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Date: Fri, 15 Sep 2006 12:37:44 -0500 (CDT)
From: Bob Friesenhahn <bfriesen@simple.dallas.tx.us>
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To: Gerben Vos <Gerben@zylab.com>
Subject: Re: [Tiff] is there alpha component present in Grayscale or
	Palettecolor image
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Status:   

On Fri, 15 Sep 2006, Gerben Vos wrote:

> Anurag Singh wrote:
>
>> I'm trying to handle tiff images with alpha component, so i want to
>> know that any tiff image which is Grayscale or Palette color can have
>> alpha component.
>
> It is possible for greyscale, just use the ExtraSamples tag as defined
> in section 18 of the TIFF spec. It is not possible for palette images;
> the ColorMap tag uses a fixed format with exactly three values per
> entry.

I would not go so far as to say it is not possible to have an alpha 
channel with a Palette-based image.  It does seem to be quite 
possible, albeit rather inconvenient/senseless if associated alpha is 
used.  I don't recall if I have ever written such an image and I have 
certainly never seen one in the real world.  It is correct that the 
color Palette itself can not contain the alpha 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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