[Gdal-dev] gdalwarp and -dstalpha

[Frank Warmerdam]

On 9/8/05, Eric Sokolowsky <eric.sokolowsky at gsfc.nasa.gov> wrote:
> I have experienced a strange (in my mind) problem with gdalwarp and the
> -dstalpha option. When I use this option on a couple of particular images
> that are reported as RGBA tiff files it turns them to grayscale. I am
> using gdal 1.2.5, but the same results seemed to occur with gdal 1.2.6.
> If the source image is RGB (no alpha) the output image is RGBA, as I
> expect. Is this normal/expected behavior? It would make more sense to me
> to have "-dstalpha" always add an alpha channel if it is not there, and
> to have a separate option "-dstnoalpha" to remove the alpha channel if
> present, instead of "-dstalpha" toggle the alpha, if this is the intended
> behavior.

Eric,

-dstalpha should use an existing alpha channel if there already is
one on the destination file.  I just tried using gdalwarp to on an RGBA
input file with -dstalpha and the output was also RGBA. 

Perhaps this was fixed since 1.2.6 (now a bit antique), or perhaps
there is something special about your situation. 

The -dstalpha is not intended to toggle presence of an alpha channel.

Best regards,
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