[gdal-dev] Support for the premultiplied alpha channels
Tamas Szekeres
szekerest at gmail.com
Sun Mar 16 17:36:22 EDT 2008
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
> >
> >
>
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