Strictly speaking, Aaron, the values ought to be encoded using floating-point.
In Photoshop, Lab values are presented as “integers”, no decimal part.
That’s Adobe’s choice. “Real” Lab values always have a fractional component.
/ Roger
From: argyllcms-bounce@xxxxxxxxxxxxx <argyllcms-bounce@xxxxxxxxxxxxx> On Behalf
Of Aaron Perelmuter
Sent: April 21, 2022 9:27 AM
To: argyllcms@xxxxxxxxxxxxx
Subject: [argyllcms] Re: ranges of LAB values
On Thu, 21 Apr 2022 at 11:17 pm, <graxx@xxxxxxxxxxxx
<mailto:graxx@xxxxxxxxxxxx> > wrote:
Lab in often encoded on 8 bit.
So, it goes from -127 to +128 but many people usually quote it on a 0 to 128
scale by “habit”.
-127 to +127 is the range of an 8 bit signed integer, if I’m not mistaking.
-Doesn’t it have to be -127 to +128 or vice-versa as -127+127+0=255 individual
values, not 256? Just wondering is all…
Cheers,
Aaron.
/ Roger
From: argyllcms-bounce@xxxxxxxxxxxxx <mailto:argyllcms-bounce@xxxxxxxxxxxxx>
<argyllcms-bounce@xxxxxxxxxxxxx <mailto:argyllcms-bounce@xxxxxxxxxxxxx> > On
Behalf Of Sylvain Ard
Sent: April 21, 2022 7:19 AM
To: argyllcms@xxxxxxxxxxxxx <mailto:argyllcms@xxxxxxxxxxxxx>
Subject: [argyllcms] Re: ranges of LAB values
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>
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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>
