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
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0778380991
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Le jeu. 21 avr. 2022 à 13:19, Sylvain Ard <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
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> 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>
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
http://sylvain-ard.fr
Entreprise individuelle SIRET : 80079243400022
Appt 26 Bât A Résidence Le Patio
83 rue de la Bugellerie
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86000 Poitiers
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