Originally Posted by sotti
If the whole point of dE is that the values below a threshold are not discernibly different, how could a value below the threshold be visually wrong?
I agree entirely. So the question is how do we calculate dE for grayscale?
If you include luminance data in those calculations, low stimulus values will result in low dEs, even though the color error is clearly visible.
To take just one example, consider the following: Assume a 2.22 target gamma, 20% stimulus, 35 ft-L peak output, no gamma error, and the following color x0.313, y0.345. This is a very greenish gray, the error easily visible.
If you ignore the level of stimulus, then the dE value (CIELUV)
Reference Luv: L100, u0, v0
Measured Luv: L100, u-7.317, v10.85
is 13.1 and the RGB balance is
This is what you would expect.
However, if you include luminance information in the dE calculation in the way I think you have suggested, then even though the RGB balance is the same and the visible error is the same, the dE value is now 2.9. This is clearly wrong.
Reference Luv: L18.11, u0, v0
Measured Luv: L18.11, u-1.325, v1.96