Re: ProPhoto RGB an ROMM-RGB in Photoshop
Re: ProPhoto RGB an ROMM-RGB in Photoshop
- Subject: Re: ProPhoto RGB an ROMM-RGB in Photoshop
- From: Marco Ugolini <email@hidden>
- Date: Mon, 28 Aug 2006 00:26:38 -0700
So, I was finally able to sit down and *try* to make sense of this puzzle.
For openers, one note of interest: the "dmdd" tag ("Device model desc tag")
in ProPhoto RGB contains the following text:
"Reference Output Medium Metric(ROMM) (Reference Output Medium
Metric(ROMM) 'Reference Output Medium Metric(ROMM)"
There it is, spelled out for all to see: "ROMM", as in "ROMM RGB". Must mean
something, at the very least a tribute if nothing else.
Anyway, let's move on:
*******
White Point in ROMM RGB:
In the White Paper from 1999 it was specified as x=0.3457, y=0.3585 (which
means that z=1-[0.3457+0.3585]=0.2958); in the PDF from the ICC, it was
listed as x=0.3457, y=0.3585; z=0.2958. So, the two papers report the same
white point numbers for ROMM RGB -- except that there is no mention of Y.
*******
White Point in ProPhoto RGB:
The "wtpt" tag (inspected in ColorThink) reports these values: Y=1.000,
x=0.346, y=0.359. Since x and y are the same (which makes z the same too),
this would appear at first to be the same white point as in ROMM RGB.
But only in theory, because there is another factor to be taken into
consideration: Y. In ProPhoto RGB the Y value is 1.0, in ROMM RGB it's 0.89.
That makes for a darker white point in ROMM RGB (L* 95.58 vs 100
respectively).
The black point is also lighter in ROMM RGB (L* 3 vs L* 0 in ProPhoto RGB).
As for the chromaticities, judging from the 3D view in the ColorThink
Grapher, they appear different for the Blue and the Red, but not by much.
Again, the x and y values for R, G and B match between the White Paper specs
for ROMM RGB and the x and y values in the ProPhoto RGB tags. But the red
and blue areas of the two profiles end up looking somewhat different in the
Grapher. Is that due to different Y values? It's hard for me to tell,
because ProPhoto RGB is a matrix-based profile (type: device: monitor),
whereas ROMM RGB is table-based (type: device: printer): the former
specifies the chromaticities in the tag table (rXYZ, gXYZ, bXYZ), but the
latter does not. So, I am not sure how to compare the two directly in that
regard.
But, whatever the causes, the fact remains that there is indeed a difference
in gamut shape between ROMM RGB and ProPhoto RGB, albeit limited. Viewed in
the ColorThink Grapher, the two profiles share what looks like a good 90+
percent of the very same gamut space, with *slight* departures in the white
and black points, as well as the blues and reds. These all look more like
*tweaks* than true substantial differences, all meant to boost the gamut of
ProPhoto RGB compared to ROMM RGB, making it somewhat larger.
I don't perceive these to be the dramatic differences that were intimated in
earlier messages, unless someone would like to explain why they should be
regarded as more substantial than that.
Regards.
--------------
Marco Ugolini
Mill Valley, CA
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