec2020 container.
Next, we will need to configure and EOTF (HDR) section and select SMPTE 2084 HDR, which is the PQ HDR10 curve.
We will need to configure the Target Curve we will be calibrating to. First we must tick the Override Targets option. When ticked, the following options become configurable:
- Diffuse White: This is normally set to 100nits (94.37844 to be more precise). It controls the scaling of the PQ curve (also called the multiplier) for lower-luminance displays by lowering this number. For example, we could set 50nits or 25nits for projectors. We could also increase this number for upto 200nits for bright-room viewing on flat panels (I can hear the purists explaining stop, but this is what some TV manufacturers do – with good reason – as the HDR10 standard designers didn’t think of it.)
- Target MinL: This controls the black level we are calibrating to. We could leave this as is which is fine. Alternatively, we can measure the black level of our display and enter the correct nit value. Or we could look up what others have measured for the display type we are calibrating. For example, we could enter 0.05 nits for high-end LCDs, 0.5 nits for low-end LCD, 0.005 for OLED and so on.
- Target MaxL: This is the maximum luminance your display is capable of reproducing. It is a good idea to measure a 100% white patch on your display in its HDR mode and then enter the number here.
By default, the target curve will hard cut content at the nit level you set for Target MaxL. To tone-map the low and high-end, you can select the BT.2390 tick box. When ticked, the following options also become configurable:
- NB Slope (Near Black Slope): This option only functions if Target MinL is something other than 0. It controls how quickly the tone curve comes out of black. The lower this is, the faster it comes out. The higher this is, the slower it comes out of black. Values between 0.1 and 3 are valid. 1 is the default.
- NW (Near White Slope -first number) this number controls the steepness (-4.5 to 4.5) of the tone curve.
- NW % (Near White Slope % – second number): It controls how early the slope starts. 0% starts it the earliest and will affect the curve earlier down while 50% limits the slope changes to the high-end.
You can monitor the tone curve as you are making changes which I described under the Monitoring HDR Tone Mapping Curve Target section further in this chapter. This allows you to play with the above options and see how they affect the target curve.
Next, let’s configure the actual HDR signal we will be calibrating. As I explained earlier, HDR10 signals can generally be mastered to 1000nits, 4000nits and 10,000 nits. Some displays will use a different tone-curve for each. However, they normally only allow you to configure the tone curve using one set of controls so you will need to decide what to optimise.
The following options are available to configure the HDR signal (and the corresponding patterns) then:
- Master MinL: the minimum luminance the signal should be able to reproduce. It is best to leave this at 0.
- Master MaxL: the maximum luminance the signal should be able to reproduce. You can set this to 1000 / 4000 / 10,000 or anything in between. But those are the most common.
- Content MaxL: the maximum luminance the content actually has. For our purposes, we should set this to the same number as Master MaxL.
- Frame Avg. MaxL: the average luminance the brightest scene in the content has. This is not really critical to set and won’t affect the tone-curve. Leave at default. The reason this is available as meta-data in HDR is so we understand whether tone-mapping will affect the scene with the most extreme brightness. The higher the Frame Avg. MaxL in the signal, the less tightly we should map highlights as general content can creep into that part of the tone curve. While it is not generally accepted as good mastering practice to have this above 400nits, some content creators have “abused” HDR and created content with very high average luminance. I am not aware of any display that takes this into consideration, but since it is available meta-data in an HDR stream, they could in the future.
HCFR Tone Curve Adjustment
There are some additional steps we will need to configure in HCFR to adjust the tone curve for HDR10.
Click Override Targets in the Preferences and then let’s adjust the following dependent on whether you have a flat screen TV or a projector:
Flat Screen TV
Leave Diffuse White as is and set Target MaxL to the maximum luminance for the display.
I would advise profiling the display for the EOTF later on and then only after that adjust the BT2390 tone mapping options (see further on re how to adjust and monitor the tone curve).
Since the tone curve might not be very configurable on the TV, you may very well be at stuck with the manufacturer’s intention for their HDR10 tone map.
However, the most important thing is that the flat panel is following the EOTF closely upto about 300 nits after which there might be tone-mapping at play that is more difficult to configure and adjust. We will look at some ways of doing this when we get to EOTF calibration.
For projectors, we will need to input both Diffuse White and Target MaxL. To do this, we have two options:
- Go into HCFR Measure Tab (our project view) and select 10-step greyscale measures window. Diffuse white is at 50%. Click on the 50% column to measure it. Then put the measured Y value into the Diffuse White box.
- Another place where a diffuse white measurement can be called up is on a Primary Saturations Measures run by changing the drop down in Measures. Click on the White column and select Measure. We will enter the measured Y value into the Diffuse White box in preferences. (While doing this, make sure that 100% pattern intensity is selected in the Pattern Generator if you are using automatic patterns).
As you configure the EOTF during calibration, you may decide to raise diffuse white and therefore make the tone-curve brighter.
Then to measure Target MaxL, we will go into Greyscale Measures, click on 100% White and select Measure. We will then enter the measured Y value into the Target MaxL box in preferences.
If you wanted to measure 0% luminance (black) and enter this into Target MinL box in preferences, you could do this. However, we will adjust near-black tone-mapping after calibration. But I want to bring your attention to this option as well in case you wanted to try both.
You could also select BT.2390 and adjust the amount of tone- mapping for the curve near black (NB) and near white (NW).
Monitoring The HDR Tone Mapping Target Curve
I want to show you a bit of a trick on how you can monitor the changes you are making to the HDR Tone Curve in the References as you are configuring the different options.
We will need to bring up the Luminance graph shown below. To do this, we can either right-click in the tab area at the very bottom and select Luminance or you can also use the icons at the top or the menu to do the same.
Since the default white line is pretty difficult to see in my opinion, right-click on the graph and select Graph Parameters. This will bring up the little dialogue box on the right.
In the Graph Preferences dialogue box, select Reference from the Graph name drop-down and then configure a more contrasty colour under Parameters. You can also change line thickness, etc. I simply selected a burnt red colour here.
There are lots of other preferences here you can play around with. But it might be best to discover these once you have a measurement line in the diagram as well (once you measured greyscale which also measures luminance and gamma).
Now as you change the HDR Parameters in Preferences, this graph will change to show you the HDR Tone Mapping Curve Target you will use to calibrate the display.
The Display Calibration Guide
If you would like to learn more about displays, and display calibration, you can get The Display calibration Guide here.