gdalwarp to clip a polygon out of a map layer using
The original map layer has a lot of empty pixels (well, transparent / alpha=0, to be precise). As it stands, my output is a GeoTIFF, where the pixels that were originally transparent remain as such, and the areas of the image that would fall outside of the cutline are also transparent.
What I want is to be able to differentiate, within that output image, between the pixels that were transparent in the source and the pixels that fell outside of the cutline, say by giving them a specific value that wouldn't exist anywhere else in the image.
Is there an easy way to do that?