[argyllcms] gamut mapping
- From: nome cognome <darkbasic4@xxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Wed, 22 Jul 2009 18:52:27 +0200
Hi all, my name's Niccolò and I'm from Italy. First of all thank you
for creating this awesome software, secondly excuse me for my bad
English.
I have some doubts about tiffgamut and collink.
I read I have to use appearance space in tiffgamut if I intend to use
the resulting gamut with collink. I can trigger it with the -pj
switch, but I cannot understand what's the intent transform (the -i
switch) in this step. Once I have extracted the gamut I'dd like to use
the perceptual intent with collink, but while extracting the gamut I
don't even know what's the aim of choosing an intent!
I read that with the aboslute intent (and the -pj switch of course) I
will have a white point mismatch between source ad destination
gamuts[1], so it should be better to use the relative intent (-ir)
instead of the absolute one[2] (-ia), but in another mail[3] is
suggested to use -ia. Why?
On the documentation I read: "Note that the CIECAM02 output space
selection by default uses the colorimetric transform of the profile
resulting in the appearance of the native device". So the default
intent is absolute or relaitive, but does not specify which one.
Furthermore I found something very strange: "The default intent will
be absolute colorimetic for L*a*b* output, and CIECAM02 appearance for
Jab output." What? What's the "CIECAM02 appearance" intent? I never
heard about it and there isn't any "CIECAM02 appearance" option for
the intent switch (-i). I'm _really_ confused!
I found this[4] message in the mailing list, you talk about Absolute,
Relative and CIECAM02 output color space.
From wikipedia[6]: "Absolute color space: a color space in which
colors are unambiguous, that is, where the interpretations of colors
in the space are colorimetrically defined without reference to
external factors. CIEXYZ and sRGB are examples of absolute color
space. The L*a*b* is sometimes referred to as absolute, though it also
needs a white point specification to make it so. A non-absolute color
space (like RGB) can be made absolute by defining its relationship to
absolute colorimetric quantities (ex. define an ICC profile).
Converting between two non-absolute color spaces or between absolute
and non-absolute color spaces is almost a meaningless concept."
From xicclu documentation[5]: "Normally the native PCS (Profile
Connection Space) of a device or abstract profile is used, but the -p
flag allows this to be overridden."
So with the -p switch I can choose the output color space, but why an
output color space defined through the -p parameter becames "Relative"
if I use a particular intent? What do you mean with "Relative" output
color space?
I consider this topic very interesting, so I surfed the web searching
for informations about it and I found lots of interesting
books[7,...,12]. Can you suggest me an easy one to start from?
The -c parameter allows choosing a set of viewing conditions (which
must be the same used with collink[2]), but which one should I choose?
pcd?
Last but not the least: noise. What's the best way to reduce the
impact of noise on the gamut size and so on the gamut mapping? I think
that the color popularity filter will help a bit, but I'm not sure if
it's the best way to achieve the goal. Do you suggest me to use the
color popularity switch, downsampling, noise reduction or a particular
filter? I don't care about processing time. I found an interesting
discussion about it[13] where someone suggested to use a median cut
quantization algorithm for gamut mapping purposes. Have been done
tests about it?
Thank you,
Niccolò (aka darkbasic)
[1]//www.freelists.org/post/argyllcms/Device-link-gamut-mapping-how-to-do-it-right,4
("I've noticed that the result of that linking is white point mismatch
between source and destination gamuts")
[2]//www.freelists.org/post/argyllcms/Device-link-gamut-mapping-how-to-do-it-right,8
("But for use with collink perceptual intent, a relative colorimetric
gamut in the same color space (ie. CIECAM02 Jab space with the same
viewing conditions) is the correct combination.")
[3]//www.freelists.org/post/argyllcms/tiffgamut-feature-request,2
[4]//www.freelists.org/post/argyllcms/tiffgamut-103,1
[5]http://www.argyllcms.com/doc/xicclu.html#p
[6]http://en.wikipedia.org/wiki/Absolute_color_space
[7]http://www.cis.rit.edu/fairchild/CAM.html
[8]http://books.google.it/books?id=8_TxzK2B-5MC&source=gbs_navlinks_s
[9]http://www3.interscience.wiley.com/journal/35037/home
[10]http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.85.4140
[11]http://www.cis.rit.edu/fairchild/PDFs/AppearanceLec.pdf
[12]http://www.polybytes.com/misc/Meet_CIECAM02.pdf
[13]//www.freelists.org/post/argyllcms/noise-and-gamut-input-profiles-with-B2A-table,10
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