With displays that prove very difficult to calibrate using the normal volumetric approach, it may be necessary to adopt a Matrix based approach to Profiling and LUT Generation.
This can be especially useful for displays that suffer stability issues such as extreme heat soak related drift.
Matrix Profiling & Calibration
With displays that prove very difficult to calibrate using the normal volumetric approach, it may be necessary to adopt a Matrix based approach to Profiling and LUT Generation, reducing the number of patches used for profiling to a minimum, but still taking some advantage of LUT application.
This can be especially useful for displays that suffer with extreme heat soak related drift, where even Pre-roll, Stabilisation and Drift patches are unable to tame the monitor, as well as where the 3D LUT implementation is of limited size, and potentially causes banding or other artefacts.
The idea is to profile with a minimum of 7 colour patches - Black, White, mid Grey, and Red, Green, & Blue, providing the absolute minimum of measurement data from which a valid profile can be saved, and from that profile a new User Colour Space extracted.
It may also help to use colour patches that are not 100% peak. In the above example we are using 90% colour, so 229, 0,0 for Red, rather than 255, 0, 0, and maybe an additional grey scale patch or two to help extract an accurate EOTF value.
Obviously, a Grey Ramp RGB Quick Profile can be used, and the colour space Extracted from that, especially with a small value set for the Grey Axis size. A value of 3 would be identical to the above Preset .csv patch list.
From the generated profile, Use Modify/Extract to generate a new User Colour Space of the native response of the monitor.
Generate a LUT using the desired target colour space as the Source, and the Extracted Colour Space as the Destination. As the LUT Generation is a Colour Space to Colour Space (Matrix to Matrix) generation it makes no difference which process option is selected.
This approach can be especially useful for HDR calibration - both PQ and HLG - when the monitor/TV suffers heat related instability.
Matrix vs. 3D LUTThe above Matrix Calibration information raises another question that does impact the accuracy of calibration.
That question is what is the difference between a 3x3 Matrix (with or without a 1D LUT), compared to 3D LUT calibration?
In simple terms, a matrix calibration will not be able calibrate all available colours the display can accurately display, while a 3D LUT calibration will.
A matrix calibration will clip the display gamut to the peak colours.
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 colour to a separate output colour, without constraint, meaning each individual colour can be calibrated without limitation or being restricted to peak primary colours - as the Gamut Coverage page shows.
