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Date: Wed, 17 Jan 2007 14:03:28 -0500
From: Frank Warmerdam <warmerdam@pobox.com>
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To: Joris <joris.at.lebbeke@skynet.be>
Subject: Re: [Tiff] Elevation Data
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Status:   

Joris wrote:
> So that leaves either b), or either agreeing a 'just anything' scheme
> doesn't work. I have some major remarks with the dynamic scheme. For
> starters, it's a two-pass scheme. Your algorithm needs to browse through
> the complete data before it's able to derive a range and make sense of
> the first pixel. That's no problem if images are small and/or buffered,
> but it doesn't scale to progressive streamed handling if big images.

Joris,

I'm not sure where progressive streaming came into this.  My OpenEV
viewer already does dynamic sampling of non-8bit TIFF files and
automatic contrast stretch.  It doesn't require too much extra time,
and it provides excellent quick display of a wide variety of scientific
data, including elevation data.

> Conversely, much imagery doesn't use its full range. Normal photographic
> material, for example, doesn't use the brightest white, nor does it
> contain patches of pure black. That's intentional, that's the actual
> image. Your dynamic range detection scheme applies something like a
> histogram equalisation, in a non-lineair space to add injury to insult,
> and that's no writer's intention.

Certainly automatic per-sample autoscaling is not appropriate for
simple photometric data.

> So, it seems that an 'anything goes as I'll detect it anyway' just isn't
> a valid option, not with interchange in mind, and not for progressive
> streamed handling of big images in any case.

I disagree.  If the objective is interchange with the family of geospatial
and scientific raster viewing applications it is still very useful.  It
is less so if the objective is display in photo viewers.  I disagree that
dynamic scaling is impractical for large images.

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.

Best regards,
-- 
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam@pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGeo, http://osgeo.org

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