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.ReaderException;
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21 import com.google.zxing.ResultPoint;
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22 import com.google.zxing.ResultPointCallback;
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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.BitMatrix;
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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 thread-safe but not reentrant. Each thread must allocate its own object.
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38 public class FinderPatternFinder {
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40 private static final int CENTER_QUORUM = 2;
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41 protected static final int MIN_SKIP = 3; // 1 pixel/module times 3 modules/center
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42 protected 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 BitMatrix image;
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46 private final Vector possibleCenters;
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47 private boolean hasSkipped;
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48 private final int[] crossCheckStateCount;
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49 private final ResultPointCallback resultPointCallback;
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52 * <p>Creates a finder that will search the image for three finder patterns.</p>
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54 * @param image image to search
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56 public FinderPatternFinder(BitMatrix image) {
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60 public FinderPatternFinder(BitMatrix image, ResultPointCallback resultPointCallback) {
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62 this.possibleCenters = new Vector();
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63 this.crossCheckStateCount = new int[5];
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64 this.resultPointCallback = resultPointCallback;
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67 protected BitMatrix getImage() {
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71 protected Vector getPossibleCenters() {
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72 return possibleCenters;
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75 FinderPatternInfo find(Hashtable hints) throws ReaderException {
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76 boolean tryHarder = hints != null && hints.containsKey(DecodeHintType.TRY_HARDER);
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77 int maxI = image.getHeight();
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78 int maxJ = image.getWidth();
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79 // We are looking for black/white/black/white/black modules in
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80 // 1:1:3:1:1 ratio; this tracks the number of such modules seen so far
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82 // Let's assume that the maximum version QR Code we support takes up 1/4 the height of the
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83 // image, and then account for the center being 3 modules in size. This gives the smallest
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84 // number of pixels the center could be, so skip this often. When trying harder, look for all
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85 // QR versions regardless of how dense they are.
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86 int iSkip = (3 * maxI) / (4 * MAX_MODULES);
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87 if (iSkip < MIN_SKIP || tryHarder) {
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91 boolean done = false;
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92 int[] stateCount = new int[5];
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93 for (int i = iSkip - 1; i < maxI && !done; i += iSkip) {
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94 // Get a row of black/white values
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100 int currentState = 0;
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101 for (int j = 0; j < maxJ; j++) {
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102 if (image.get(j, i)) {
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104 if ((currentState & 1) == 1) { // Counting white pixels
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107 stateCount[currentState]++;
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108 } else { // White pixel
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109 if ((currentState & 1) == 0) { // Counting black pixels
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110 if (currentState == 4) { // A winner?
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111 if (foundPatternCross(stateCount)) { // Yes
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112 boolean confirmed = handlePossibleCenter(stateCount, i, j);
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114 // Start examining every other line. Checking each line turned out to be too
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115 // expensive and didn't improve performance.
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118 done = haveMultiplyConfirmedCenters();
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120 int rowSkip = findRowSkip();
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121 if (rowSkip > stateCount[2]) {
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122 // Skip rows between row of lower confirmed center
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123 // and top of presumed third confirmed center
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124 // but back up a bit to get a full chance of detecting
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125 // it, entire width of center of finder pattern
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127 // Skip by rowSkip, but back off by stateCount[2] (size of last center
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128 // of pattern we saw) to be conservative, and also back off by iSkip which
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129 // is about to be re-added
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130 i += rowSkip - stateCount[2] - iSkip;
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135 // Advance to next black pixel
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138 } while (j < maxJ && !image.get(j, i));
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139 j--; // back up to that last white pixel
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141 // Clear state to start looking again
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148 } else { // No, shift counts back by two
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149 stateCount[0] = stateCount[2];
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150 stateCount[1] = stateCount[3];
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151 stateCount[2] = stateCount[4];
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157 stateCount[++currentState]++;
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159 } else { // Counting white pixels
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160 stateCount[currentState]++;
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164 if (foundPatternCross(stateCount)) {
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165 boolean confirmed = handlePossibleCenter(stateCount, i, maxJ);
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167 iSkip = stateCount[0];
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169 // Found a third one
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170 done = haveMultiplyConfirmedCenters();
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176 FinderPattern[] patternInfo = selectBestPatterns();
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177 ResultPoint.orderBestPatterns(patternInfo);
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179 return new FinderPatternInfo(patternInfo);
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183 * Given a count of black/white/black/white/black pixels just seen and an end position,
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184 * figures the location of the center of this run.
