Re: [Lcms-user] General question about CIE Lab and TIFF

[Aaron B. <bo...@gm...>]

Thanks, Mark That's quite helpful. Is there a way of storing the illumanant
or other CIE information in the TIFF ?
And are there any other TIFF tags I should be reading besides photometric
tag ?

On Sun, Aug 18, 2019 at 3:53 AM Mark Allen <m.f...@pl...> wrote:

> Hi,
>
> If you’re talking about TIFFTAG_PHOTOMETRIC (262) then the
> PHOTOMETRIC_CIELAB (8) and PHOTOMETRIC_ICCLAB (9) just tell what color
> space the pixel data uses. The only difference in the two is whether the A
> and B components are signed or unsigned.
>
> The original implementation of LAB in TIFF defined CIELAB. It uses signed
> values –128..127 for the A and B components.
>
> Apparently some people implemented it improperly and assumed unsigned
> values for A and B of 0..255. Adobe tried to “solve” the problem by adding
> the ICCLAB version to TIFF which uses unsigned 0..255 for A and B. At least
> that’s how I remember it. This happened more than 20 years ago.
>
> There’s two formats so you can have both signed and unsigned versions of A
> and B. Signed values seems to cause problems for some software so they can
> use the unsigned version instead. Other than that the two LABs are the same.
>
> You can see the “official” version in the two main TIFF documents.
> https://www.adobe.io/open/standards/TIFF.html  The addition of ICCLAB is
> mentioned in the TIFF technical notes:
> https://www.adobe.io/content/dam/udp/en/open/standards/tiff/TIFFPM6.pdf
>
> Mark Allen
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