2 * Copyright 2007 ZXing authors
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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.qrcode.detector;
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19 import com.google.zxing.DecodeHintType;
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20 import com.google.zxing.MonochromeBitmapSource;
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21 import com.google.zxing.ReaderException;
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22 import com.google.zxing.common.BitArray;
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23 import com.google.zxing.common.Collections;
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24 import com.google.zxing.common.Comparator;
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25 import com.google.zxing.common.GenericResultPoint;
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27 import java.util.Hashtable;
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28 import java.util.Vector;
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31 * <p>This class attempts to find finder patterns in a QR Code. Finder patterns are the square
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32 * markers at three corners of a QR Code.</p>
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34 * <p>This class is not thread-safe and should not be reused.</p>
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36 * @author srowen@google.com (Sean Owen)
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38 final class FinderPatternFinder {
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40 private static final int CENTER_QUORUM = 2;
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41 private static final int MIN_SKIP = 3; // 1 pixel/module times 3 modules/center
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42 private static final int MAX_MODULES = 57; // support up to version 10 for mobile clients
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43 private static final int INTEGER_MATH_SHIFT = 8;
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45 private final MonochromeBitmapSource image;
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46 private final Vector possibleCenters;
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47 private boolean hasSkipped;
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50 * <p>Creates a finder that will search the image for three finder patterns.</p>
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52 * @param image image to search
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54 FinderPatternFinder(MonochromeBitmapSource image) {
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56 this.possibleCenters = new Vector();
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59 FinderPatternInfo find(Hashtable hints) throws ReaderException {
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60 boolean tryHarder = hints != null && hints.containsKey(DecodeHintType.TRY_HARDER);
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61 int maxI = image.getHeight();
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62 int maxJ = image.getWidth();
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63 // We are looking for black/white/black/white/black modules in
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64 // 1:1:3:1:1 ratio; this tracks the number of such modules seen so far
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66 // Let's assume that the maximum version QR Code we support takes up 1/4 the height of the
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67 // image, and then account for the center being 3 modules in size. This gives the smallest
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68 // number of pixels the center could be, so skip this often. When trying harder, look for all
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69 // QR versions regardless of how dense they are.
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70 int iSkip = (int) (maxI / (MAX_MODULES * 4.0f) * 3);
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71 if (iSkip < MIN_SKIP || tryHarder) {
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75 boolean done = false;
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76 int[] stateCount = new int[5];
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77 for (int i = iSkip - 1; i < maxI && !done; i += iSkip) {
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78 // Get a row of black/white values
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79 BitArray blackRow = image.getBlackRow(i, null, 0, maxJ);
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85 int currentState = 0;
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86 for (int j = 0; j < maxJ; j++) {
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87 if (blackRow.get(j)) {
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89 if ((currentState & 1) == 1) { // Counting white pixels
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92 stateCount[currentState]++;
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93 } else { // White pixel
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94 if ((currentState & 1) == 0) { // Counting black pixels
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95 if (currentState == 4) { // A winner?
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96 if (foundPatternCross(stateCount)) { // Yes
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97 boolean confirmed = handlePossibleCenter(stateCount, i, j);
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99 iSkip = 1; // Go back to examining each line
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101 done = haveMulitplyConfirmedCenters();
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103 int rowSkip = findRowSkip();
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104 if (rowSkip > stateCount[2]) {
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105 // Skip rows between row of lower confirmed center
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106 // and top of presumed third confirmed center
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107 // but back up a bit to get a full chance of detecting
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108 // it, entire width of center of finder pattern
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110 // Skip by rowSkip, but back off by stateCount[2] (size of last center
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111 // of pattern we saw) to be conservative, and also back off by iSkip which
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112 // is about to be re-added
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113 i += rowSkip - stateCount[2] - iSkip;
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118 // Advance to next black pixel
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121 } while (j < maxJ && !blackRow.get(j));
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122 j--; // back up to that last white pixel
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124 // Clear state to start looking again
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131 } else { // No, shift counts back by two
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132 stateCount[0] = stateCount[2];
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133 stateCount[1] = stateCount[3];
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134 stateCount[2] = stateCount[4];
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140 stateCount[++currentState]++;
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142 } else { // Counting white pixels
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143 stateCount[currentState]++;
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147 if (foundPatternCross(stateCount)) {
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148 boolean confirmed = handlePossibleCenter(stateCount, i, maxJ);
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150 iSkip = stateCount[0];
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152 // Found a third one
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153 done = haveMulitplyConfirmedCenters();
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159 FinderPattern[] patternInfo = selectBestPatterns();
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160 GenericResultPoint.orderBestPatterns(patternInfo);
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162 return new FinderPatternInfo(patternInfo);
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166 * Given a count of black/white/black/white/black pixels just seen and an end position,
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167 * figures the location of the center of this run.
