/* * 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 com.squareup.gifencoder; 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<Color> quantize(Multiset<Color> 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<Color> clusterCentroids = new HashSet<>(); for (Cluster cluster : clusters) { clusterCentroids.add(Color.getCentroid(cluster.colors)); } return clusterCentroids; } private static final class Cluster { final Multiset<Color> colors; double largestSpread; int componentWithLargestSpread; Cluster(Multiset<Color> 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 (Color color : colors) { min = Math.min(min, color.getComponent(component)); max = Math.max(max, color.getComponent(component)); } return max - min; } Collection<Cluster> split() { List<Color> 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<Color> { final int component; ColorComponentComparator(int component) { this.component = component; } @Override public int compare(Color a, Color b) { double componentDifference = a.getComponent(component) - b.getComponent(component); if (componentDifference == 0) { return ArbitraryComparator.INSTANCE.compare(a, b); } return (int) Math.signum(componentDifference); } } }