This is completely correct, except for the float part.
Using 16bits per component (uint16 for L, int16 for a and b) also
preserves the fractional component.
But no-one should be representing Lab in 8bits unless they don't care
about accuracy at all! Think of it this way - if values are rounded (or,
worse, truncated) to 8bits there is a worst-case error of almost 1 on
each of L, a and b. So a deltaE 76 of sqrt(3) = 1.7, from quantization
error alone!
On 2022-04-21 09:23, graxx@xxxxxxxxxxxx wrote:
--
Sylvain,
CIE Lab is independent of any computer implementation.
If you look at CIE Publications 15-2 which defines Colorimetry, you won’t find any mention of how the numbers should be represented inside a computer memory. CIE Lab numbers are just numbers and don’t need computers to exist.
Actually, you need “floating-point” variables to correctly represent CIE Lab, otherwise you loose the fractional part.
/ Roger
*From:*argyllcms-bounce@xxxxxxxxxxxxx <argyllcms-bounce@xxxxxxxxxxxxx> *On Behalf Of *Sylvain Ard
*Sent:* April 21, 2022 7:33 AM
*To:* argyllcms@xxxxxxxxxxxxx
*Subject:* [argyllcms] Re: ranges of LAB values
I want to understand, it's all. I think you're right the good formula is 255*(0 to 1 value)-128 because I saw on Internet that unsigned bytes come from -128 to 127 and I think that CieLab is standard unsigned byte.
Do you know my imagemagick commands ?
Sylvain Ard
0549507724
0778380991
sylvain.ard@xxxxxxxxx
http://sylvain-ard.fr
Entreprise individuelle SIRET : 80079243400022
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Le jeu. 21 avr. 2022 à 13:29, Aaron Perelmuter <aaron.746@xxxxxxxxx> a écrit :
I suppose that depends on from where one derives their
definitions. I would generally do 256*x-128 as 0>>255 contains 256
integers. Also, do the math and substitute either 127 or 128 and I
doubt the resulting colour will be in any way noticeably different.
I am somewhat confused though, if you already know the answer to
your question, why ask? Please don’t misunderstand what I’m
saying, I mean, it’s better to be as specific as you possibly can
in terms of what you’re asking. If you understand something to be
255*value-127, why not ask about the time you saw something very
slightly different to that? Just makes it easier to give the exact
answer you’re searching for is all.
Keep the questions coming, we can all learn from them.
On Thu, 21 Apr 2022 at 9:19 pm, Sylvain Ard
<sylvain.ard@xxxxxxxxx> wrote:
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
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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
<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>
