On 2022-04-21 13:39, graxx@xxxxxxxxxxxx wrote:
Sure, but numbers can be encoded.
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 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>
<mailto: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
Appt 26 Bât A Résidence Le Patio
83 rue de la Bugellerie
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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>
--
Chris Lilley
@svgeesus
Technical Director @ W3C
W3C Strategy Team, Core Web Design
W3C Architecture & Technology Team, Core Web & Media