2 * Copyright 2007 Google Inc.
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4 * Licensed under the Apache License, Version 2.0 (the "License");
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5 * you may not use this file except in compliance with the License.
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6 * You may obtain a copy of the License at
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8 * http://www.apache.org/licenses/LICENSE-2.0
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10 * Unless required by applicable law or agreed to in writing, software
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11 * distributed under the License is distributed on an "AS IS" BASIS,
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12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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13 * See the License for the specific language governing permissions and
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14 * limitations under the License.
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17 package com.google.zxing.common;
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20 * <p>Encapsulates logic that estimates the optimal "black point", the luminance value
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21 * which is the best line between "white" and "black" in a grayscale image.</p>
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23 * <p>TODO: Include reference to paper with similar ideas</p>
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25 * @author srowen@google.com (Sean Owen)
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27 public final class BlackPointEstimator {
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29 private BlackPointEstimator() {
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33 * <p>Given an array of <em>counts</em> of luminance values (i.e. a histogram), this method
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34 * decides which bucket of values corresponds to the black point -- which bucket contains the
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35 * count of the brightest luminance values that should be considered "black".</p>
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37 * @param luminanceBuckets an array of <em>counts</em> of luminance values
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38 * @return index within argument of bucket corresponding to brightest values which should be
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39 * considered "black"
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41 public static int estimate(int[] luminanceBuckets) {
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43 int numBuckets = luminanceBuckets.length;
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45 // Find tallest peak in histogram
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47 int firstPeakSize = 0;
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48 for (int i = 0; i < numBuckets; i++) {
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49 if (luminanceBuckets[i] > firstPeakSize) {
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51 firstPeakSize = luminanceBuckets[i];
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55 // Find second-tallest peak -- well, another peak that is tall and not
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56 // so close to the first one
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58 int secondPeakScore = 0;
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59 for (int i = 0; i < numBuckets; i++) {
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60 int distanceToBiggest = i - firstPeak;
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61 // Encourage more distant second peaks by multiplying by square of distance
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62 int score = luminanceBuckets[i] * distanceToBiggest * distanceToBiggest;
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63 if (score > secondPeakScore) {
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65 secondPeakScore = score;
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69 // Put firstPeak first
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70 if (firstPeak > secondPeak) {
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71 int temp = firstPeak;
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72 firstPeak = secondPeak;
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76 // Find a valley between them that is low and close to the midpoint of the two peaks
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77 int bestValley = firstPeak;
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78 int bestValleyScore = 0;
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79 for (int i = firstPeak + 1; i < secondPeak; i++) {
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80 // Encourage low valleys near the mid point between peaks
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81 int score = (firstPeakSize - luminanceBuckets[i]) * (i - firstPeak) * (secondPeak - i);
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82 if (score > bestValleyScore) {
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84 bestValleyScore = score;
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