Handle missing n5 blocks (#104)

handle missing n5 blocks by creating empty arrays

TODO consider handling this in the CachedCellImg or possibly re-using
   arrays to reduce RAM consumption/ better utilize cache

Co-authored-by: Christian Tischer <christian.tischer@embl.de>

src/main/java/bdv/img/n5/N5ImageLoader.java

@@ -89,11 +89,7 @@ import net.imglib2.type.volatiles.VolatileUnsignedLongType;
 import net.imglib2.type.volatiles.VolatileUnsignedShortType;
 import net.imglib2.util.Cast;
 import net.imglib2.view.Views;
-import org.janelia.saalfeldlab.n5.DataBlock;
-import org.janelia.saalfeldlab.n5.DataType;
-import org.janelia.saalfeldlab.n5.DatasetAttributes;
-import org.janelia.saalfeldlab.n5.N5FSReader;
-import org.janelia.saalfeldlab.n5.N5Reader;
+import org.janelia.saalfeldlab.n5.*;
 
 import static bdv.img.n5.BdvN5Format.DATA_TYPE_KEY;
 import static bdv.img.n5.BdvN5Format.DOWNSAMPLING_FACTORS_KEY;
@@ -352,7 +348,39 @@ public class N5ImageLoader implements ViewerImgLoader, MultiResolutionImgLoader
 		@Override
 		public A loadArray( final long[] gridPosition ) throws IOException
 		{
-			return createArray.apply( n5.readBlock( pathName, attributes, gridPosition ) );
+			final DataBlock< ? > dataBlock = n5.readBlock( pathName, attributes, gridPosition );
+
+			if ( dataBlock == null )
+				return createEmptyArray( gridPosition );
+			else
+				return createArray.apply( dataBlock );
+		}
+
+		private A createEmptyArray( long[] gridPosition )
+		{
+			final int[] blockSize = attributes.getBlockSize();
+			final int n = blockSize[ 0 ] * blockSize[ 1 ] * blockSize[ 2 ];
+			switch ( attributes.getDataType() )
+			{
+				case UINT8:
+				case INT8:
+					return createArray.apply( new ByteArrayDataBlock( blockSize, gridPosition, new byte[ n ] ) );
+				case UINT16:
+				case INT16:
+					return createArray.apply( new ShortArrayDataBlock( blockSize, gridPosition, new short[ n ] ) );
+				case UINT32:
+				case INT32:
+					return createArray.apply( new IntArrayDataBlock( blockSize, gridPosition, new int[ n ] ) );
+				case UINT64:
+				case INT64:
+					return createArray.apply( new LongArrayDataBlock( blockSize, gridPosition, new long[ n ] ) );
+				case FLOAT32:
+					return createArray.apply( new FloatArrayDataBlock( blockSize, gridPosition, new float[ n ] ) );
+				case FLOAT64:
+					return createArray.apply( new DoubleArrayDataBlock( blockSize, gridPosition, new double[ n ] ) );
+				default:
+					throw new UnsupportedOperationException("Data type not supported: " + attributes.getDataType());
+			}
 		}
 	}