[gdal-dev] Support for the premultiplied alpha channels

[Tamas Szekeres]

I've found that the tif and the png formats use different alpha
representations by default so I consider this as a bug in GDAL not to
represent this behaviour, and created a ticket for it.

http://trac.osgeo.org/gdal/ticket/2279


Best regards,

Tamas



2008/3/16, Tamas Szekeres <szekerest at gmail.com>:
> Here is an example to demonstrate the effect of  the incorrect alpha
>  consideration. I've composed a tiff created with adobe photoshop (and
>  loaded by GDAL C#) with a white-gray hatch background.
>
>  In the first case I've read the image as Pixelformat.Format32bppArgb
>  (with non premultiplied alpha):
>  http://vbkto.dyndns.org:1280/non_pre_overlay.png
>
>  In the second case I've loaded the same raster into Pixelformat.Format32bppPArgb
>  http://vbkto.dyndns.org:1280/pre_overlay.png
>
>  The latter is the correct.
>
>  Best regards,
>
>  Tamas
>
>
>
>  2008/3/16, Tamas Szekeres <szekerest at gmail.com>:
>
> > Frank,
>  >
>  >  I really got confused because of this :-((  It seems the the various
>  >  formats use different alpha representations by default. In this regard
>  >  I really don't know how to take care of this in an application without
>  >  having to consider the format actually used.
>  >  For example tiff uses pre-multiplied alpha by default (related to
>  >  EXTRASAMPLE_ASSOCALPHA = 1), but however, other formats like png uses
>  >  non premultiplied by default (AFAIK). Because of this reason the upper
>  >  level application won't really know which relationship to be used when
>  >  combining these images.
>  >  This is not an issue for GDAL itself but a significant problem for the
>  >  upper level applications. The drivers should at least denote which
>  >  alpha representation is actually used.
>  >
>  >
>  >  Best regards,
>  >
>  >  Tamas
>  >
>  >
>  >
>  >
>  >  2008/3/16, Frank Warmerdam <warmerdam at pobox.com>:
>  >
>  > > Tamas Szekeres wrote:
>  >  >  > Folks,
>  >  >  >
>  >  >  > Do we have any support to use bitmaps with premultiplied alpha
>  >  >  > channels? I think there are some raster formats that can convey these
>  >  >  > types of images and the viewers can take care of this option.
>  >  >  >
>  >  >  > The only thing I'd like to see is to be able to determine if the image
>  >  >  > actually uses premultiplied alpha regardless of whether the drivers
>  >  >  > actually support this or not at the moment.
>  >  >  >
>  >  >  > But currently I don't see which is the right place to denote this
>  >  >  > option. I consider this is a dataset wide property rather that
>  >  >  > attached to a single band.
>  >  >
>  >  >
>  >  > Tamas,
>  >  >
>  >  >  There is no GDAL way of expressing whether image bands are premultipled
>  >  >  with alpha (unfortunately).
>  >  >
>  >  >  Best regards,
>  >  >
>  >  > --
>  >  >  ---------------------------------------+--------------------------------------
>  >  >  I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
>  >  >  light and sound - activate the windows | http://pobox.com/~warmerdam
>  >  >  and watch the world go round - Rush    | President OSGeo, http://osgeo.org
>  >  >
>  >  >
>  >
>