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From: "Joris" <joris.at.lebbeke@skynet.be>
To: "Frank Warmerdam" <warmerdam@pobox.com>
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Subject: Re: [Tiff] Elevation Data
Date: Wed, 17 Jan 2007 20:00:55 +0100
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Status:   

Frank,

> > > Libtiff uses the names PHOTOMETRIC_MINISWHITE and
> > > PHOTOMETRIC_MINISBLACK.  This implies something different than
> > > BlackIsZero.  Does that help?
> >
> > It's confusing. I usually use the names MinIsBlack and MinIsWhite...
> > What's in a name?
>
> Craig Bruce referred to these as ZeroIsBlack which is quite distinct
> from MinIsBlack if you are thinking about signed integer values as he
> was.

I'm starting to see where you guys are coming from...

But still, ZeroIsBlack and MinIsBlack are different names for the same
grayscale we *all know* encodes black as zero. It is unsigned. I don't
care if Craig feels it's elevation data. He's writing grayscale.

Well, I do care if Crag feels it's elevation data, of course, because I
feel we need to come to a solution to this problem that satisfies us
all. But if it's grayscale, it *is* grayscale, and that comes with all
known properties. So either

a) we need to find something that sits good with renderers seeing the
grayscale, and still sits good with Craig re-interpreting it as
elevation data.
b) or we need to simply say it's impossible, and refer him to an
Undefined ExtraSample, which is really intended for this sort of purpose
in the first place. The possible trouble here may be that there is no
NULL Photometric, with 0 image data samples, leaving all other samples
as ExtraSamples and making it clear for image renderers there's nothing
there to their liking. So if he hasn't got an actual image to write
along with his elevation data, and doesn't much care to write a dummy
image channel, we're stuck with option a).

In another message, you wrote
> There is a certain inherent tension between those who think of TIFFs
and
> images in general in photometric terms, and those who think of them as
a
> container for transporting all sorts of gridded scientific data.  But
we
> need to recognize they are both legitimate applications.

Of course they are both legitimate applications. But I don't know about
there being room for tension. There certainly is no Purpose WORD tag
with values Imaging = 1 and Scientific = 2. So the same data, hits both
family of readers.

There is the ExtraSamples, where you can put anything, and it's clear to
the photographic imagers this isn't intended for them to render. But
then there's the base samples, and the rules that apply to the
Photometric and all chosen for these, do apply.

I keep repeating myself. Though there may be no need to do so. I came up
with a recommendation that I think agrees with the spec, agrees with
imaging needs, and allows Craig to redefine ranges to match his view of
metres metric and sea level = 0 elevation data. Is there anything in
that recomendation (http://www.asmail.be/msg0055261416.html) that you
see conflicts with any needs or with de facto usage (as far as the
latters is consistent)?


Best regards,

Joris Van Damme
info@awaresystems.be
http://www.awaresystems.be/
Download your free TIFF tag viewer for windows here:
http://www.awaresystems.be/imaging/tiff/astifftagviewer.html

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