[gdal-dev] Warp to GeoTIFF with Transparency?

[Even Rouault]

Le mardi 18 septembre 2012 05:15:03, Nik Sands a écrit :
> I'm very new to GDAL so please be gentle.  My searching has found many
> references to this problem, but no complete solutions that are explained
> clearly enough for me to work through.

The Warp API is powerful, and thus admitedly complex to master.

> 
> I'm using the GDAL API to read in a source image (could be from a variety
> of formats), warp it to Spherical Mercator, and then write out the
> reprojected image as a GeoTIFF.  I would like the parts of the warped TIFF
> that are not included in the original image to be represented by fully
> transparent pixels instead of the black pixels that are being generated.
> 
> I've set "ALPHA" to "YES" in the warp options but this has not helped. 
> I've also tried adding an extra transparent band, but this made the entire
> output image transparent.

Strange, perhaps you should retry.

To have a transparency band added in the output of the warping, you need to 
set the band number in the GDALCreate() call to 1 + 
GDALGetRasterCount(hSrcDS), and also set psWarpOptions->nDstAlphaBand = 1 + 
GDALGetRasterCount(hSrcDS)

This should be equivalent to the -dstalpha option of gdalwarp. By the way, if 
I were you, I would try to find the gdalwarp command line that fits my need, and 
then study its code source to extract what is needed for your use case.