[gdal-dev] gdalwarper destination alpha mask

[Glenn Burkhardt]

I'm trying to track down a discrepancy between GDAL 1.7.2 and GDAL 
2.4.0.  The NASA Worldwind code makes a call to gdal.ReprojectImage() 
using a dataset generated from an input file to a working destination 
dataset that's empty, except that the alpha band has been filled with 
0xFF.  In the case I'm examining, the projection is 1 to 1; no change.  
The source file has an appropriate alpha mask to clip the image so only 
relevant data is shown.

When using the 1.7.2 release, the image is shown with a proper overlay 
on the lower priority layers, with the portions with alpha mask of 0 
being completely transparent.  But the more recent 2.4.0 release has a 
black boundary around the image, indicating that the alpha mask was set 
to 0xff for the whole image.

It looks like there's a difference in behavior in the Java call

                         gdal.ReprojectImage(srcDS, destDS, s_srs_wkt, 
t_srs_wkt, gdalconst.GRA_Average);

For the 1.7.2 release, the alpha mask in the destination data set is 
filled in correctly, matching the alpha band in the source. But in the 
2.4.0 release, the destination alpha band is filled with 0xFF, which 
sort of explains the black boundary.  Curiously, for the 1.7.2 release, 
it doesn't matter if the destination alpha band was filled with zeros or 
0xFF.  For the 2.4.0 release, if the destination dataset alpha band is 
filled with zeros, the image displays correctly, and the reprojected 
destination dataset alpha band matches the source.

I'm trying to understand what the purpose is of the destination alpha 
density mask in 'gdalwarpper.cpp'.  In the file 'gdalwarpoperation.cpp', 
I see a note that says "Note that detailed semantics of the masks should 
be found in

GDALWarpKernel."  And in 'gdalwarpkernel.cpp': "A detailed explanation 
of the semantics of the validity and density masks, and their effects on 
resampling kernels is needed here."

So if there's a place where the semantics are explained, please send me 
link.  And, if this rings a bell about a change that was made some time 
between the 1.7.2 (April 23, 2010) and the 2.4.0, please let me know.

Thanks.