2002.03.05 20:31 "Re: 16 bit LAB in TIFF - documentation update", by Chris Cox
This is the official update for 16 bit LAB data in TIFF files, part of the documentation update coming out with the Photoshop 7 SDK.
I'm afraid I'm going to abuse your generosity also. But this is a draft, so picking up problems is good, right?
- The numeric range values for a* and b* conflict with the description - if you multiply the max 8 bit values by 256 as described, you don't end up with the numbers given.
For which encoding?
CIE LAB (PhotometricInterpretation 8):
-128 => -128
127 => 127
256 * -128 => -32766
256 * 127 => 32512
(plus some margin)
ICC LAB (PhotometricInterpretation 9):
-128 + 128 => 0
127 + 128 => 255
256 * -128 + 32768 = 0
256 * 127 + 32768 = 65280
256 * (-128 + 128) = 0
256 * (127 + 128) = 65280
(plus some margin -- identical to the ICC.1:1998-09 encoding)
The math works on my calculator....
- Why does the WhitePoint tag not apply? How do you figure out whether a Lab encoded file is D50, or D65, or anything else come to that. I never understood that statement in TIFF 6. Martí Maria reads TIFF6 as stating that Lab data must be D65, but it only says that is generally used.
That part I didn't change - that came from the original LAB in TIFF notes. It could be optional -- but Adobe and ICC prefer D50, while other software vendors seem to prefer D65.