...BTW, I have a suspicion that you are mistaking Andrew's conclusions...both gamut warnings are taking the image colors and determining whether or not they are in/out of gamut based on the display or output profile. I think you are making things more complicated than they are (or are misunderstanding how LR4 works).
In any case, what you really want to do is learn how to soft proof. Everything else is, well less useful. (I was gonna say bullshyte but resisted, ya know:~)
I’m not sure I understand your answer, it’s a bit ambiguous to me, so let me try to make myself more clear. I agree some of this is borderline shyte but it does have implications of consequence.
Each pixel in the original image has a color I’ll call Lab(Original). This value does not change.
During softproofing Lightroom creates a proof copy of the original image. The pixels change as we make adjustments to the proof. Label the color of the pixels in the proof Lab(Proof). The Lab(Proof) values may or may not be the same as the Lab(Original) values, depending on adjustments the user makes.
Using the printer profile, LR can also calculate what each pixel on the print will look like to a standard observer. Call the calculated color of the printed pixels Lab(Print).
LR calculates the LR(Print) values and uses those to generate the display we see during softproofing. We are looking at the colors that will be on the print. (Within limits).
To send an image to the printer or to calculate the Lab(Print) values, LR must bring Lab(Proof) values that fall outside the printer gamut into gamut. Depending on the rendering intent, Lab(Proof) values within the printer gamut may also be changed. In fact, nearly all values probably change regardless of rendering intent since tone mapping is required to squeeze the image’s dynamic range into the printer’s smaller dynamic range. So most, if not all, of the Lab(Print) values are different than the Lab(Proof) values, though not by much in the ideal case.
The gamut warning on the right side of the histogram shows us where the Lab(Proof) values fall outside the printer’s gamut.
The gamut warning on the left shows us what? It would be nice if it showed where the Lab(Print) values fall outside of the display’s gamut. But what Andrew Rodney found (as I understood him; forgive me Andrew if I got this wrong) is that this warning instead shows where the Lab(Original) values fall outside the display’s gamut. Or perhaps the Lab(Proof) values, but certainly not the Lab(Print) values.
Why is this important? If I knew that I cannot see on my display how some of the colors will print, I might alter my workflow. If it’s a small print I might just print it and see how it turns out. If it’s a large print I might print just a small portion. Or make changes to the proof image. Etc. But if I do not know that I’m not seeing the print colors I might be surprised by the print.
I learned several years ago from you and Michael to keep softproofing simple (“From Camera To Print – Fine Art Printing”) and that has worked well for me. But here is a new tool from Lightroom that could help the process if it is designed to show us when the print colors cannot be displayed.
So that is the question: does the LR gamut warning on the left show us if the Original colors cannot be displayed? Or the Proof colors? Or the Print colors? Or something else?