Differences between ICC V2 v V4 and Perceptual v Relative Intent on B&W

[Doug Gray]

Nor are you ever likely to, for printer profiles.

Synthetic display profiles like sRGB etc, yes, because the parametrized curves let the straight segment of the sRGB luminance curve be more efficiently encoded than a LUT, which was the primary reason for adding this tag.

Parametric curves do reduce the size of sRGB profiles since 1D linear LUTs aren't needed. Data rates were slower, storage was more expensive, and images smaller back in 2001 when V4 was first introduced. The few KBs difference is far less material these days.

However, the floating point 3DLUTs in V4.3 are much improved. They are no longer forced into having the same number of grid points in each dimension and have more preprocessing capability with matrix transforms on both ends to get the most out of the LUTs.  That can make a big difference for the same level of accuracy. OTOH, most printer profiles aren't attached to images and the V2 16 bit 3D LUTs can be made as accurate as desired. Size is hardly an issue these days. These changes primarily benefit print devices and were put in to improve them.

[Doug Gray]

I've not tried to keep up with it, but I know at one stage HP made some profiles using the PRMG.

The main technical problem with the PRMG is the same one that's always been with the idea of pre-computed A2B and B2A tables:
you can't implement a full range of intents properly. Anything using the PRMG (or any other common intermediate gamut where the source and destination profiles are arbitrarily mixed) is going to end up with what I would call a saturation intent. That's because all you can really do is map the source space gamut to the PRMG, and then the PRMG to the destination spaces gamut. So if the destination has a larger gamut in some places than the source, you get gamut expansion. In practice this may not be much of an issue for typical RGB display space to printer space, but it makes it a not-so-general gamut mapping strategy.

Another issue I've noted before, is that you get two gamut mapping transforms being concatenated. Each transform involves a degree of inaccuracy, so the inaccuracy is doubled. And if the two profiles are created by different software, there is no guarantee that the two transforms cancel out very well where they should.

It's never likely to be resolved. It isn't even clear to me how it could be done.  Interesting idea though. In the abstract.

[Doug Gray]

Really ? Where ?L=3.1 is the black point of the PRM, independent of the use or not of the PRMG.

You are right. L=3.1 is the PRM either way. I worded it incorrectly. The problem is that Adobe Photoshop, doing a Perceptual->Device->Perceptual, using only a neutral tone curve, produces significantly more inversion error using V4 profiles than V2 profiles. Adobe adds in an L=3.1 base level before going to the profile which then clips it out. So the printer is printing close to the same way V2 profiles print. However, when doing an inversion, consideration of the PRM is not done resulting in increased error. See the attached plots of the inversion error in V2 and V4 Perceptual. I believe the V4 profiles are created correctly and the increased error is from Photoshop.

The scaling of the colorimetric table black point was not something that was specified or even allowed in the V2 specification (i.e. See ICCV2.4 page 66 Equations A.1 to A.3), irrespective of how some may have mis-implemented it, so this is not something that V4 specification has really brought to the table.

The ICC says there were ambiguities in spec'ing the V2 Colorimetric tables and V4 specifically clarifies that:
http://www.color.org/advantagesv4.pdf

In v2 profile builders were allowed to modify measurement data prior to building the relative colorimetric tables for a profile. This sometimes led to differences in the way in which colorimetric data could be interpreted when a colorimetric match is required.

I credit the V4 development with correcting this ambiguity and further, strongly urging V2 profile makers to do the same. For the most part it seems they have. As I said, I've only seen this screwed up BtoA1 table in older canned profiles.

Neutral tone curve error of Perceptual Roundtrip of L=0 to L=100 for V2 and V4 profiles using Photoshop CC x64 in Win 10. Orange is the V4 error, blue is the V2 error. I1Profiler.

L values were in increments of .1. The X axis data is labeled 10x actual L values.  Too lazy to scale the axis.

[GWGill]

However, the floating point 3DLUTs in V4.3 are much improved. They are no longer forced into having the same number of grid points in each dimension and have more preprocessing capability with matrix transforms on both ends to get the most out of the LUTs.  That can make a big difference for the same level of accuracy. OTOH, most printer profiles aren't attached to images and the V2 16 bit 3D LUTs can be made as accurate as desired. Size is hardly an issue these days. These changes primarily benefit print devices and were put in to improve them.

Marginal improvements at best, so return on investment in using them (by creating software that emits them) is low.

[NickXavi]

Well, if Digitaldog, Doug Gray and GWGill, believe that today the version 2 is the best option, what I can add?

But is hard to understand that a company like X-Rite, can advise on the i1Profiler's Help that, except in case of incompatibility, the best option is version 4.

X-Rite says textually "In general, version 4 profiles will give greater precision"

Marketing perhaps, according to Digitaldog?

Reviewed the comments, I'm thinking of changing my profiles to version 2.

Thanks!

[Doug Gray]

Marginal improvements at best, so return on investment in using them (by creating software that emits them) is low.

True. As it is accuracy is quite good at the inner parts of a gamut with 16 bit LUTs. The iffy areas are near the gamut edges and that can be reduced easily if needed by expanding the grid point size. Storage is cheap and printer profiles aren't usually attached to images. Even where they are images have grown so much in the last decade it makes the profile size small by comparison. I've also noticed that the vendors take the shortest path to being able to label profiles "V4." I've never seen a shaping matrix, newly part of V4 3D LUT paths, that is anything other than pass through. So they just take the same V2 data and plop it into V4 with the PRM front end change for Perceptual and voila, they have a profile they can call V4.

I mostly use Rel Col and that is identical in I1Profiler V2 and V4 profiles so why switch.

[digitaldog]

Well, if Digitaldog, Doug Gray and GWGill, believe that today the version 2 is the best option, what I can add?

What I recommend you do is build a few profiles both ways, run some tests.

[GWGill]

Well, if Digitaldog, Doug Gray and GWGill, believe that today the version 2 is the best option, what I can add?

Since V4 is essentially a super-set of V2, there's no reason they can't work just as well, but getting inter-interoperability right is hard, particularly when a vendor gets locked into something by not wanting to cause incompatibility with it's own previous versions.

[NickXavi]

Hi all,

I tested versions 2 and 4 of some profiles and the truth is that the differences are almost negligible.

Higher out of gamut of the V4 (very small), slightly higher volume for V2 is slightly seen and slightly diferences in Dmax for V4 (max 0,02).

Maybe in the Round Trip Colorimetric is where a difference can be seen for V2. Attached is an example Hahnemuhle Fine Art Baryta.

[Doug Gray]

Hi all,

I tested versions 2 and 4 of some profiles and the truth is that the differences are almost negligible.

Higher out of gamut of the V4 (very small), slightly higher volume for V2 is slightly seen and slightly diferences in Dmax for V4 (max 0,02).

Maybe in the Round Trip Colorimetric is where a difference can be seen for V2. Attached is an example Hahnemuhle Fine Art Baryta.

In Perceptual (BtoA0) only the L curve is clipped in V4 (starts ramping at L=3.1) but the a* and b* color portions are not so this could produce small differences as well in the non-neutral areas. It might also affect "gamut volume," a rather nebulous concept outside of Absolute Colorimetry.  I've only investigated Perceptual and Colorimetric intents for B&W purposes. I don't see any difference at all in Colorimetric roundtrips.  The differences in Perceptual are minimal. What I don't see is any reason to use V4 printer profiles at this time but I doubt V4 profiles would be visually different. At least on the I1Profiler.