I better go take more programming lessons because I never knew that “12.45”
could be represented using an “integer” type. I completely confess my
ignorance. I use “int” to hold numbers that “don’t have” decimal parts, such as
65536, and “double” to hold numbers that “do have” decimal parts such as
“12.45” or “87.92”.
I’m sorry I completely missed your point but I don’t feel offended.
/ Roger
From: argyllcms-bounce@xxxxxxxxxxxxx <argyllcms-bounce@xxxxxxxxxxxxx> On Behalf
Of Chris Lilley
Sent: April 21, 2022 1:10 PM
To: argyllcms@xxxxxxxxxxxxx
Subject: [argyllcms] Re: ranges of LAB values
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 <mailto: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 <mailto:argyllcms-bounce@xxxxxxxxxxxxx>
<mailto:argyllcms-bounce@xxxxxxxxxxxxx> <argyllcms-bounce@xxxxxxxxxxxxx> On
Behalf Of Sylvain Ard
Sent: April 21, 2022 7:33 AM
To: argyllcms@xxxxxxxxxxxxx <mailto: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 <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 13:29, Aaron Perelmuter <aaron.746@xxxxxxxxx
<mailto: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
<mailto: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 <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>
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>
--
Chris Lilley
@svgeesus
Technical Director @ W3C
W3C Strategy Team, Core Web Design
W3C Architecture & Technology Team, Core Web & Media
