Originally Posted by fight4yu
So, are you saying that the cube itself contains the grayscale (0-100%, 21 steps) already? I am not sure I understand what you mean by do grayscale with the cube in unity, then calibrate the cube.
An oversimplified view of the video path basically goes like this.
HDMI in -> Cube 3D LUT -> Ramps 1D Lut -> HDMI Out
The cube covers literally every single RGB triplet, so everything after the cube is no longer at reference values anymore. Things that aren't at reference values can't be calibrated correctly.
So you reset the cube to default (unity), so that you have a clean path to the 1D LUT. Basically this is like calibrating the display before you calibrate the radiance. Just one step further. Calibrate the TV with reference patterns. Calibrate the 1D LUTs in the radiance, Calibrate the 3D LUT.
The CalMAN workflow for building 3D LUTS already handles this correctly, by forcing a 3D LUT reset before doing a 1D Ramps AutoCal.