Hermie, Dennis and Hendrik,
Still referring to bjanes' red flower
in ProPhoto RGB and the question about possible image degradation from conversion to sRGB, the following tests might be interesting though the results are somehow slightly contradictive:
I.) Numerical Difference
File (a.) left in pRGB
Copied file (b.) converted pRGB > sRGB > pRGB,
/> Create an overlay of both files in Difference blend mode
/> Flatten the layers and update the histogram
On my screen the resulting image looks very dark (not to say largely black). The average RGB = 1.19 level (deviation 3.12). This seems to be in line with above Delta E plots. All in all the numerical difference caused by this conversion pRGB > sRGB seems to be quite low.
Even if it would be higher (e.g. with a more saturated flower), said conversion can be seen as a reduction of color saturation by purpose. I mean, for any reason which shall not be questioned here, ‘we’ want to have this image ‘in’ this smaller gamut. So something has to change, at least as far as oog colors are concerned. Some changes in particular on the color channels a &b simply belong to this conversion. Accordingly, parts of the Delta E can not really count regarding the loss of details. Or?
Anyway, in this case here there’s only a minor numerical difference. So where is the major loss from conversion ?
II.) Perceivable Difference
Again the idea is to bypass any possible gamut limitations of the monitor, now without relying on the implementation of the Desat-Monitor-Colors function:
File (a.) left in pRGB
Copied file (b.) converted pRGB > sRGB > pRGB
/> With both files (separately) add a Hue/Sat.-layer and set the saturation slider down to minus50%. Change this layer to Saturation blend mode, respectively.
/> Flatten the layers.
/> Create an overlay of both files in Normal blend mode
/> Toggle the upper layer to watch for perceivable image degradation.
Results are quite interesting. Yes, there is a some degradation and loss of details (in particularly bottom left). But, it’s not at all that the whole fine structure would have been wiped out. The appreciated reader might wish to check and judge this effect.
Somewhere it should be mentioned that this is not the fault of ProPhoto RGB. There’s a basic input/output mismatch (in this case with sRGB for output) along the whole digital imaging chain, at least for those of us who occasionally capture colorful flowers, etc.
Just two further thoughts:
Finally I tend to accept the second test, at least in the sense of a worst case representation. On this basis it might be interesting to think again about possible improvements, how to fine-tune the conversion regarding the maintenance of fine details (of course without intending to sacrifice other attributes). Or, probably more efficient, given that many details are silently still present in sRGB something like an USM + Saturation Mask seems to point in right direction…
Regarding channel clipping of the histogram (due to RelCol as opposed to exposure) I’d still say that this greatly overemphasizes the problem. Or, the other way round, the conversion pRGB > sRGB is not so damaging as one might expect. Bottom line for me is to be careful with any global saturation slider on a pRGB-basis (like in ACR). For example, with some vibrant flowers as part of a landscape, it’s probably better to limit any further increase of saturation e.g. via selections /masks in Photoshop; e.g. to facilitate a deeper blue for the sky.
Sorry for this long post.
Have a nice weekend! Peter