Optimizing 3D LUT Dark Detail
By Joel Barsotti and Tom Schulte
In some displays, a D65 grayscale point cannot be achieved at the lowest luminance levels, because D65 is outside the display’s native color gamut. If ultimate grayscale accuracy is targeted at those lowest luminance levels during 3D LUT calibration, inconsistent color transitions will be created, causing color artifacts in shadow detail.
This paper presents a method of achieving the most accurate and consistent color possible for all displays at low luminance levels, assuring the best possible display performance, with no shadow color artifacts.
Low Luminance Color Gamut
If you were to plot an LCD display’s native color gamut at different luminance levels, you would see that at luminance levels typically below about 25-30%, the display has reduced color gamut area. In other words, the display doesn’t achieve 100% saturation at those lower luminance levels.
You would also see that the display’s native black point is almost never at D65. That means that at lower luminance levels (typically below 5-10%) D65 will surely be outside the display’s reduced gamut area.
Therefore, when trying to correct neutral gray to D65 below about 5-10% while calibrating an LCD display with a 3D LUT, the grayscale point is naturally going to go to the edge of the display’s RGB signal cube.
At the luminance levels where the grayscale point transitions to or from the edge of the signal cube, inconsistent color and grayscale transitions will be created, causing shadow color artifacts. Because a display’s black point is usually bluer than D65, the color artifacts tend to be some red/green combination.
Therefore, trying to correct neutral gray as closely as possible to D65 is a problem at low luminance levels where D65 is outside the display’s gamut, because it creates inconsistent color artifacts in the display’s shadow details.
Shadow Consistency
Calman’s core calibration algorithms are the best in the industry, and they are constantly being fine-tuned to deal with the peculiarities of the real world physics of electronic displays. One such peculiarity is that, at the low luminance levels associated with shadow detail, a display’s native color gamut may not contain the D65 grayscale point.
As described above, if ultimate grayscale accuracy is targeted at all luminance levels during 3D LUT calibration, inconsistent color and grayscale transitions will often be created in the display’s shadow detail. Instead, at a display’s lowest luminance levels where D65 is outside the display’s color gamut and can’t be achieved, the target needs to be maintaining consistent color transitions in the shadow detail, avoiding color artifacts.
To accomplish this, when Calman renders 3D LUT calibration data from a display’s profile data, it determines the lowest luminance level at which D65 is still within the display’s color gamut. Calman’s 3D LUT rendering engine then calculates consistent color transitions for all gamut colors from that level down to the black point.
This automatic switch to targeting color consistency avoids color or grayscale discontinuities and keeps the grayscale point inside the display’s achievable gamut at the lowest luminance levels.
Conclusion
Calman’s advanced 3D LUT calibration algorithm avoids creating color artifacts at a display’s lowest luminance levels where D65 is unachievable by automatically switching its correction target from ultimate color accuracy to ultimate color consistency. This shadow consistency feature of Calman’s 3D LUT algorithm assures the most accurate images possible on any display, with no shadow color artifacts.