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From: Bob Friesenhahn <bfriesen@simple.dallas.tx.us>
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Cc: tiff@lists.maptools.org
Subject: Re: [Tiff] Question about transfer function
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On Tue, 8 May 2018, Larry Gritz wrote:

> I'm embarrassed to say that today I stumbled on a statement in the 
> TIFF 6.0 spec that had somehow eluded me previously in nearly 30 
> years of dealing with TIFF files. To wit:
>
> "Be aware that the PhotometricInterpretation value of 0 or 1 
> (grayscale) implies linear data because no gamma is specified. The 
> PhotometricInterpretation value of 2 (RGB data) specifies the NTSC 
> gamma of 2.2 as a default. If data is written as something other 
> than the default, then a GrayResponseCurve field or a 
> TransferFunction field must be present to define the deviation. For 
> grayscale data, be sure that the densities in the GrayResponseCurve 
> are consistent with the PhotometricInterpretation field and the 
> HalftoneHints field."

In the real world of semi-random sources and software, it is most 
likely that a grayscale image has a gamma of approximately 2.2.  Lots 
of software uses Rec.601 or Rec.709 Luma calculations to produce a 
gray image from RGB, or might even just select one of the RGB channels 
(e.g. green) and use it as the gray channel.  GraphicsMagick still 
uses Rec.601 Luma transforms by default although it is likely that 
Rec.709 is a better choice.

TIFF 6.0 is very old.  Since then ICC color profiles and other means 
have emerged to provide colorspace information.

Bob
-- 
Bob Friesenhahn
bfriesen@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/
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