plantuml/src/net/sourceforge/plantuml/quantization/MedianCutQuantizer.java

/*
 * Copyright (C) 2015 Square, Inc.
 *
 * Licensed 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 net.sourceforge.plantuml.quantization;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
import java.util.TreeSet;

/**
 * Implements median cut quantization.
 *
 * <p>
 * The algorithm works as follows:
 *
 * <ul>
 * <li>Begin with one cluster containing all the original colors.</li>
 * <li>Find the cluster containing the greatest spread along a single color
 * component (red, green or blue).</li>
 * <li>Find the median of that color component among colors in the cluster.</li>
 * <li>Split the cluster into two halves, using that median as a threshold.</li>
 * <li>Repeat this process until the desired number of clusters is reached.</li>
 * </ul>
 */
public final class MedianCutQuantizer implements ColorQuantizer {
	public static final MedianCutQuantizer INSTANCE = new MedianCutQuantizer();

	private MedianCutQuantizer() {
	}

	@Override
	public Set<QColor> quantize(Multiset<QColor> originalColors, int maxColorCount) {
		TreeSet<Cluster> clusters = new TreeSet<>(new ClusterSpreadComparator());
		clusters.add(new Cluster(originalColors));

		while (clusters.size() < maxColorCount) {
			Cluster clusterWithLargestSpread = clusters.pollFirst();
			clusters.addAll(clusterWithLargestSpread.split());
		}

		Set<QColor> clusterCentroids = new HashSet<>();
		for (Cluster cluster : clusters) {
			clusterCentroids.add(QColor.getCentroid(cluster.colors));
		}
		return clusterCentroids;
	}

	private static final class Cluster {
		final Multiset<QColor> colors;
		double largestSpread;
		int componentWithLargestSpread;

		Cluster(Multiset<QColor> colors) {
			this.colors = colors;
			this.largestSpread = -1;
			for (int component = 0; component < 3; ++component) {
				double componentSpread = getComponentSpread(component);
				if (componentSpread > largestSpread) {
					largestSpread = componentSpread;
					componentWithLargestSpread = component;
				}
			}
		}

		double getComponentSpread(int component) {
			double min = Double.POSITIVE_INFINITY;
			double max = Double.NEGATIVE_INFINITY;
			for (QColor color : colors) {
				min = Math.min(min, color.getComponent(component));
				max = Math.max(max, color.getComponent(component));
			}
			return max - min;
		}

		Collection<Cluster> split() {
			List<QColor> orderedColors = new ArrayList<>(colors);
			Collections.sort(orderedColors, new ColorComponentComparator(componentWithLargestSpread));
			int medianIndex = orderedColors.size() / 2;
			return Arrays.asList(new Cluster(new HashMultiset<>(orderedColors.subList(0, medianIndex))),
					new Cluster(new HashMultiset<>(orderedColors.subList(medianIndex, orderedColors.size()))));
		}
	}

	/**
	 * Orders clusters according to their maximum spread, in descending order.
	 */
	static final class ClusterSpreadComparator implements Comparator<Cluster> {
		@Override
		public int compare(Cluster a, Cluster b) {
			double spreadDifference = b.largestSpread - a.largestSpread;
			if (spreadDifference == 0) {
				return ArbitraryComparator.INSTANCE.compare(a, b);
			}
			return (int) Math.signum(spreadDifference);
		}
	}

	/**
	 * Orders colors according to the value of one particular component, in
	 * ascending order.
	 */
	static final class ColorComponentComparator implements Comparator<QColor> {
		final int component;

		ColorComponentComparator(int component) {
			this.component = component;
		}

		@Override
		public int compare(QColor a, QColor b) {
			double componentDifference = a.getComponent(component) - b.getComponent(component);
			if (componentDifference == 0) {
				return ArbitraryComparator.INSTANCE.compare(a, b);
			}
			return (int) Math.signum(componentDifference);
		}
	}
}