/* * Copyright 2007 ZXing authors * * 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.google.zxing.qrcode.detector; import com.google.zxing.DecodeHintType; import com.google.zxing.MonochromeBitmapSource; import com.google.zxing.ReaderException; import com.google.zxing.ResultPoint; import com.google.zxing.common.BitArray; import com.google.zxing.common.Collections; import com.google.zxing.common.Comparator; import java.util.Hashtable; import java.util.Vector; /** *

This class attempts to find finder patterns in a QR Code. Finder patterns are the square * markers at three corners of a QR Code.

* *

This class is not thread-safe and should not be reused.

* * @author Sean Owen */ final class FinderPatternFinder { private static final int CENTER_QUORUM = 2; private static final int MIN_SKIP = 3; // 1 pixel/module times 3 modules/center private static final int MAX_MODULES = 57; // support up to version 10 for mobile clients private static final int INTEGER_MATH_SHIFT = 8; private final MonochromeBitmapSource image; private final Vector possibleCenters; private boolean hasSkipped; private final int[] crossCheckStateCount; /** *

Creates a finder that will search the image for three finder patterns.

* * @param image image to search */ FinderPatternFinder(MonochromeBitmapSource image) { this.image = image; this.possibleCenters = new Vector(); this.crossCheckStateCount = new int[5]; } FinderPatternInfo find(Hashtable hints) throws ReaderException { boolean tryHarder = hints != null && hints.containsKey(DecodeHintType.TRY_HARDER); int maxI = image.getHeight(); int maxJ = image.getWidth(); // We are looking for black/white/black/white/black modules in // 1:1:3:1:1 ratio; this tracks the number of such modules seen so far // Let's assume that the maximum version QR Code we support takes up 1/4 the height of the // image, and then account for the center being 3 modules in size. This gives the smallest // number of pixels the center could be, so skip this often. When trying harder, look for all // QR versions regardless of how dense they are. int iSkip = (int) (maxI / (MAX_MODULES * 4.0f) * 3); if (iSkip < MIN_SKIP || tryHarder) { iSkip = MIN_SKIP; } boolean done = false; int[] stateCount = new int[5]; BitArray blackRow = new BitArray(maxJ); for (int i = iSkip - 1; i < maxI && !done; i += iSkip) { // Get a row of black/white values blackRow = image.getBlackRow(i, blackRow, 0, maxJ); stateCount[0] = 0; stateCount[1] = 0; stateCount[2] = 0; stateCount[3] = 0; stateCount[4] = 0; int currentState = 0; for (int j = 0; j < maxJ; j++) { if (blackRow.get(j)) { // Black pixel if ((currentState & 1) == 1) { // Counting white pixels currentState++; } stateCount[currentState]++; } else { // White pixel if ((currentState & 1) == 0) { // Counting black pixels if (currentState == 4) { // A winner? if (foundPatternCross(stateCount)) { // Yes boolean confirmed = handlePossibleCenter(stateCount, i, j); if (confirmed) { // Start examining every other line. Checking each line turned out to be too // expensive and didn't improve performance. iSkip = 2; if (hasSkipped) { done = haveMulitplyConfirmedCenters(); } else { int rowSkip = findRowSkip(); if (rowSkip > stateCount[2]) { // Skip rows between row of lower confirmed center // and top of presumed third confirmed center // but back up a bit to get a full chance of detecting // it, entire width of center of finder pattern // Skip by rowSkip, but back off by stateCount[2] (size of last center // of pattern we saw) to be conservative, and also back off by iSkip which // is about to be re-added i += rowSkip - stateCount[2] - iSkip; j = maxJ - 1; } } } else { // Advance to next black pixel do { j++; } while (j < maxJ && !blackRow.get(j)); j--; // back up to that last white pixel } // Clear state to start looking again currentState = 0; stateCount[0] = 0; stateCount[1] = 0; stateCount[2] = 0; stateCount[3] = 0; stateCount[4] = 0; } else { // No, shift counts back by two stateCount[0] = stateCount[2]; stateCount[1] = stateCount[3]; stateCount[2] = stateCount[4]; stateCount[3] = 1; stateCount[4] = 0; currentState = 3; } } else { stateCount[++currentState]++; } } else { // Counting white pixels stateCount[currentState]++; } } } if (foundPatternCross(stateCount)) { boolean confirmed = handlePossibleCenter(stateCount, i, maxJ); if (confirmed) { iSkip = stateCount[0]; if (hasSkipped) { // Found a third one done = haveMulitplyConfirmedCenters(); } } } } FinderPattern[] patternInfo = selectBestPatterns(); ResultPoint.orderBestPatterns(patternInfo); return new FinderPatternInfo(patternInfo); } /** * Given a count of black/white/black/white/black pixels just seen and an end position, * figures the location of the center of this run. */ private static float centerFromEnd(int[] stateCount, int end) { return (float) (end - stateCount[4] - stateCount[3]) - stateCount[2] / 2.0f; } /** * @param stateCount count of black/white/black/white/black pixels just read * @return true iff the proportions of the counts is close enough to the 1/1/3/1/1 ratios * used by finder patterns to be considered a match */ private static boolean foundPatternCross(int[] stateCount) { int totalModuleSize = 0; for (int i = 0; i < 5; i++) { int count = stateCount[i]; if (count == 0) { return false; } totalModuleSize += count; } if (totalModuleSize < 7) { return false; } int moduleSize = (totalModuleSize << INTEGER_MATH_SHIFT) / 7; int maxVariance = moduleSize / 2; // Allow less than 50% variance from 1-1-3-1-1 proportions return Math.abs(moduleSize - (stateCount[0] << INTEGER_MATH_SHIFT)) < maxVariance && Math.abs(moduleSize - (stateCount[1] << INTEGER_MATH_SHIFT)) < maxVariance && Math.abs(3 * moduleSize - (stateCount[2] << INTEGER_MATH_SHIFT)) < 3 * maxVariance && Math.abs(moduleSize - (stateCount[3] << INTEGER_MATH_SHIFT)) < maxVariance && Math.abs(moduleSize - (stateCount[4] << INTEGER_MATH_SHIFT)) < maxVariance; } private int[] getCrossCheckStateCount() { crossCheckStateCount[0] = 0; crossCheckStateCount[1] = 0; crossCheckStateCount[2] = 0; crossCheckStateCount[3] = 0; crossCheckStateCount[4] = 0; return crossCheckStateCount; } /** *