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186 private static float centerFromEnd(int[] stateCount, int end) {
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187 return (float) (end - stateCount[4] - stateCount[3]) - stateCount[2] / 2.0f;
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191 * @param stateCount count of black/white/black/white/black pixels just read
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192 * @return true iff the proportions of the counts is close enough to the 1/1/3/1/1 ratios
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193 * used by finder patterns to be considered a match
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195 protected static boolean foundPatternCross(int[] stateCount) {
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196 int totalModuleSize = 0;
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197 for (int i = 0; i < 5; i++) {
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198 int count = stateCount[i];
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202 totalModuleSize += count;
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204 if (totalModuleSize < 7) {
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207 int moduleSize = (totalModuleSize << INTEGER_MATH_SHIFT) / 7;
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208 int maxVariance = moduleSize / 2;
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209 // Allow less than 50% variance from 1-1-3-1-1 proportions
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210 return Math.abs(moduleSize - (stateCount[0] << INTEGER_MATH_SHIFT)) < maxVariance &&
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211 Math.abs(moduleSize - (stateCount[1] << INTEGER_MATH_SHIFT)) < maxVariance &&
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212 Math.abs(3 * moduleSize - (stateCount[2] << INTEGER_MATH_SHIFT)) < 3 * maxVariance &&
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213 Math.abs(moduleSize - (stateCount[3] << INTEGER_MATH_SHIFT)) < maxVariance &&
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214 Math.abs(moduleSize - (stateCount[4] << INTEGER_MATH_SHIFT)) < maxVariance;
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217 private int[] getCrossCheckStateCount() {
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218 crossCheckStateCount[0] = 0;
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219 crossCheckStateCount[1] = 0;
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220 crossCheckStateCount[2] = 0;
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221 crossCheckStateCount[3] = 0;
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222 crossCheckStateCount[4] = 0;
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223 return crossCheckStateCount;
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227 * <p>After a horizontal scan finds a potential finder pattern, this method
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228 * "cross-checks" by scanning down vertically through the center of the possible
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229 * finder pattern to see if the same proportion is detected.</p>
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231 * @param startI row where a finder pattern was detected
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232 * @param centerJ center of the section that appears to cross a finder pattern
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233 * @param maxCount maximum reasonable number of modules that should be
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234 * observed in any reading state, based on the results of the horizontal scan
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235 * @return vertical center of finder pattern, or {@link Float#NaN} if not found
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237 private float crossCheckVertical(int startI, int centerJ, int maxCount,
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238 int originalStateCountTotal) {
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239 BitMatrix image = this.image;
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241 int maxI = image.getHeight();
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242 int[] stateCount = getCrossCheckStateCount();
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244 // Start counting up from center
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246 while (i >= 0 && image.get(centerJ, i)) {
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253 while (i >= 0 && !image.get(centerJ, i) && stateCount[1] <= maxCount) {
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257 // If already too many modules in this state or ran off the edge:
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258 if (i < 0 || stateCount[1] > maxCount) {
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261 while (i >= 0 && image.get(centerJ, i) && stateCount[0] <= maxCount) {
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265 if (stateCount[0] > maxCount) {
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269 // Now also count down from center
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271 while (i < maxI && image.get(centerJ, i)) {
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278 while (i < maxI && !image.get(centerJ, i) && stateCount[3] < maxCount) {
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282 if (i == maxI || stateCount[3] >= maxCount) {
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285 while (i < maxI && image.get(centerJ, i) && stateCount[4] < maxCount) {
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289 if (stateCount[4] >= maxCount) {
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293 // If we found a finder-pattern-like section, but its size is more than 40% different than
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294 // the original, assume it's a false positive
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295 int stateCountTotal = stateCount[0] + stateCount[1] + stateCount[2] + stateCount[3] +
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297 if (5 * Math.abs(stateCountTotal - originalStateCountTotal) >= 2 * originalStateCountTotal) {
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301 return foundPatternCross(stateCount) ? centerFromEnd(stateCount, i) : Float.NaN;
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305 * <p>Like {@link #crossCheckVertical(int, int, int, int)}, and in fact is basically identical,
