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Date: Thu, 17 Apr 2008 16:48:19 -0500 (CDT)
From: Bob Friesenhahn <bfriesen@simple.dallas.tx.us>
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To: Chris Cox <ccox@adobe.com>
Subject: Re: [Tiff] a question about creating transparency of the image in TIFF
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On Thu, 17 Apr 2008, Chris Cox wrote:

> There are 2 types of alpha in TIFF: associated (transparency) and
> unassociated (undefined extra channels).
>
> Also, the TIFF spec. states that images with associated alpha (transparency)
> must be premultiplied.

The TIFF specification does not prevent a colormapped file with 
transparency but the colors in the colormap would then need to be 
premultiplied, which could be a challenge to surmount.  As a result, 
no one does this.

Popular software like Photoshop does not place any interpretation on 
unassociated alpha, so such channels are just "extra" channels that 
only a Photoshop expert could figure out.

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

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