LG OLED Matrix Gamut Calibration vs. 3D LUT: What is the difference between a 3x3 Matrix (with or without a 1D LUT), compared to a 3D LUT calibration?
The matrix calibration will not be able calibrate all available colors the display can accurately display because it will clip the display gamut to the peak colors, while a 3D LUT calibration will.
3D LUT Calibration
The 'clipping' is due to a 3x3 matrix defining a six sided 3 dimensional shape, with flat sides, while a 3D LUT will map each and every input color to a separate output color, without constraint, meaning each individual 'color' can be 'calibrated' without limitation or being restricted to peak primary colors.
The following 3D Cubes show the difference between the 3D LUT and 3x3 Matrix, with the Matrix unable to allow calibration outside of the calibrated primary colors.
3D LUT based Cube
Matrix based Cube
Originally Posted by Wjboshart
Regardless of the state of the autocal feature, I will say even ootb, these tvs are still very accurate. Basically only thing I’ve had to do is dial down excess blue in whiter than white. Don’t get me wrong, I would like to more accurately hit reference target points, but even just with balancing 2pt controls colors at max were dE 2.4 on my set
WRGB OLED's displays are NOT accurate volumetrically at all. It will require more advanced methodology from typical/limited patch-set and classic verification methods.
A great approach to assessing any display, to better understand a given display's underlying capabilities and issues that will affect potential calibration accuracy is to profile the native (un-calibrated display state) to itself, so measuring Primary colors and white point and generating a target color space with those values, a full volumetric profile of the display should map each and every measurement accurately to the target color space.
For LightSpace users, its relatively easy to perform, by profiling the display with a large cube based profile (with the display set to its native, un-calibrated setting), generating a new Color Space with the peak RGB & W values, as well as gamma, taken from the profile, and then generating a LUT with the Source as the new color space, and Destination as the actual profile.
The closer the LUT is to 'unity' (full cube with equaled spaced dots, when you will look the 3D Cube Graphics LUT Viewer) the better the underlying capabilities of the display.
For displays with inherent underlying non-linear inaccuracy, as shown above when a display is profiled to itself (and not directly associated with thermal instability, which requires the use of 'Stabilisation' patches to overcome), a very large profiling sequence will be required, in combination with a very large 3D calibration LUT to overcome the non-linear issues.
WRGB OLED technology will inherently suffer such issues, due to the inclusion of the 'white' pixel, as this will distort the standard RGB color channel relationship.
If you look at this image...
...you can see the way the ‘vectors’ change direction, and how multiple ‘input’ points all point towards the same actual color point.
(The colored dot is the measured color point, color coded for dE accuracy, while the ‘vector line’ shows where the point should actually be...
The output of the display is not linear and predictable when the input signal is changed in a linear and predictable way.
The causes display volumetric non-linearity.
In its simplest form, a displayed color is simply the sum of its components C= R+G+B.
If it is ‘linear’, then the combination is predictable in that changes in the input red signal has no effect on the light coming from the blue or green channel.
This is mostly true for old school CRT's, and for the most part LCD's and even LED's.
With a WOLED display, it has 4 emitters.
So, when the red signal is adjust, it will change the emission from the red LED, and may be also the white LED depending on the overall RGB signal that it is receiving.
This breaks the simple C= R+G+B rule.
The device now has a complex signal to light behavior.
This means the display is very non-linear in its response.
If you have a non-linear system, you need a full 3d model to make it do what you want it to. The greater the non-linearity, the bigger the size of the 3D LUT required to manage it.
(Actually, if you had direct access to RGB and W, then you would be best generating a 4 channel cube! This is often done internally in printers that have more than 3 inks; same basic idea.)
All above data coming from SDR profiling data, stuff are lot of worse in HDR mode, lets see about this below....
... by comparing WRGB OLED @ HDR mode (without Tone-Mapping active) 800nits peak vs. LCD @ HDR mode (without Tone-Mapping active) comparison will be used to better demonstrate the problem.
To be able to perform 21-Point Cube profiling in HDR mode (without tone-mapping activated), Maciej Koper (mkoper - hdtvpolska.com); popular and very expert calibrator/TV reviewer from Poland, performed a lot of measurements using LightSpace and testing the Stabilization feature, to improve panel stability over the time.
The following shows a volumetric comparison between WOLED and LCD displays when used for HDR, with a peak Luma of 700-800 nits, again with both displays profiled to themselves to provide relative data that is easy to compare.
These examples have been generated with special display assessment software used in-house by Light Illusion
The accuracy of HDR displays is something of a difficult area for many to assess, as the existing metrics used to define calibration accuracy are just not capable of showing true volumetric issues.
Therefore, measuring the RGB primaries, and white point, for any display, and generating a target color space with those values, a full volumetric profile of the display should map each and every measurement accurately to the target color space.
The 3D charts below are also color coded, with green measurements showing points that have a sub-1 dE. Orange points are between 1 and 2.3 dE. Red points are above 2.3 dE.
As can very easily be seen the WOLED has major volumetric issues when used for HDR, as the color gamut luma cannot match the grey scale luma, causing a clipping of RGB Separation.
The LCD display has relatively consistent volumetric accuracy throughout.
In the above 3D CIE graphs the LCD display shows a relatively good/acceptable level of underlying display volumetric capability, while the WOLED graph shows issues throughout, that become increasingly worse as display brightness increases.
The second set of graphs have dE tangent lines included, which show the dE error for each and every point. The difference is obvious.
The 'Cube graphs are 'normalized' versions of the CIE graphs, and help visualize the volumetric issue with any display. The WOLED graph shows just how much volumetric accuracy is missing.
And again, we can add dE tangent lines to help highlight the errors.
RGB Separation Charts
RGB Separation compares each primary R, G, and B patch of the same stimulus value (for example Red 128,0,0, Green 0,128,0, and Blue 0,0,128) to the equivalent grey scale patch (128,128,128), matching the individual RGB patch measured values to the expected color matrix combination for the equivalent grey patch.
Any error in the graph can show the display is suffering color decoupling issues with the display's separate RGB color channels, or can show that the color channels just can't match the grey scale luma levels, as in this case.
Again, looking at the above, a good display would profile to itself near perfectly... and would have an RGB Separation graph where the RGB plots follow perfectly the target line (all overlapping).
As can be seen, the LCD is 'ok', while not perfect, but the WOLED has major issues that are easily apparent, due to the inclusion of the 'white' pixel, as this distorts the standard RGB color channel relationship - excessively at HDR brightness levels. (if you sum the Y of 100% patch of R+G+B primaries you get 400nits while the same time if you display a 100% White patch you get 800nit...with WOLED's)
In very simple terms, what this means is the WOLED can never be calibrated for HDR... ...but calibrated with 3D LUT in SDR mode, the recommendation is up 105-110 nits, there will be to ABL limiting and displays are more stable overs the time.
All these issues has been ignored/missed from site/forum/you-tube TV reviewers, from calibration companies... while Hollywood clients ''which are very happy from their results