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306 * except it reads horizontally instead of vertically. This is used to cross-cross
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307 * check a vertical cross check and locate the real center of the alignment pattern.</p>
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309 private float crossCheckHorizontal(int startJ, int centerI, int maxCount,
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310 int originalStateCountTotal) {
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311 BitMatrix image = this.image;
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313 int maxJ = image.getWidth();
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314 int[] stateCount = getCrossCheckStateCount();
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317 while (j >= 0 && image.get(j, centerI)) {
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324 while (j >= 0 && !image.get(j, centerI) && stateCount[1] <= maxCount) {
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328 if (j < 0 || stateCount[1] > maxCount) {
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331 while (j >= 0 && image.get(j, centerI) && stateCount[0] <= maxCount) {
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335 if (stateCount[0] > maxCount) {
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340 while (j < maxJ && image.get(j, centerI)) {
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347 while (j < maxJ && !image.get(j, centerI) && stateCount[3] < maxCount) {
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351 if (j == maxJ || stateCount[3] >= maxCount) {
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354 while (j < maxJ && image.get(j, centerI) && stateCount[4] < maxCount) {
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358 if (stateCount[4] >= maxCount) {
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362 // If we found a finder-pattern-like section, but its size is significantly different than
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363 // the original, assume it's a false positive
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364 int stateCountTotal = stateCount[0] + stateCount[1] + stateCount[2] + stateCount[3] +
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366 if (5 * Math.abs(stateCountTotal - originalStateCountTotal) >= originalStateCountTotal) {
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370 return foundPatternCross(stateCount) ? centerFromEnd(stateCount, j) : Float.NaN;
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374 * <p>This is called when a horizontal scan finds a possible alignment pattern. It will
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375 * cross check with a vertical scan, and if successful, will, ah, cross-cross-check
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376 * with another horizontal scan. This is needed primarily to locate the real horizontal
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377 * center of the pattern in cases of extreme skew.</p>
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379 * <p>If that succeeds the finder pattern location is added to a list that tracks
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380 * the number of times each location has been nearly-matched as a finder pattern.
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381 * Each additional find is more evidence that the location is in fact a finder
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384 * @param stateCount reading state module counts from horizontal scan
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385 * @param i row where finder pattern may be found
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386 * @param j end of possible finder pattern in row
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387 * @return true if a finder pattern candidate was found this time
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389 protected boolean handlePossibleCenter(int[] stateCount, int i, int j) {
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390 int stateCountTotal = stateCount[0] + stateCount[1] + stateCount[2] + stateCount[3] +
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392 float centerJ = centerFromEnd(stateCount, j);
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393 float centerI = crossCheckVertical(i, (int) centerJ, stateCount[2], stateCountTotal);
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394 if (!Float.isNaN(centerI)) {
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396 centerJ = crossCheckHorizontal((int) centerJ, (int) centerI, stateCount[2], stateCountTotal);
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397 if (!Float.isNaN(centerJ)) {
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398 float estimatedModuleSize = (float) stateCountTotal / 7.0f;
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399 boolean found = false;
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400 int max = possibleCenters.size();
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401 for (int index = 0; index < max; index++) {
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402 FinderPattern center = (FinderPattern) possibleCenters.elementAt(index);
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403 // Look for about the same center and module size:
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404 if (center.aboutEquals(estimatedModuleSize, centerI, centerJ)) {
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405 center.incrementCount();
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411 ResultPoint point = new FinderPattern(centerJ, centerI, estimatedModuleSize);
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412 possibleCenters.addElement(point);
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413 if (resultPointCallback != null) {
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414 resultPointCallback.foundPossibleResultPoint(point);
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424 * @return number of rows we could safely skip during scanning, based on the first
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425 * two finder patterns that have been located. In some cases their position will
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426 * allow us to infer that the third pattern must lie below a certain point farther
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427 * down in the image.
