1999.07.19 16:03 "RE: White point in CIELab TIFF files", by Martin Bailey
The TIFF 6 spec is more than a little vague about what white point should be used for L*a*b* images, and there's no specified tag to put that data into the file.
There is a tag to store White point, the tag is:
Tag = 318 (13Eh)
Type = RATIONAL
N = 2The chromacity of the white point of the image. This is the chromacity when each of the primaries has its ReferenceWhite value. The value is described using the 1931 CIE xy chromacity diagram and only the chromacity is specified.
TIFF 6.0 specification has a "Section23: CIE L*a*b* images" where it says that "White point: does not apply" but I have seen several TIFF whith photometric = CIELAB and white point specified as D50 in this tag.
Hmmm. I knew about the tag, but the Lab files I have don't use it and I'd seen the comment about not applying.
Ho hum. I can feel some experimentation coming on. I need to do that to figure out how 256 levels relates to a range of -127 to +127 anyway :-)
Regards
Martin Bailey
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