[Gdal-dev] gdalwarp nodata

[Frank Warmerdam]

Rich,

OK, I have finally connected up the alpha support properly through
the warp architecture - at least I think I have.  It is now possible
to overlay an image with alpha values on another image with alpha values
and get proper mixing, and proper setting of merged alpha values in the
output image.

You might want to grab a nightly snapshot or build from CVS if you are
interested in taking advantage of this.

gdalwarp now has -srcalpha and -dstalpha switches. -srcalpha will be
implicitly enabled if the last band of thei input file is recognised as
being an alpha band.  Likewise -dstalpha will be automatically set if the
destination file already exists, and the last band (or perhaps the next
band after the bands being transferred?) is marked as an alpha band.

If you are creating an output file and want an alpha band you need to set
the -dstalpha explicitly, even if the source file has an alpha band.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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and watch the world go round - Rush    | Geospatial Programmer for Rent