[argyllcms] Re: Perceptual intent
- From: <robert@xxxxxxxxxxxxxxxxxx>
- To: <argyllcms@xxxxxxxxxxxxx>
- Date: Thu, 18 Dec 2014 15:17:31 -0000
Graeme Gill wrote:
>If you don't know what it's doing, then I don't think
>anything certain can be concluded from it's reports.
Well ... exactly the same thing could be said of i1Profiler, Argyll
(although I suppose I could look at the code, so there is a definite plus
there), ColorThink Pro, Photoshop .... At some stage you have to trust what
expert companies produce, and Imatest (which produces GamutVision) is a very
reputable company in the measurement and testing of image systems.
>If you define the source gamut as the colorspace the images are encoded in
>(which is the assumption that happens 99% of the time), then this is very
>easy,
>since there is nothing that forces an image to occupy the full gamut of the
>space it is encoded in.
OK, I understand ... when you said "occupy the source gamut" you meant fully
occupy the source gamut. The thing is that a particular image could have 99%
of colours well within sRGB, but with the one, critical, colour being
outside of even Beta RGB, but within the printer gamut (a lot of 'experts'
like Andrew Rodney, Digital Dog, recommend using ProPhoto for that reason).
If sRGB is chosen then the critical colour gets dumbed down, if ProPhoto is
chosen then all the other colours are dumbed down(assuming a perceptual
mapping). Of course, it could be that your image-specific mapping would
deal with that situation very well.
>If perceptual intent is used, then just portion of
>the image gamut outside that of the destination will fill the destination -
>portions of the image gamut that are within the destination will stay
>within it.
Stay within it, yes, but also get compressed in order to try to maintain the
relationship between the colours.
>Using relative colorimetric, the values are clipped by at least the black
>level of the
>printer, which is about 3 dE. Using a crude offset of the black, that makes
>the expected perceptual output roughly:
>3, 3 + 8 = 12, 21 + 3 = 25.
>which is not so far from what you get.
>i.e. you expect the black to be raised, so as to not loose
>shadow detail the way you are loosing it to clipping using relative
>colorimetric.
Well, the black point with perceptual is the same in the perceptual and
relative mappings, so I don't see why you say that the relative mapping is
clipping the colours more that the perceptual mapping. What is happening is
that greys are being raised in the perceptual mapping while the black point
is maintained. BTW, I did have BPC on.
Your figures are consistent with mine, using my profile. However what I
would like to understand is why the perceptual darks are opened up so much
more than the relative darks. Here are some figures:
Input Lab:
0 0 0
5 0 0
10 0 0
15 0 0
20 0 0
25 0 0
xicclu -fb -ir iPF6400-Canson-Baryta-310-Argyll-2584.icc < LabIn.txt >
RGBOutRel.txt
xicclu -ff -ir iPF6400-Canson-Baryta-310-Argyll-2584.icc < RGBOutRel.txt >
LabOutRel.txt
LabOutRel:
0.000000 0.000000 0.005768 [RGB] -> Lut -> 3.142155 -0.047737 0.872621 [Lab]
0.017459 0.022376 0.035043 [RGB] -> Lut -> 4.995929 -0.062729 0.493997 [Lab]
0.068113 0.088819 0.102322 [RGB] -> Lut -> 9.973725 -0.012720 -0.023000
[Lab]
0.123088 0.156227 0.169959 [RGB] -> Lut -> 14.928571 0.014240 0.068149 [Lab]
0.174376 0.216059 0.234942 [RGB] -> Lut -> 19.934737 0.043087 0.070440 [Lab]
0.220649 0.267695 0.289002 [RGB] -> Lut -> 24.874757 0.024867 0.083745 [Lab]
xicclu -fb -ip iPF6400-Canson-Baryta-310-Argyll-2584.icc < LabIn.txt >
RGBOutPerc.txt
xicclu -ff -ir iPF6400-Canson-Baryta-310-Argyll-2584.icc < RGBOutPerc.txt >
LabOutPerc.txt
LabOutPerc:
0.000000 0.000000 0.000000 [RGB] -> Lut -> 3.060566 -0.115180 1.191879 [Lab]
0.093685 0.113280 0.118837 [RGB] -> Lut -> 12.107669 0.325751 1.103462 [Lab]
0.154788 0.190790 0.196084 [RGB] -> Lut -> 17.785377 -0.098978 1.349102
[Lab]
0.201016 0.243456 0.253762 [RGB] -> Lut -> 22.622330 -0.108052 1.351999
[Lab]
0.246967 0.293316 0.307076 [RGB] -> Lut -> 27.415151 -0.057629 1.217678
[Lab]
0.297312 0.346474 0.361547 [RGB] -> Lut -> 32.296640 -0.061477 1.196553
[Lab]
There is a huge difference in the darks between the two and I don't
understand why the perceptual darks are so much lighter that the relative
darks.
Feeding the same data through the i1Profiler profile gives this:
xicclu -fb -ip iPF6400-Canson-Baryta-310-i1Profiler-2584.icc < LabIn.txt >
RGBOutPercI1.txt
xicclu -ff -ir iPF6400-Canson-Baryta-310-i1Profiler-2584.icc <
RGBOutPercI1.txt > LabOutPercI1.txt
0.000009 0.000000 0.001556 [RGB] -> Lut -> 3.772275 0.080011 0.654983 [Lab]
0.030916 0.047660 0.042261 [RGB] -> Lut -> 8.063481 0.100195 0.542403 [Lab]
0.088553 0.109315 0.113235 [RGB] -> Lut -> 12.388256 0.050054 0.704928 [Lab]
0.137472 0.164338 0.170508 [RGB] -> Lut -> 16.814056 0.048684 0.358926 [Lab]
0.184870 0.217493 0.233959 [RGB] -> Lut -> 21.173848 0.031764 0.464404 [Lab]
0.233533 0.274958 0.295613 [RGB] -> Lut -> 25.718078 0.044263 0.353982 [Lab]
So the darks are opened up more than the Argyll relative darks, but a lot
less than the Argyll perceptual darks.
I don't know if the Argyll perceptual mapping is a problem or not ... it can
be fixed with a curve in Photoshop. However, when toggling from perceptual
to relative soft-proofing in Photoshop, the perceptual proof is way lighter
than the relative proof. It just seems that your perceptual mapping shifts
the whole image up, but it holds the L=0 black down.
>That image doesn't seem to be available at present.
Sorry, it's here:
http://www.irelandupclose.com/customer/argyll/perctest.jpg
Robert
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