[gdal-dev] Warp to GeoTIFF with Transparency?

[Nik Sands]

Thank you very kindly, Even.  It works perfectly now.  I can't recall exactly what I'd tried previously when adding the additional band, but I think I must have neglected to include the

	psWarpOptions->nDstAlphaBand = GDALGetRasterCount(hSrcDS) + 1;

I thought I'd tried that before, but it certainly fixed it this time.

So again... thanks very much for your assistance.  It's very much appreciated.

Cheers,
Nik.


On 19/09/2012, at 7:12 AM, Even Rouault <even.rouault at mines-paris.org> wrote:

> 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.