Alternate Patch Sets
LightSpace can use any user defined Patch Set, either as a Quick Profile or as a full Characterisation.
Example Patch Sets for both can be downloaded via the Downloads
page, and information on their use can be found in the Display Profiling User Guide
Reduced Gamut Profiling
However, there are further uses for User Patch Sets, such as deliberately reducing the Gamut Area to be profiled, which can be of use with displays that have a smaller gamut than the required colour space target, with internal colour management that cannot be disabled. Such displays will suffer gamut edge compression, such as JVC Wide Gamut projectors, that are colour managed to Rec2020 that cannot be disabled, as described in the Error! page, Pre Calibration Issues
The first step is to define the actual gamut area the display is actively using within its target colour space. For example, using a JVC projector set to Rec2020 wide gamut, the R, G, and b peak xy values can be extracted, leaving the white point and gamma as per the Rec2020 colour space.
To define the actual gamut area of the display profile with a Grey Only, or Primary Only Quick Profile, and use the Points Info pop-up window to interrogate the max Red, Green, and Blue values, as shown.
The extracted colour space can then be used to define a gamut reduction LUT, using Convert Colour Space with Source as the Extracted colour space, and the Destination as the target colour space - Rec2020 for the JVC projector mentioned here.
The gamut reduction LUT can then be used as an 'Active LUT' within the Characterisation menu, and 'Export Colour List' used to export a CVS file with the reduced gamut patch set.
The patch set will not yet work as desired, as there are no 100% patches at all. So, 100% Red, Green, and Blue patches need to be added to the end of the list using Excel.
You will then have a reduced volumetric gamut profile set, with Peak RGB patches at 100%, so precisely locating the gamut area to be profiled.
The patch set will therefore not attempt to profile outside the display's available gamut, so will avoid any edge compression issues, such as suffered when a display is pre-calibrated to a larger gamut than it can actually attain.
But, by also having the three 100% RGB patches in the profile set means the calibration LUT will correctly target the desired colour space - Rec2020 in the above examples.
Note: When exporting the reduced gamut patch set from with the Characterisation menu it may be preferable to reduce the cube size - say 19^3, or 17^3, so when the final calibration LUT is being generated it will have approximately the same volumetric granularity as a standard 21^3 profile. However, a gamut reduced patch set can be made to any size, and even with a different percentage reduction on the different gamut edges, etc.
In some circumstances it may be helpful to slightly reduce the gamut further - say 5% or so, just to make sure the reduced gamut patch set is fully within the display's available gamut coverage.
The following example shows the extracted JVC gamut Green being reduced buy 5%, with the new xy values shown. The same would need to be done for Red and Blue, and a new 'Colour Space' saved using the new RGB xy values.
Note: The Gamut Reduction will be based on the relative distance of Red, Green, and Blue from the colour space white point.
While the above example only added back the three 100% Red, Green, and Blue patches, it may be preferable to add back 100% CMY as well, or all Primary Red, Green, and Blue patches, or even add back all the patches on the gamut edge!
(Any patch with a zero value for one of the RGB colour channels will be a gamut edge colour, so XXX,XXX,0 or XXX,0,XXX or 0,XXX,XXX...)
Additionally, it may be preferable to focus on getting the reduced gamut patch set best suited to the display first, finding the best Red, Green, and Blue gamut reduction values that on their own produce the best calibration with the best overall calibrated gamut.
(The calibrated gamut will be smaller than the actual max gamut of the display, as there are no 100% gamut patches.)
When the ideal reduced gamut set has been defined, the 100% gamut patches can be added back, to restore the the missing gamut.
From the above it should be obvious that altering just the Gamma value when generating a LUT to be used to export a User Patch Set will 'bias' the patches more towards the blacks, or the whites.
In the above example, the default Gamma of 2.4 for a Rec709 display will effectively be cancelled out, making the patches more equally spaced in linear light terms.