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Message-ID: <4B38F010.4060104@gmail.com>
Date: Mon, 28 Dec 2009 18:51:12 +0100
From: Manlio Perillo <manlio.perillo@gmail.com>
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To: Lee Howard <faxguy@howardsilvan.com>
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Subject: Re: [Tiff] bug in tiff2pdf, when handling Lab images
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Lee Howard ha scritto:
> Manlio Perillo wrote:
> [...]
>> The problem is with tiff2pdf writing two times the WhitePoint entry in
>> the Lab dictionary:
>>
>> /ColorSpace [/Lab <<
>> /WhitePoint [0.9643 1.0000 0.8251]
>> [0.9643 1.0000 0.8251]
>> /Range [-127 127 -127 127]
>>  
>>>> ]
> 
> If you look at the code this appears to be intentional:
> 

Ok, maybe I have found the real cause of the problem.

The TIFF 6.0 specification states that, for L*a*b* color space
(PhotometricInterpretation):

  ...
  WhitePoint: does not apply
  ...
  The reference white for this data type is the perfect reflecting
  diffuser (100% diffuse reflectance at all visible wavelengths).
  The L* range is from 0 (perfect absorbing black) to 100 (perfect
  reflecting diffuse white). The a* and b* ranges will be represented as
  signed 8 bit values having the range -127 to +127.
  ...
  Generally, D65 illumination is used and a perfect reflecting diffuser
  is used for the reference white.

D65 is also the default value of the WhitePoint field.


The Adobe Photoshop TIFF Technical Notes, instead, states:

  WhitePoint: default is CIE D50, may be omitted if default value


So in tiff2pdf code both cases are considered; IMHO one of the cases
should have been commented but the author forgot to do so.

The Photoshop technical notes are the one to follow, since the original
TIFF 6.0 specification is wrong.



Regards   Manlio
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