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

[Mark A. <m.f...@pl...>]

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