After a horizontal scan finds a potential finder pattern, this method * "cross-checks" by scanning down vertically through the center of the possible * finder pattern to see if the same proportion is detected.

* * @param startI row where a finder pattern was detected * @param centerJ center of the section that appears to cross a finder pattern * @param maxCount maximum reasonable number of modules that should be * observed in any reading state, based on the results of the horizontal scan * @return vertical center of finder pattern, or {@link Float#NaN} if not found */ private float crossCheckVertical(int startI, int centerJ, int maxCount, int originalStateCountTotal) { MonochromeBitmapSource image = this.image; int maxI = image.getHeight(); int[] stateCount = getCrossCheckStateCount(); // Start counting up from center int i = startI; while (i >= 0 && image.isBlack(centerJ, i)) { stateCount[2]++; i--; } if (i < 0) { return Float.NaN; } while (i >= 0 && !image.isBlack(centerJ, i) && stateCount[1] <= maxCount) { stateCount[1]++; i--; } // If already too many modules in this state or ran off the edge: if (i < 0 || stateCount[1] > maxCount) { return Float.NaN; } while (i >= 0 && image.isBlack(centerJ, i) && stateCount[0] <= maxCount) { stateCount[0]++; i--; } if (stateCount[0] > maxCount) { return Float.NaN; } // Now also count down from center i = startI + 1; while (i < maxI && image.isBlack(centerJ, i)) { stateCount[2]++; i++; } if (i == maxI) { return Float.NaN; } while (i < maxI && !image.isBlack(centerJ, i) && stateCount[3] < maxCount) { stateCount[3]++; i++; } if (i == maxI || stateCount[3] >= maxCount) { return Float.NaN; } while (i < maxI && image.isBlack(centerJ, i) && stateCount[4] < maxCount) { stateCount[4]++; i++; } if (stateCount[4] >= maxCount) { return Float.NaN; } // If we found a finder-pattern-like section, but its size is more than 20% different than // the original, assume it's a false positive int stateCountTotal = stateCount[0] + stateCount[1] + stateCount[2] + stateCount[3] + stateCount[4]; if (5 * Math.abs(stateCountTotal - originalStateCountTotal) >= originalStateCountTotal) { return Float.NaN; } return foundPatternCross(stateCount) ? centerFromEnd(stateCount, i) : Float.NaN; } /** *

