Wish I could help you with image magick but I have no experience with this
product.
/ Roger
From: argyllcms-bounce@xxxxxxxxxxxxx <argyllcms-bounce@xxxxxxxxxxxxx> On Behalf
Of Sylvain Ard
Sent: April 21, 2022 7:25 AM
To: argyllcms@xxxxxxxxxxxxx
Subject: [argyllcms] Re: ranges of LAB values
what I want to do is créate a profile with scanin and IT8 file (with LAB
values), TIFF RGB image of the color chart and CHT file, then use image magick
command :
magick in.tiff +profile ICC -profile scanner_profile.icc -profile
some_lab_profile.icc -define quantum:format=floating-point -depth 32 out.tiff
to convert a TIFF of scanned object to LAB TIFF with the profile of scanin and
standard LAB profile
and then do
magick out.tiff -crop 1x1+12+34 +repage ftxt:
to extract LAB value of a pixel of the scanned object image
This "-crop" makes a one-pixel image, from the pixel at (12,34). "ftxt:" gives
the value in each channel, on a nominal scale from 0 to QuantumRange. I use IM
Q32, QuantumRange is 2^32-1 which is 4294967295. If you use Q16, then your
QuantumRange is 65535.
so the values extracted are not in the range -127-128 for A and B and 0-100 for
L
I'm feared there is a problem
Sylvain Ard
0549507724
0778380991
sylvain.ard@xxxxxxxxx <mailto:sylvain.ard@xxxxxxxxx>
http://sylvain-ard.fr
Entreprise individuelle SIRET : 80079243400022
Appt 26 Bât A Résidence Le Patio
83 rue de la Bugellerie
86000 Poitiers
Le jeu. 21 avr. 2022 à 13:19, Sylvain Ard <sylvain.ard@xxxxxxxxx
<mailto:sylvain.ard@xxxxxxxxx> > a écrit :
isn't it rather -127 - 128?
because to match 0-1 values to near 127 values we do 255*value-127 ?
Sylvain Ard
0549507724
0778380991
sylvain.ard@xxxxxxxxx <mailto:sylvain.ard@xxxxxxxxx>
http://sylvain-ard.fr
Entreprise individuelle SIRET : 80079243400022
Appt 26 Bât A Résidence Le Patio
83 rue de la Bugellerie
86000 Poitiers
Le jeu. 21 avr. 2022 à 13:15, Aaron Perelmuter <aaron.746@xxxxxxxxx
<mailto:aaron.746@xxxxxxxxx> > a écrit :
In most cases, for all intents and purposes, Lab is understood to be within the
following ranges: L*0>>100;a*-128>>127;b*-128>>127
As was mentioned in a reply to a previous list message, technically Lab values
can and do sometimes go well outside of the ranges listed but in almost every
instance, for the vast majority of people and use cases, Lab is understood to
be within the bounds listed above.
As was also mentioned in another reply to a list message, it certainly helps to
be able to give a more accurate or more pertinent answer if you can state your
reasons why you’d like to know what it is you’re asking about.
I’m not 100% sure about this but ttbomk, there’s no such thing as a v2, v4 or
any other version number for Lab profiles, for tiff or any other file format.
Is there even a Lab profile anyway? What’s the exact file name for the Lab
profiles you’re asking about? Finally, IT8 files consists of a great many
different sub-types which are used for myriad purposes. Most commonly IT.7/3
and IT8.7/4 are often used for profiling CMYK processes, IT8.7/2 (used to be)
often used for creating scanner profiles and so on - I say used to be because
not many people in this day and age are creating scanner profiles or even
own/use a scanner. Regardless, there are several flavours of IT8 type files but
again, in almost every case the Lab range is as listed above.
Best Regards,
Aaron.
On Thu, 21 Apr 2022 at 8:45 pm, Sylvain Ard <sylvain.ard@xxxxxxxxx
<mailto:sylvain.ard@xxxxxxxxx> > wrote:
Hi I would like to know the ranges of L,A and B values in : ICC profiles v2,
TIFF images with LAB profile v2 and v4, IT8 CGATS files and their matches
because the ranges are differents.
Thank you
Best regards
Sylvain Ard
0549507724
0778380991
sylvain.ard@xxxxxxxxx <mailto:sylvain.ard@xxxxxxxxx>
http://sylvain-ard.fr
Entreprise individuelle SIRET : 80079243400022
Appt 26 Bât A Résidence Le Patio
83 rue de la Bugellerie
<https://www.google.com/maps/search/83+rue+de+la+Bugellerie+86000+Poitiers?entry=gmail&source=g>
86000 Poitiers
<https://www.google.com/maps/search/83+rue+de+la+Bugellerie+86000+Poitiers?entry=gmail&source=g>
