Add an alpha layer to GeoTIFF raster data

Note, an alpha layer is always added (to single and 3 band GeoTIFF
files) since gdalwarp doesn't add a new alpha band if one is already
present.

Fixes #570

lib/robots/dor_repo/gis_assembly/normalize_data.rb

@@ -179,6 +179,8 @@ def normalize
             # if using 8-bit color palette, convert into RGB
             convert_8bit_to_rgb if eight_bit?
 
+            add_alpha_channel
+
             compute_statistics
           end
 
@@ -270,6 +272,17 @@ def eight_bit?
             false
           end
 
+          def add_alpha_channel
+            # NOTE: gdalwarp is smart enough not to add a new alpha channel (band) if one is already there.
+            # If we want to improve the performance of the normalize step, and many GeoTIFFs already
+            # have alpha channels, then we could introspect on the GeoTIFF file with gdalinfo and skip
+            # this call to gdalwarp if one is already present.
+            logger.info "normalize-data: adding alpha channel for #{output_filepath}"
+            temp_filepath = "#{output_dir}/#{geo_object_name}_alpha.tif"
+            system_with_check("#{Settings.gdal_path}gdalwarp -dstalpha #{output_filepath} #{temp_filepath}")
+            FileUtils.mv(temp_filepath, output_filepath)
+          end
+
           def compute_statistics
             system_with_check("#{Settings.gdal_path}gdalinfo -mm -stats -norat -noct #{output_filepath}")
           end

spec/robots/dor_repo/gis_assembly/normalize_data_spec.rb

@@ -177,6 +177,10 @@
         expect(Kernel).to have_received(:system).with(
           "gdal_translate -expand rgb /tmp/normalize_bb021mm7809/raw8bit.tif /tmp/normalize_bb021mm7809/EPSG_4326/MCE_FI2G_2014.tif -co 'COMPRESS=LZW'"
         )
+        # Adds an alpha channel
+        expect(Kernel).to have_received(:system).with(
+          'gdalwarp -dstalpha /tmp/normalize_bb021mm7809/EPSG_4326/MCE_FI2G_2014.tif /tmp/normalize_bb021mm7809/EPSG_4326/MCE_FI2G_2014_alpha.tif'
+        )
         # Stats
         expect(Kernel).to have_received(:system).with(
           'gdalinfo -mm -stats -norat -noct /tmp/normalize_bb021mm7809/EPSG_4326/MCE_FI2G_2014.tif'