MATH-1270

Visualization of quantization error.

src/changes/changes.xml

@@ -57,7 +57,7 @@ If the output is not quite correct, check for invisible trailing spaces!
       <action dev="erans" type="add" issue="MATH-1270">
         Various SOFM visualizations (in package "o.a.c.m.ml.neuralnet.twod.util"):
         Unified distance matrix, hit histogram, smoothed data histograms,
-        topographic error.
+        topographic error, quantization error.
       </action>
       <action dev="erans" type="add" issue="MATH-1268">
         New interfaces to be implemented by algorithms that visualizes properties

src/main/java/org/apache/commons/math3/ml/neuralnet/twod/util/QuantizationError.java

@@ -0,0 +1,76 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.commons.math3.ml.neuralnet.twod.util;
+
+import org.apache.commons.math3.ml.neuralnet.MapUtils;
+import org.apache.commons.math3.ml.neuralnet.Neuron;
+import org.apache.commons.math3.ml.neuralnet.twod.NeuronSquareMesh2D;
+import org.apache.commons.math3.ml.distance.DistanceMeasure;
+
+/**
+ * Computes the quantization error histogram.
+ * Each bin will contain the average of the distances between samples
+ * mapped to the corresponding unit and the weight vector of that unit.
+ */
+public class QuantizationError implements MapDataVisualization {
+    /** Distance. */
+    private final DistanceMeasure distance;
+
+    /**
+     * @param distance Distance.
+     */
+    public QuantizationError(DistanceMeasure distance) {
+        this.distance = distance;
+    }
+
+    /** {@inheritDoc} */
+    @Override
+    public double[][] computeImage(NeuronSquareMesh2D map,
+                                   Iterable<double[]> data) {
+        final int nR = map.getNumberOfRows();
+        final int nC = map.getNumberOfColumns();
+
+        final LocationFinder finder = new LocationFinder(map);
+
+        // Hit bins.
+        final int[][] hit = new int[nR][nC];
+        // Error bins.
+        final double[][] error = new double[nR][nC];
+
+        for (double[] sample : data) {
+            final Neuron best = MapUtils.findBest(sample, map, distance);
+
+            final LocationFinder.Location loc = finder.getLocation(best);
+            final int row = loc.getRow();
+            final int col = loc.getColumn();
+            hit[row][col] += 1;
+            error[row][col] += distance.compute(sample, best.getFeatures());
+        }
+
+        for (int r = 0; r < nR; r++) {
+            for (int c = 0; c < nC; c++) {
+                final int count = hit[r][c];
+                if (count != 0) {
+                    error[r][c] /= count;
+                }
+            }
+        }
+
+        return error;
+    }
+}