I'm using gdal_merge.py to merge a few GeoTIFF's into a GeoPackage. The source files are UTM33 projection, 3 bands (RGB) without Nodata tags. They make up a non-rectangular mosaic, so gdal_merge correctly sets alpha=0 on the areas without data in the resulting GeoPackage.
However, it also inexplicably fills in a bit of black border. I notice that the black borders only appear along the "internal borders" i.e., not around the outer bounding box of the GeoPackage. Does anyone know if and how this can be fixed?
Note: I know that other GDAL tools such as GDALWarp is probably better for this particular case, but I am using this as a test for processes with very large amounts of source data. In such cases, gdal_merge is the best option I've found.
gdal_merge -of GPKG -o output.gpkg srcfile1.tif srcfile2.tif srcfile3.tif srcfile4.tif