Like {@link #crossCheckVertical(int, int, int, int)}, and in fact is basically identical, * except it reads horizontally instead of vertically. This is used to cross-cross * check a vertical cross check and locate the real center of the alignment pattern.

*/ private float crossCheckHorizontal(int startJ, int centerI, int maxCount, int originalStateCountTotal) { MonochromeBitmapSource image = this.image; int maxJ = image.getWidth(); int[] stateCount = getCrossCheckStateCount(); int j = startJ; while (j >= 0 && image.isBlack(j, centerI)) { stateCount[2]++; j--; } if (j < 0) { return Float.NaN; } while (j >= 0 && !image.isBlack(j, centerI) && stateCount[1] <= maxCount) { stateCount[1]++; j--; } if (j < 0 || stateCount[1] > maxCount) { return Float.NaN; } while (j >= 0 && image.isBlack(j, centerI) && stateCount[0] <= maxCount) { stateCount[0]++; j--; } if (stateCount[0] > maxCount) { return Float.NaN; } j = startJ + 1; while (j < maxJ && image.isBlack(j, centerI)) { stateCount[2]++; j++; } if (j == maxJ) { return Float.NaN; } while (j < maxJ && !image.isBlack(j, centerI) && stateCount[3] < maxCount) { stateCount[3]++; j++; } if (j == maxJ || stateCount[3] >= maxCount) { return Float.NaN; } while (j < maxJ && image.isBlack(j, centerI) && stateCount[4] < maxCount) { stateCount[4]++; j++; } if (stateCount[4] >= maxCount) { return Float.NaN; } // If we found a finder-pattern-like section, but its size is significantly different than // the original, assume it's a false positive int stateCountTotal = stateCount[0] + stateCount[1] + stateCount[2] + stateCount[3] + stateCount[4]; if (5 * Math.abs(stateCountTotal - originalStateCountTotal) >= originalStateCountTotal) { return Float.NaN; } return foundPatternCross(stateCount) ? centerFromEnd(stateCount, j) : Float.NaN; } /** *

This is called when a horizontal scan finds a possible alignment pattern. It will * cross check with a vertical scan, and if successful, will, ah, cross-cross-check * with another horizontal scan. This is needed primarily to locate the real horizontal * center of the pattern in cases of extreme skew.

