Fix FastGauss

There was an error, in one of the equations for calculating a determinant.

Author: maarzt

src/main/java/net/imglib2/algorithm/convolution/fast_gauss/FastGaussCalculator.java

@@ -239,7 +239,7 @@ private Parameters( final int _M, final double sigma )
 
 				final double D11 = r_3 - r_5 * C_35 - r_7 * C_37;
 				final double D21 = p_3 - p_5 * C_35 - p_7 * C_37;
-				final double D31 = p_3 * r_5 + p_5 * r_7 + C_37 - p_5 * r_3 - p_7 * r_5 * C_37;
+				final double D31 = p_3 * r_5 + p_5 * r_7 * C_37 - p_5 * r_3 - p_7 * r_5 * C_37;
 				final double D41 = p_5 * r_7 * C_35 + p_7 * r_3 - p_3 * r_7 - p_7 * r_5 * C_35;
 
 				final double D12 = r_1 - r_5 * C_15 - r_7 * C_17;

src/test/java/net/imglib2/algorithm/gauss3/GaussTest.java

@@ -18,7 +18,6 @@
 import net.imglib2.util.Localizables;
 import net.imglib2.util.Util;
 import net.imglib2.view.Views;
-import org.junit.Ignore;
 import org.junit.Test;
 
 import java.util.function.BiFunction;
@@ -53,7 +52,6 @@ public void testGauss3()
 		assertImagesEqual( 24, secondDerivativeX( expected ), secondDerivativeX( result ) );
 	}
 
-	@Ignore( "The FastGauss currently deals poorly with an offset in the image." )
 	@Test
 	public void testFastGauss()
 	{
@@ -129,6 +127,8 @@ private RandomAccessibleInterval< T > secondDerivativeX( RandomAccessibleInterva
 	private void assertImagesEqual( int expectedSnr, RandomAccessibleInterval< ? extends RealType< ? > > a, RandomAccessibleInterval< T > b )
 	{
 		double actualSnr = snr( a, b );
+		if ( Double.isNaN( actualSnr ) )
+			fail( "Either the expected or the actual image contains NaN values." );
 		if ( expectedSnr > actualSnr )
 			fail( "The SNR is lower than expected, expected: " + expectedSnr + " dB actual: " + actualSnr + " dB" );
 	}