Nope. Rel. Col. is specified only for in gamut colors. There are no requirements on mapping out of gamut colors. There is only advice that out of gamut colors should be unsurprising. For instance mapping an OOG cyan to red would be legal but probably surprising. However, there is no technical definition of unsurprising. I think most profile vendors map to the nearest dE76 on the gamut surface but don't know for certain.
Thanks for the desaturation tips Doug — forgot about that one. When I finish processing a photo it’s almost always in Photoshop, and when I see OOG colors I can’t easily or happily resolve, my preferred method is to save a downsized “deleteme.tif” copy of the image file, load it up in ColorThink and look at the image datapoints that lay outside the printer gamut. That gives me a great idea where the problem colors lie in terms of luminosity, saturation and hue and the spaces I have available to shift the problem areas around. I then typically use Tony Kuyper’s luminosity masks and the hue panel to nudge those areas into compliance.
The histogram is easy enough, but the problem with the OOG warnings in PS is they are not accurate enough. A troublesome image that inspired my post here is a 4’ high picture of a sunlit bamboo forest I’m working on. At some point in the back and forth fine tuning everything, there is a point where balancing the luminosity and saturation (hue doesn’t work well with this one) will degrade the overall IQ. I can see there is data in some of the leaves that I could tap into, but condensing the image’s luminosity or saturation comes at great pain and/or is practically impossible without doing damage on a larger scale.
Anyway, I am going to reprocess this particular image today to preserve/amplify the luminosity differences accessible in my file (essentially by hand blending multiple versions of the processed RAW file, 16 bit 51MB) and that might be my only practical solution for this shot. I need to do that anyway. The realization that I was headed that way yesterday made me wonder if different versions of RelCol might retain more detail in the leaves. I can imagine, for example, some algorithm slightly amplifying luminosity differences between midtone OOG colors, something like a high pass filter, that would compensate in appearance at least in luminosity for more flattened chromaticity in the OOG color rendering.
In any event, it seems like potentially a good idea at the moment to have something like that built into a RelCol rendering pipeline, and because I have a number of print pipelines residing on my computer it made me wonder about any known differences among manufacturers. I know the answer to this question if there is one would yield almost imperceptible results, but in 4’ long prints, a few changes like this can be perceptible.