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169 private static float centerFromEnd(int[] stateCount, int end) {
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170 return (float) (end - stateCount[4] - stateCount[3]) - stateCount[2] / 2.0f;
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174 * @param stateCount count of black/white/black/white/black pixels just read
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175 * @return true iff the proportions of the counts is close enough to the 1/1/3/1/1 ratios
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176 * used by finder patterns to be considered a match
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178 private static boolean foundPatternCross(int[] stateCount) {
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179 int totalModuleSize = 0;
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180 for (int i = 0; i < 5; i++) {
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181 int count = stateCount[i];
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185 totalModuleSize += count;
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187 if (totalModuleSize < 7) {
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190 int moduleSize = (totalModuleSize << INTEGER_MATH_SHIFT) / 7;
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191 int maxVariance = moduleSize / 2;
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192 // Allow less than 50% variance from 1-1-3-1-1 proportions
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193 return Math.abs(moduleSize - (stateCount[0] << INTEGER_MATH_SHIFT)) < maxVariance &&
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194 Math.abs(moduleSize - (stateCount[1] << INTEGER_MATH_SHIFT)) < maxVariance &&
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195 Math.abs(3 * moduleSize - (stateCount[2] << INTEGER_MATH_SHIFT)) < 3 * maxVariance &&
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196 Math.abs(moduleSize - (stateCount[3] << INTEGER_MATH_SHIFT)) < maxVariance &&
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197 Math.abs(moduleSize - (stateCount[4] << INTEGER_MATH_SHIFT)) < maxVariance;
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201 * <p>After a horizontal scan finds a potential finder pattern, this method
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202 * "cross-checks" by scanning down vertically through the center of the possible
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203 * finder pattern to see if the same proportion is detected.</p>
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205 * @param startI row where a finder pattern was detected
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206 * @param centerJ center of the section that appears to cross a finder pattern
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207 * @param maxCount maximum reasonable number of modules that should be
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208 * observed in any reading state, based on the results of the horizontal scan
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209 * @return vertical center of finder pattern, or {@link Float#NaN} if not found
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211 private float crossCheckVertical(int startI, int centerJ, int maxCount, int originalStateCountTotal) {
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212 MonochromeBitmapSource image = this.image;
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214 int maxI = image.getHeight();
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215 int[] stateCount = new int[5];
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217 // Start counting up from center
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219 while (i >= 0 && image.isBlack(centerJ, i)) {
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226 while (i >= 0 && !image.isBlack(centerJ, i) && stateCount[1] <= maxCount) {
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230 // If already too many modules in this state or ran off the edge:
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231 if (i < 0 || stateCount[1] > maxCount) {
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234 while (i >= 0 && image.isBlack(centerJ, i) && stateCount[0] <= maxCount) {
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238 if (stateCount[0] > maxCount) {
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242 // Now also count down from center
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244 while (i < maxI && image.isBlack(centerJ, i)) {
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251 while (i < maxI && !image.isBlack(centerJ, i) && stateCount[3] < maxCount) {
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255 if (i == maxI || stateCount[3] >= maxCount) {
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258 while (i < maxI && image.isBlack(centerJ, i) && stateCount[4] < maxCount) {
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262 if (stateCount[4] >= maxCount) {
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266 // If we found a finder-pattern-like section, but its size is more than 20% different than
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267 // the original, assume it's a false positive
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268 int stateCountTotal = stateCount[0] + stateCount[1] + stateCount[2] + stateCount[3] + stateCount[4];
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269 if (5 * Math.abs(stateCountTotal - originalStateCountTotal) >= originalStateCountTotal) {
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273 return foundPatternCross(stateCount) ? centerFromEnd(stateCount, i) : Float.NaN;
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277 * <p>Like {@link #crossCheckVertical(int, int, int, int)}, and in fact is basically identical,
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278 * except it reads horizontally instead of vertically. This is used to cross-cross
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279 * check a vertical cross check and locate the real center of the alignment pattern.</p>
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281 private float crossCheckHorizontal(int startJ, int centerI, int maxCount, int originalStateCountTotal) {
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282 MonochromeBitmapSource image = this.image;
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284 int maxJ = image.getWidth();
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285 int[] stateCount = new int[5];