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429 private int findRowSkip() {
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430 int max = possibleCenters.size();
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434 FinderPattern firstConfirmedCenter = null;
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435 for (int i = 0; i < max; i++) {
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436 FinderPattern center = (FinderPattern) possibleCenters.elementAt(i);
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437 if (center.getCount() >= CENTER_QUORUM) {
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438 if (firstConfirmedCenter == null) {
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439 firstConfirmedCenter = center;
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441 // We have two confirmed centers
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442 // How far down can we skip before resuming looking for the next
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443 // pattern? In the worst case, only the difference between the
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444 // difference in the x / y coordinates of the two centers.
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445 // This is the case where you find top left last.
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447 return (int) (Math.abs(firstConfirmedCenter.getX() - center.getX()) -
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448 Math.abs(firstConfirmedCenter.getY() - center.getY())) / 2;
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456 * @return true iff we have found at least 3 finder patterns that have been detected
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457 * at least {@link #CENTER_QUORUM} times each, and, the estimated module size of the
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458 * candidates is "pretty similar"
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460 private boolean haveMultiplyConfirmedCenters() {
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461 int confirmedCount = 0;
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462 float totalModuleSize = 0.0f;
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463 int max = possibleCenters.size();
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464 for (int i = 0; i < max; i++) {
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465 FinderPattern pattern = (FinderPattern) possibleCenters.elementAt(i);
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466 if (pattern.getCount() >= CENTER_QUORUM) {
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468 totalModuleSize += pattern.getEstimatedModuleSize();
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471 if (confirmedCount < 3) {
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474 // OK, we have at least 3 confirmed centers, but, it's possible that one is a "false positive"
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475 // and that we need to keep looking. We detect this by asking if the estimated module sizes
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476 // vary too much. We arbitrarily say that when the total deviation from average exceeds
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477 // 5% of the total module size estimates, it's too much.
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478 float average = totalModuleSize / (float) max;
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479 float totalDeviation = 0.0f;
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480 for (int i = 0; i < max; i++) {
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481 FinderPattern pattern = (FinderPattern) possibleCenters.elementAt(i);
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482 totalDeviation += Math.abs(pattern.getEstimatedModuleSize() - average);
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484 return totalDeviation <= 0.05f * totalModuleSize;
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488 * @return the 3 best {@link FinderPattern}s from our list of candidates. The "best" are
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489 * those that have been detected at least {@link #CENTER_QUORUM} times, and whose module
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490 * size differs from the average among those patterns the least
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491 * @throws ReaderException if 3 such finder patterns do not exist
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493 private FinderPattern[] selectBestPatterns() throws ReaderException {
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495 int startSize = possibleCenters.size();
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496 if (startSize < 3) {
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497 // Couldn't find enough finder patterns
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498 throw ReaderException.getInstance();
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501 // Filter outlier possibilities whose module size is too different
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502 if (startSize > 3) {
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503 // But we can only afford to do so if we have at least 4 possibilities to choose from
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504 float totalModuleSize = 0.0f;
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505 for (int i = 0; i < startSize; i++) {
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506 totalModuleSize += ((FinderPattern) possibleCenters.elementAt(i)).getEstimatedModuleSize();
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508 float average = totalModuleSize / (float) startSize;
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509 for (int i = 0; i < possibleCenters.size() && possibleCenters.size() > 3; i++) {
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510 FinderPattern pattern = (FinderPattern) possibleCenters.elementAt(i);
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511 if (Math.abs(pattern.getEstimatedModuleSize() - average) > 0.2f * average) {
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512 possibleCenters.removeElementAt(i);
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518 if (possibleCenters.size() > 3) {
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519 // Throw away all but those first size candidate points we found.
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520 Collections.insertionSort(possibleCenters, new CenterComparator());
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521 possibleCenters.setSize(3);
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524 return new FinderPattern[]{
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525 (FinderPattern) possibleCenters.elementAt(0),
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526 (FinderPattern) possibleCenters.elementAt(1),
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527 (FinderPattern) possibleCenters.elementAt(2)
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532 * <p>Orders by {@link FinderPattern#getCount()}, descending.</p>
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534 private static class CenterComparator implements Comparator {
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535 public int compare(Object center1, Object center2) {
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536 return ((FinderPattern) center2).getCount() - ((FinderPattern) center1).getCount();
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