AWARE [SYSTEMS]
AWare Systems, , Home TIFF and LibTiff Mailing List Archive

LibTiff Mailing List

TIFF and LibTiff Mailing List Archive
August 2007

Previous Thread
Next Thread

Previous by Thread
Next by Thread

Previous by Date
Next by Date

Contact

The TIFF Mailing List Homepage
Archive maintained by AWare Systems



New Datamatrix section



Valid HTML 4.01!



Thread

2007.08.06 22:08 "Transparency ... associated or unassociated alpha is more compatible?", by William Gallafent
2007.08.06 22:27 "Re: Transparency ... associated or unassociated alpha is more compatible?", by Bob Friesenhahn
2007.08.06 22:28 "Re: Transparency ... associated or unassociated alpha is morecompatible?", by Chris Cox
2007.08.06 22:32 "Re: Transparency ... associated or unassociated alpha is morecompatible?", by Chris Cox
2007.08.06 22:48 "Re: Transparency ... associated or unassociated alpha is morecompatible?", by Bob Friesenhahn
2007.08.06 22:48 "Re: Transparency ... associated or unassociated alpha is morecompatible?", by Chris Cox

2007.08.06 22:32 "Re: Transparency ... associated or unassociated alpha is morecompatible?", by Chris Cox

PNG and SGI RGB support associated alpha (aka transparency), just not
premultiplied.

TGA is supposed to support associated alpha (transparency), but most users
of TGA don't seem to understand the difference and get transparency and
general (unassociated) alpha channels confused.  When Photoshop 7 shipped
with transparency support in TGA format, the users got so confused that
Adobe had to go back to using unassociated alpha.  The TGA users still can't
agree on what they want, much less what they understand.

Associated Alpha == transparency
Unassociated Alpha == using the extra channel(s) for something else
Premulitplication (black matte) / Matting can only apply to transparency,
and varies with the file format or sometimes flags within the file format.

Chris



-----Original Message-----
From: tiff-bounces@lists.maptools.org on behalf of Bob Friesenhahn
Sent: Mon 8/6/2007 3:27 PM
To: William Gallafent
Cc: tiff@lists.maptools.org
Subject: Re: [Tiff] Transparency ... associated or unassociated alpha is
morecompatible?
 
On Mon, 6 Aug 2007, William Gallafent wrote:

> I'm experimenting with TIFF export for RGBA images, bit depth
> eight and sixteen. I need to decide which style of alpha
> channel to use in order to maximise interoperability with other

As best I can tell, alpha is only meaningful in TIFF for the 
associated alpha case.  It is the only case where the purpose/use of 
the alpha is defined.  One is only left to assume the intention of the 
alpha channel for unassociated alpha.

PNG, SGI RGB, Targa, and many other formats supporting an alpha 
channel use unassociated alpha.  In my own experience, unassociated 
alpha causes less headache (outside of TIFF) since so many other 
formats and algorithms work better with unassociated alpha.  As an 
example of where associated alpha causes a headache, consider the 
algorithm necessary to save the image to a palette-based image format.

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