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288 while (j >= 0 && image.isBlack(j, centerI)) {
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295 while (j >= 0 && !image.isBlack(j, centerI) && stateCount[1] <= maxCount) {
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299 if (j < 0 || stateCount[1] > maxCount) {
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302 while (j >= 0 && image.isBlack(j, centerI) && stateCount[0] <= maxCount) {
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306 if (stateCount[0] > maxCount) {
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311 while (j < maxJ && image.isBlack(j, centerI)) {
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318 while (j < maxJ && !image.isBlack(j, centerI) && stateCount[3] < maxCount) {
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322 if (j == maxJ || stateCount[3] >= maxCount) {
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325 while (j < maxJ && image.isBlack(j, centerI) && stateCount[4] < maxCount) {
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329 if (stateCount[4] >= maxCount) {
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333 // If we found a finder-pattern-like section, but its size is significantly different than
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334 // the original, assume it's a false positive
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335 int stateCountTotal = stateCount[0] + stateCount[1] + stateCount[2] + stateCount[3] + stateCount[4];
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336 if (5 * Math.abs(stateCountTotal - originalStateCountTotal) >= originalStateCountTotal) {
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340 return foundPatternCross(stateCount) ? centerFromEnd(stateCount, j) : Float.NaN;
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344 * <p>This is called when a horizontal scan finds a possible alignment pattern. It will
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345 * cross check with a vertical scan, and if successful, will, ah, cross-cross-check
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346 * with another horizontal scan. This is needed primarily to locate the real horizontal
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347 * center of the pattern in cases of extreme skew.</p>
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349 * <p>If that succeeds the finder pattern location is added to a list that tracks
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350 * the number of times each location has been nearly-matched as a finder pattern.
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351 * Each additional find is more evidence that the location is in fact a finder
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354 * @param stateCount reading state module counts from horizontal scan
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355 * @param i row where finder pattern may be found
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356 * @param j end of possible finder pattern in row
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357 * @return true if a finder pattern candidate was found this time
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359 private boolean handlePossibleCenter(int[] stateCount,
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362 int stateCountTotal = stateCount[0] + stateCount[1] + stateCount[2] + stateCount[3] + stateCount[4];
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363 float centerJ = centerFromEnd(stateCount, j);
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364 float centerI = crossCheckVertical(i, (int) centerJ, stateCount[2], stateCountTotal);
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365 if (!Float.isNaN(centerI)) {
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367 centerJ = crossCheckHorizontal((int) centerJ, (int) centerI, stateCount[2], stateCountTotal);
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368 if (!Float.isNaN(centerJ)) {
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369 float estimatedModuleSize = (float) stateCountTotal / 7.0f;
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370 boolean found = false;
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371 int max = possibleCenters.size();
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372 for (int index = 0; index < max; index++) {
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373 FinderPattern center = (FinderPattern) possibleCenters.elementAt(index);
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374 // Look for about the same center and module size:
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375 if (center.aboutEquals(estimatedModuleSize, centerI, centerJ)) {
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376 center.incrementCount();
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382 possibleCenters.addElement(new FinderPattern(centerJ, centerI, estimatedModuleSize));
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391 * @return number of rows we could safely skip during scanning, based on the first
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392 * two finder patterns that have been located. In some cases their position will
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393 * allow us to infer that the third pattern must lie below a certain point farther
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394 * down in the image.
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396 private int findRowSkip() {
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397 int max = possibleCenters.size();
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401 FinderPattern firstConfirmedCenter = null;
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402 for (int i = 0; i < max; i++) {
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403 FinderPattern center = (FinderPattern) possibleCenters.elementAt(i);
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404 if (center.getCount() >= CENTER_QUORUM) {
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405 if (firstConfirmedCenter == null) {
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406 firstConfirmedCenter = center;
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408 // We have two confirmed centers
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409 // How far down can we skip before resuming looking for the next
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410 // pattern? In the worst case, only the difference between the
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411 // difference in the x / y coordinates of the two centers.
