Originally Posted by ihd
This has been a very interesting and illuminating AVS thread, and my thanks to all who've made it so. It's the first I've felt compelled to subscribe and post a response to - basically, the conclusions that this particular beginner in calibration has reached (or jumped to!) from reading thus far.
The next three paragraphs only discuss initial display characterisation or profiling, something made much easier with spacediver's helpful graph, with labelled hues, in post #349.
If a display is reproducing input colours faithfully, then hues within a perceptual low-resolution region (to the left on the graph) will be reproduced as colours belonging in that region, and such non-linearities as exist (e.g. hue 1 switching places with hue 3) will be imperceptible. But what if the reproduction is sufficiently non-linear that these input colours, when displayed, resemble those further to the right, i.e. hues which can be resolved by the human visual system? Or if they are reproduced as any other colour outside a given low-resolution region, and contrast visibly with others within it? (In the latter case, hues 1 and 3 changing places might also have consequences, depending on the adjacent, contrasting colour.) So unless one samples these perceptually insignificant hues reasonably well one cannot know if the colours displayed actually correspond to the input patch values, so that theoretically insignificant regions, once reproduced, no longer remain so.
I'm unsure how important this is in practice - I guess it depends how severely actual displays deviate within narrow bands. Another variable might be screen materials and/or ambient light (who knows what effect those LEDs in your equipment stack might have
One would expect however that an initial display characterisation of 'reasonable' granularity would (a) identify areas of the colour space within which a given display is most non-linear and provide the means to design a custom patch set whose granularity (sampling frequency) is weighted in favour of these high-error areas, and that (b) such patch sets could also be weighted to favour, or not favour, perceptually significant regions, and their post-calibration colour renditions compared and assessed.
Now at the assessment or display verification stage, weighting in favour of entirely new, and preferably the most perceptually significant / visually resolvable regions surely makes sense. By this point we should know very well where the reproduction errors are in the display device: all we want now is to confirm that we have made the best corrections to these that we can, and to examine the calibrated display's performance most closely within areas that are visually/perceptually significant. We are no longer concerned that an input of hue 1 will result in hue 3 (or hue 5, for that matter): if we found such an error in the sampling process during initial characterisation, we have corrected for it now. Why now go back and sample these colours again? At best we'll discover that we didn't correct them as much as we hoped.
(And perhaps there was always a smaller probability that errors in perceptually less significant areas would lead to poor visual results, as device errors in these regions would have to be larger to have an impact - which some of the calibration results using perceptually-optimised patch sets in this thread seem to attest to.)
So my hopefully not too unreasonable take on this is that, during initial display profiling, one needs to not disregard (or at least not completely disregard) perceptually insignificant colour regions, because we don't yet know that the device will display them as intended - at that point, all we know is that, if reproduced correctly
, fine gradations in those regions will be imperceptible. Or to put it another way, all we know is that those input patches might not be so difficult to tell apart once displayed on a non-linear device. But once we have corrected for the difference between the patches and what a given device actually displays on-screen, then those displayed colour regions revert to perceptual insignificance. We now won't be able to visually differentiate the patch for hue 1 from the patch for hue 3, and that is as it should be. And now we can focus our patch sets on verifying that the finest-grained differences are being reproduced correctly.
I suppose one might think that if hue 3 in the perceptually insignificant region can be displayed inaccurately enough for this to become significant, then this can happen in the opposite direction - somehow during calibration hue 7 is transformed into hue 3, and this is missed during verification because hue 3 isn't sampled in the post-calibration verification sampling scheme, biased as this is towards perceptual space. But this shouldn't matter, because while we probably won't be sampling hue 3, we likely will be sampling hue 7, so that any errors in 7's reproduction will be caught that way (and of course we do want to catch those).