[gdal-dev] GDAL alpha channels

[Craig Bruce]

I have run into a problem using GDAL to read TIFF images with a alpha
channels.  GDAL returns the source TIFF sample values verbatim regardless
of whether the alpha channel is associated (pre-multiplied) or unassociated
within the TIFF image samples.  (In each alpha model, the same pixel
value with the same alpha value will have different sample values.)

GDAL does not appear to enforce a specific alpha model in its interface
(pre-multiplied or non-pre-multiplied) and does not appear to offer any
indication of which is currently being used.  This creates a dead end as
far as the GDAL interface goes.  My system and most image formats (e.g.,
PNG) use the non-pre-multiplied alpha model.  TIFF can use either.

What is a user supposed to do about this?

--------------------------+----------------------+--------------------------
Dr. Craig S. Bruce        | Ph 819-771-8303 x205 |             CubeWerx Inc.
Senior Software Developer |   Fax 819-771-8388   |  Gatineau, Québec, Canada
csbruce at cubewerx.com      |  http://csbruce.com/ |  http://www.cubewerx.com/
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