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412 // This is the case where you find top left first. Draw it out.
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414 return (int) (Math.abs(firstConfirmedCenter.getX() - center.getX()) -
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415 Math.abs(firstConfirmedCenter.getY() - center.getY()));
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423 * @return true iff we have found at least 3 finder patterns that have been detected
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424 * at least {@link #CENTER_QUORUM} times each, and, the estimated module size of the
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425 * candidates is "pretty similar"
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427 private boolean haveMulitplyConfirmedCenters() {
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428 int confirmedCount = 0;
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429 float totalModuleSize = 0.0f;
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430 int max = possibleCenters.size();
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431 for (int i = 0; i < max; i++) {
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432 FinderPattern pattern = (FinderPattern) possibleCenters.elementAt(i);
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433 if (pattern.getCount() >= CENTER_QUORUM) {
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435 totalModuleSize += pattern.getEstimatedModuleSize();
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438 if (confirmedCount < 3) {
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441 // OK, we have at least 3 confirmed centers, but, it's possible that one is a "false positive"
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442 // and that we need to keep looking. We detect this by asking if the estimated module sizes
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443 // vary too much. We arbitrarily say that when the total deviation from average exceeds
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444 // 15% of the total module size estimates, it's too much.
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445 float average = totalModuleSize / max;
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446 float totalDeviation = 0.0f;
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447 for (int i = 0; i < max; i++) {
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448 FinderPattern pattern = (FinderPattern) possibleCenters.elementAt(i);
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449 totalDeviation += Math.abs(pattern.getEstimatedModuleSize() - average);
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451 return totalDeviation <= 0.15f * totalModuleSize;
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455 * @return the 3 best {@link FinderPattern}s from our list of candidates. The "best" are
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456 * those that have been detected at least {@link #CENTER_QUORUM} times, and whose module
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457 * size differs from the average among those patterns the least
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458 * @throws ReaderException if 3 such finder patterns do not exist
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460 private FinderPattern[] selectBestPatterns() throws ReaderException {
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461 Collections.insertionSort(possibleCenters, new CenterComparator());
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463 int max = possibleCenters.size();
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464 while (size < max) {
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465 if (((FinderPattern) possibleCenters.elementAt(size)).getCount() < CENTER_QUORUM) {
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472 // Couldn't find enough finder patterns
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473 throw new ReaderException("Could not find three finder patterns");
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477 // Found just enough -- hope these are good!
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478 return new FinderPattern[]{
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479 (FinderPattern) possibleCenters.elementAt(0),
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480 (FinderPattern) possibleCenters.elementAt(1),
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481 (FinderPattern) possibleCenters.elementAt(2)
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485 possibleCenters.setSize(size);
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487 // Hmm, multiple found. We need to pick the best three. Find the most
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488 // popular ones whose module size is nearest the average
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490 float averageModuleSize = 0.0f;
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491 for (int i = 0; i < size; i++) {
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492 averageModuleSize += ((FinderPattern) possibleCenters.elementAt(i)).getEstimatedModuleSize();
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494 averageModuleSize /= (float) size;
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496 // We don't have java.util.Collections in J2ME
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497 Collections.insertionSort(possibleCenters, new ClosestToAverageComparator(averageModuleSize));
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499 return new FinderPattern[]{
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500 (FinderPattern) possibleCenters.elementAt(0),
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501 (FinderPattern) possibleCenters.elementAt(1),
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502 (FinderPattern) possibleCenters.elementAt(2)
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507 * <p>Orders by {@link FinderPattern#getCount()}, descending.</p>
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509 private static class CenterComparator implements Comparator {
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510 public int compare(Object center1, Object center2) {
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511 return ((FinderPattern) center2).getCount() - ((FinderPattern) center1).getCount();
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516 * <p>Orders by variance from average module size, ascending.</p>
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518 private static class ClosestToAverageComparator implements Comparator {
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519 private final float averageModuleSize;
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521 private ClosestToAverageComparator(float averageModuleSize) {
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522 this.averageModuleSize = averageModuleSize;
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525 public int compare(Object center1, Object center2) {
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526 return Math.abs(((FinderPattern) center1).getEstimatedModuleSize() - averageModuleSize) <
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527 Math.abs(((FinderPattern) center2).getEstimatedModuleSize() - averageModuleSize) ?
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