New ICC v4 sRGB Color Space May Prevent Clipping Converting From ProPhoto RGB

Question

Just discovered and tried out this new sRGB color space downloaded from this page:
http://www.color.org/srgbprofiles.xalter
Here's a cropped demo with histograms of a raw image I've been working on in ProPhotoRGB in ACR 3.7 and CS2:
http://www.flickr.com/photos/26078880@N02/2874068887/sizes/o/
Please disregard the PRMG name in the demo. I got confused as to which was which on that site and thought that this version of sRGB was a PRMG=(Perceptual Reference Medium Gamut) profile. It's called sRGB_v4_ICC_preference.icc.
Note the different color shifts using the Perceptual intent and unchecking Black Point Compensation=(BPC). Neat little features embedded in this profile, but not sure about how it renders certain colors close to clipping in the shadows. That site doesn't recommend mixed use of ICC v4 with v2 color spaces and output device profiles but I did it anyway just to see what it does.
Who knows this may be the equivalent of handing scissors to children, but I'm just one of those curious children and thought I'ld share anyway.

Answer 1

I'll try to respond to a few items above.
The digital values in a Pro Photo RGB file should generally be the same as for a ROMM RGB file. The exception is that all possible Pro Photo RGB values are "legal" but not all ROMM RGB values are "legal." ISO 22028-2 restricts the legal ROMM values to those with PCS LAB values between 0 and 100 L* and -128 to +128 a* and b*.
There is no enforcement if you use the illegal values in a ROMM file but as some of these values do not represent possible colors this is not advisable. I think it is good practice to try to stay mostly inside the ICC v4 PRM gamut with Pro Photo/ROMM images. You can check this using the gamut warning profile on the ICC site.
If you have a Pro Photo RGB image you should just assign the ROMM profile to it. Converting to ROMM RGB should not change anything in the ideal sense but there is the possibility to introduce rounding errors and mismatch black points.
The main difference between the Pro Photo RGB profile and the ROMM RGB profile is the former includes black scaling to zero (as is common with v2 color space and display profiles) and the latter does not (as is required with v4 profiles).
In the duck picture, the reason for the difference is the ROMM profile black is at L*=3 so this is where the lowest blacks land when converting MRC to v2 sRGB (which has the sRGB blacks scaled to L*=0). In this case leaving BPC off is analogous to turning on "Simulate Black Ink" in Photoshop Proof Setup.
All the ACR color space choices are v2 profiles with black scaling. If you want a v4 profile embedded you have to assign it (if you have a corresponding v4 profile) or convert to it.
Both the sRGB and PRM gamuts fit within the ROMM RGB legal encoding range, but if you use a v2 profile with black scaling you should always turn on BPC when combining with a v4 profile. Otherwise the v4 profile will think the v2 profile represents a device with an infinite dynamic range. When BPC is on the Color Engine scales the black point of the source profile to the black point of the destination profile.
The sRGB gamut extends outside the PRM gamut in some places, and the PRM gamut extends outside the sRGB gamut in other places. If you convert using a colorimetric intent in either direction some of the gamut will be clipped. The purpose of the sRGB v4 perceptual transforms is to minimize clipping in both directions.
I can't do this justice here but basically scene-referred images are encodings of the scene colors and output-referred images are encodings of the picture colors on some medium for which the picture colors have been optimized. You can make a scene-referred image by setting the ACR sliders (except the white balance sliders) to zero and the curves tab to linear. You make an output-referred image when you adjust the sliders to non-zero values to make a nice picture as viewed on some medium. For example you might adjust the sliders differently to get the best results printing colorimetrically on glossy photo paper vs. on plain office paper.
Often an important part of the transform to output-referred includes a midtone contrast and saturation boost. While there will likely be some highlight and may be some shadow compression, it is misleading to think of this transform only as a compression to some output medium dynamic range. In some cases the output medium dynamic range is larger than that of the scene.
Usually the transform to output-referred is more complicated than a simple gamma function.
Regardless of the image state (scene-referred or output-referred) the sRGB, Adobe RGB, or Pro Photo RGB nonlinearities will be applied to create the image data that you open into Photoshop.
We are working in the ICC to prepare more information for posting on this topic.