Hi Bart,
I wanted to go back to this technique because it's really quick and easy using an action - and it makes sense to me. You are specifically targeting OOG colors (so it's not just a saturation map as with the HSL filter technique).
Hi Robert,
Yes, I'm targeting OOG colors because they tend to be pushed to the 255 (in 8-bit) boundary in the smaller gamut colorspace. However, we're not getting only those OOG colors, but also smaller differences e.g. due to converting back and forth between different illuminants and some other color shifts. But the more extreme differences will indeed allow to address the OOG colors.
Anyway, what I found is that the technique doesn't just give OOG colors, but also dark-point clipping, so there's a need to do a small levels adjustment on the mask to remove the dark shadow (but now that I say that I see that I'm being ridiculous because these dark areas won't be printed anyway
.
Spot on! We are altering colors, so it is more about taming the magnitude of difference, and the relevance of those colors.
The fact that we also get some colors with smaller differences will allow a sort of Perceptual (I'm using this very loosely) adjustment of
adjacent colors that were still more or less in gamut. Afterall, we cannot
only wrestle the OOG colors into gamut submission, because that would essentially be similar to a Relative Colorimetric intent, and would lose color differentiation/detail.
What I would like to understand (and from a post I saw on another thread I can see that you know your maths) is what happens exactly when we use this technique. I'll try to explain what I think happens and perhaps you would be good enough to set me straight if I've got it wrong.
OK, when we convert from the working space to the paper profile using Relative, every OOG pixel will be shifted to the nearest in-gamut color. What that color is depends on the CMM and the profile, but it could include saturation, hue and lightness shifts. So lets say that a particular pixel gets shifted from red (Lab 50,80,70) to orange (Lab 50,50,70) (could happen, right?).
Then, when we convert back to the working space, the CMM and reverse profile may or may not shift that color again. Let’s say that this particular color is inside the gamut of the print but outside the gamut of the working space. Then there could be a shift to, say, Lab 50,60,70, to bring it back into the working-space. Correct?
If we then use the difference between the round-trip image and the original, we’ll get a difference … but the difference won’t be quite correct.
You've got it. It won't be absolutely correct, but it will be somewhat weighted towards the magnitude of the absolute difference.
I assume, and I could be absolutely and entirely wrong here (!!) that in the round-trip, the white point shift that occurs in the working-space to print profile direction will be reversed correctly in the print profile back to working-space conversion. But if it’s not, then there are a whole load of colors that will show a difference even though they aren’t OOG at all. Presumably what actually happens is down to how good the CMM and profile are … but do you know what is supposed to happen?
Correct again, however, the largest differences will still be in the clipped colors.
At any rate this doesn’t mean that the mask that’s generated isn’t 90% correct (and if it is it’s good enough for me!).
That's it. It won't be 100% correct, but it will have a weighting towards the OOG colors.
But I would really like to understand what actually happens (or is supposed to happen) when we convert from one profile to another … and then back again. I would seriously appreciate an explanation … either from you or from anyone who knows and has the time to give it!
Solutions like Gamutvision and Colorthink will allow to get much closer, however it remains to be seen how that accuracy in converted into a practical solution. Identifying a problem is nice, identifying it very accurately is nicer, but not being able to solve the issue is worthless.
Based on the OOG weighted initial mask, we can adjust the mask, and we can adjust the corrective layer on which that mask will be used. We can make an overall desaturated layer and apply the (adjusted) mask to that, but we can also choose to do something different.
Cheers,
Bart