/// <summary>
/// Compare every cluster to every other cluster and decide if we should merge them based on
/// whether the radius of the combined cluster is less than the sum of the radii of the original clusters.
/// </summary>
/// <returns>True if any merges were performed, false otherwise.</returns>
bool MergeByRadius()
{
int mergeCount = 0;
if (MergeableShrinkage <= 0 || Clusters.NumPartitions == 1)
{
return(false);
}
Timer.Start("Merge by radius");
var Radii = new Dictionary <string, ClusterRadius>();
foreach (var label in Clusters.ClassLabels())
{
Radii[label] = new ClusterRadius(Clusters.PointsInClass(label).ToList());
}
var potentialMerges = new List <RadiusMergeCandidate>();
var minShrinkage = double.MaxValue;
foreach (var label1 in Clusters.ClassLabels())
{
foreach (var label2 in Clusters.ClassLabels().Where(label => label1.CompareTo(label) == -1))
{
var potentialMerge = new RadiusMergeCandidate(
Clusters,
label1,
Radii[label1],
label2,
Radii[label2]
);
minShrinkage = Math.Min(minShrinkage, potentialMerge.Shrinkage);
if (potentialMerge.Shrinkage <= MergeableShrinkage)
{
potentialMerges.Add(potentialMerge);
}
}
}
//TODO: Should we process merges from low shrinkage to high, and regenerate results after each merge?
// This is in case merging A + B is allowed and B + C is allowed, but
// after A + B are merged to form D, C + D are not a good merge.
// For now, process all merges that pass the Shrinkage test.
foreach (var potentialMerge in potentialMerges)
{
if (Merge(potentialMerge.Point1, potentialMerge.Point2, true))
{
mergeCount++;
}
}
Logger.Info($"{mergeCount} cluster pairs successfully merged by radius, with {potentialMerges.Count} expected.");
Logger.Info($"Radius shrinkage values: Best {minShrinkage} vs Permitted {MergeableShrinkage}");
Timer.Stop("Merge by radius");
return(mergeCount > 0);
}
/// <summary>
/// Assuming this ClusterRadius to be for a combination of two clusters, compare its radius to the
/// sum of radii for the two clusters tentatively combined to form it and return the ratio as the shrinkage.
///
/// This is to be used to help the DensityClassifier decide if two clusters it has carved out of a single original cluster
/// should really be put back together.
///
/// Instead of comparing the maximum radius or the mean radius, the two values are blended together.
/// The reason is that outliers can inflate the radius of a cluster, while the mean could grossly understate
/// the spatial extent of a cluster.
/// </summary>
/// <returns>The shrinkage. If the radius of the combined cluster exceeds the sum of the radii of its parts,
/// this is a value greater than one.
/// If the combination has a lower radius, this value is less than one.
/// Only if the value is lower than one is it appropriate to merge the cluster.
/// How much less than one is a good value is a matter for further research.
/// </returns>
/// <param name="part1">Statistics about the first component cluster.</param>
/// <param name="part2">Statistics about the second component cluster.</param>
public double Shrinkage(ClusterRadius part1, ClusterRadius part2)
{
var combined = this;
var partsMaximumSum = part1.MaximumRadius + part2.MaximumRadius;
var partsMeanSum = part1.MeanRadius + part2.MeanRadius;
//TODO: Research how to weight the values. Use 2:1 weighting for now. 1:1 did not work as well as expected.
var partsSumWeightedRadius = (2 * partsMaximumSum + partsMeanSum) / 3;
var combinedWeightedRadius = (2 * combined.MaximumRadius + combined.MeanRadius) / 3;
if (combinedWeightedRadius == 0)
{
return(0);
}
return(combinedWeightedRadius / partsSumWeightedRadius);
}
public RadiusMergeCandidate(
Classification <UnsignedPoint, string> clusters,
string label1,
ClusterRadius radius1,
string label2,
ClusterRadius radius2
)
{
Label1 = label1;
Point1 = clusters.PointsInClass(Label1).First();
Label2 = label2;
Point2 = clusters.PointsInClass(Label2).First();
CombinedRadius = new ClusterRadius(clusters.PointsInClass(Label1), clusters.PointsInClass(Label2));
Shrinkage = CombinedRadius.Shrinkage(radius1, radius2);
}