CIE primary and white point specification in TIFF (vs ISO IIF)

CIE primary and white point specification in TIFF (vs ISO IIF)

Post by Mark Sorns » Wed, 15 Jul 1992 06:14:27



        I've been comparing how TIFF V6.0 and the (proposed) ISO Image
    Interchange Facility (IIF) [draft as of 27 March 1992] specify the CIE
    chromaticity and luminance values for the white point and each of the
    primaries.

        TIFF only specifies the CIE x and y chromaticity values for the
    white point and the primaries, whereas the IIF spec specifies x,y and
    Y, as well as the CIE standard observer (2 degrees 1931 or 10 degrees
    1976).

        Since the TIFF spec says that the chromaticity values apply to the
    1931 CIE xy chromaticity diagram, there's no ambiguity there [although
    I do have a question about how the specification of the observer
    effects the identification of each of the primaries and the white
    point, and consequently, the 'color' of any pixel].

        My most pressing question is this:  since TIFF does not specify
    what the Y value is for either the white point (though I assume this
    can be assumed to be 1.0, yes?) or any of the primaries, what should
    any reader of TIFF 6.0 files assume the Y value to be (for the white
    point and the primaries)?  *CAN* a reader even assume a value for Y for
    the white point and the primaries?

        I'd appreciate e-mail replies [or e-mail copies of posted replies]
    since I don't read this group very often.  Thanx in advance.

                                                                -mark.

 
 
 

1. White point & Black point

I've been asked for some stock images, and for them to be scanned with a
White point of less than 252 and a Black point no less than 3.
This keeps the rgb values slightly less than the full 256 range, and helps
limit extreme values that will not print. And it helps limit errors when
values fall outside the boundaries of an 8 bit file.
However, I have some files that are already scanned and saved as 16 bit. I
do not want to have to go back and scan again, so how do I accomplish the
same in PS?
Do I go to Levels and move the bottom Output Levels slider in at both ends
by 3 points?
And if so, is this only true for files which have a Levels histogram showing
that the values touch the end on one or both sides ? If they dont touch the
end then they're less than the maximum/extreme of 0 & 256 anyway - correct?

=========

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