After performing a 3D LUT based calibration users can sometimes be tempted to tweak the display's manual controls.
Such adjustments will always break the 3D LUT calibration, regardless of where in the display's image processing chain the controls exits.
Display Adjustments Post Calibration
There is an incorrect assumption that having the display's manual controls after the 3D LUT means they can be used post calibration, without affecting the accuracy of the 3D LUT calibration. That assumption is incorrect, as while not as bad as having the manual controls before the 3D LUT, altering controls post LUT will still break the calibration result.
The issue is that the 3D LUT is correcting any non-linear irregularities in both the display's image processing electronics, as well as the actual screen. Therefore making any changes either before or after the LUT will break the relationship between the correction the 3D LUT is generating, and the location of the errors within the display.
Manual Controls post-LUT
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Input Signal
Input video signal
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Input 1D pre-LUT
Input 3x 1D RGB LUT for gamma and colour temp
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3D LUT
3D LUT calibration
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Manual Controls
Display manual controls
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1D post-LUT
1D LUT for screen linearity
(Often a single 1D gamma only LUT) -
Screen
Display screen
In the above signal path the issues are relatively obvious, as it is often assumed the screen has a purely linear response to input signal changes. But, that is not the case, as is defined by the need to often have an input 1D LUT to attempt to linearise the screen's response.
Regardless, changing the output from the 3D LUT via manual controls (brightness, contract, gain, etc) before the signal gets to the screen will break the final display calibration as the 3D LUT output (and the pre-1D LUT) expect the screen to be exactly as it was when profiled for LUT generation.
It should also be noted that if a display has a 3D LUT followed by a 1D LUT, the 1D LUT cannot be adjusted after the 3D LUT has been generated and uploaded. With a 1D LUT followed by a 3D LUT, the 3D LUT cannot be adjusted after the 1D LUT has been generated and uploaded.
Any post calibration signal path adjustments can only ever be at the beginning of the calibration chain, although even then unexpected results are likely!
