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I've tried just about every single TileMill compositing operation on the vector and raster layers in question, and I have to admit that I can't figure it out: How the heck can I mask a raster with a vector in TileMill/CartoCSS?

Basically, I have a raster, generated by QGIS (and masked in QGIS with a vector shapefile of the U.S.). I ran it through GDAL to colorize it and re-project it into web mercator for TileMill. Somewhere in the process, everything outside the QGIS mask became a solid white pixel, and I just can't seem to trim that white off. I've brought the same shapefile into TileMill, and that's what I'm trying to use to trim the raster.

As usual, I feel like there's something basic I didn't do—maybe in the QGIS GeoTIFF export process?—that would explain TileMill's stubborn refusal to do away with the gross white square around my map, but Google is failing me. Any ideas?

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If I understand your problem correctly, there may be another workaround to this as well by using the comp-op operations (which can be quite confusing).

Let's say you had your #raster layer and a #shp poly layer. The shp layer is polys defining areas that you WANT to see from the raster. Anything outside the shp polys will be masked with the following method:

#shp{polygon-comp-op: src-out;} #raster{raster-comp-op: dst-over;}

So now, the raster will only show through your shp poly and will be completely transparent everywhere else (in your case, any area outside the US shp poly)

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Turns out I was asking the wrong question—this wasn't a problem for Tilemill to solve. I used the dstalpha command in GDAL, and it seems to have taken care of the issue. The command in question is:

gdalwarp -dstalpha -srcnodata 255 -dstnodata 0 SOURCE.tif DESTINATION.tif 

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