* *

If that succeeds the finder pattern location is added to a list that tracks * the number of times each location has been nearly-matched as a finder pattern. * Each additional find is more evidence that the location is in fact a finder * pattern center * * @param stateCount reading state module counts from horizontal scan * @param i row where finder pattern may be found * @param j end of possible finder pattern in row * @return true if a finder pattern candidate was found this time */ private boolean handlePossibleCenter(int[] stateCount, int i, int j) { int stateCountTotal = stateCount[0] + stateCount[1] + stateCount[2] + stateCount[3] + stateCount[4]; float centerJ = centerFromEnd(stateCount, j); float centerI = crossCheckVertical(i, (int) centerJ, stateCount[2], stateCountTotal); if (!Float.isNaN(centerI)) { // Re-cross check centerJ = crossCheckHorizontal((int) centerJ, (int) centerI, stateCount[2], stateCountTotal); if (!Float.isNaN(centerJ)) { float estimatedModuleSize = (float) stateCountTotal / 7.0f; boolean found = false; int max = possibleCenters.size(); for (int index = 0; index < max; index++) { FinderPattern center = (FinderPattern) possibleCenters.elementAt(index); // Look for about the same center and module size: if (center.aboutEquals(estimatedModuleSize, centerI, centerJ)) { center.incrementCount(); found = true; break; } } if (!found) { possibleCenters.addElement(new FinderPattern(centerJ, centerI, estimatedModuleSize)); } return true; } } return false; } /** * @return number of rows we could safely skip during scanning, based on the first * two finder patterns that have been located. In some cases their position will * allow us to infer that the third pattern must lie below a certain point farther * down in the image. */ private int findRowSkip() { int max = possibleCenters.size(); if (max <= 1) { return 0; } FinderPattern firstConfirmedCenter = null; for (int i = 0; i < max; i++) { FinderPattern center = (FinderPattern) possibleCenters.elementAt(i); if (center.getCount() >= CENTER_QUORUM) { if (firstConfirmedCenter == null) { firstConfirmedCenter = center; } else { // We have two confirmed centers // How far down can we skip before resuming looking for the next // pattern? In the worst case, only the difference between the // difference in the x / y coordinates of the two centers. // This is the case where you find top left last. hasSkipped = true; return (int) (Math.abs(firstConfirmedCenter.getX() - center.getX()) - Math.abs(firstConfirmedCenter.getY() - center.getY())) / 2; } } } return 0; } /** * @return true iff we have found at least 3 finder patterns that have been detected * at least {@link #CENTER_QUORUM} times each, and, the estimated module size of the * candidates is "pretty similar" */ private boolean haveMulitplyConfirmedCenters() { int confirmedCount = 0; float totalModuleSize = 0.0f; int max = possibleCenters.size(); for (int i = 0; i < max; i++) { FinderPattern pattern = (FinderPattern) possibleCenters.elementAt(i); if (pattern.getCount() >= CENTER_QUORUM) { confirmedCount++; totalModuleSize += pattern.getEstimatedModuleSize(); } } if (confirmedCount < 3) { return false; } // OK, we have at least 3 confirmed centers, but, it's possible that one is a "false positive" // and that we need to keep looking. We detect this by asking if the estimated module sizes // vary too much. We arbitrarily say that when the total deviation from average exceeds // 15% of the total module size estimates, it's too much. float average = totalModuleSize / max; float totalDeviation = 0.0f; for (int i = 0; i < max; i++) { FinderPattern pattern = (FinderPattern) possibleCenters.elementAt(i); totalDeviation += Math.abs(pattern.getEstimatedModuleSize() - average); } return totalDeviation <= 0.05f * totalModuleSize; } /** * @return the 3 best {@link FinderPattern}s from our list of candidates. The "best" are * those that have been detected at least {@link #CENTER_QUORUM} times, and whose module * size differs from the average among those patterns the least * @throws ReaderException if 3 such finder patterns do not exist */ private FinderPattern[] selectBestPatterns() throws ReaderException { Collections.insertionSort(possibleCenters, new CenterComparator()); int size = 0; int max = possibleCenters.size(); while (size < max) { if (((FinderPattern) possibleCenters.elementAt(size)).getCount() < CENTER_QUORUM) { break; } size++; } if (size < 3) { // Couldn't find enough finder patterns throw ReaderException.getInstance(); } if (size > 3) { // Throw away all but those first size candidate points we found. possibleCenters.setSize(size); // We need to pick the best three. Find the most // popular ones whose module size is nearest the average float averageModuleSize = 0.0f; for (int i = 0; i < size; i++) { averageModuleSize += ((FinderPattern) possibleCenters.elementAt(i)).getEstimatedModuleSize(); } averageModuleSize /= (float) size; // We don't have java.util.Collections in J2ME Collections.insertionSort(possibleCenters, new ClosestToAverageComparator(averageModuleSize)); } return new FinderPattern[]{ (FinderPattern) possibleCenters.elementAt(0), (FinderPattern) possibleCenters.elementAt(1), (FinderPattern) possibleCenters.elementAt(2) }; } /** *

Orders by {@link FinderPattern#getCount()}, descending.

*/ private static class CenterComparator implements Comparator { public int compare(Object center1, Object center2) { return ((FinderPattern) center2).getCount() - ((FinderPattern) center1).getCount(); } } /** *

Orders by variance from average module size, ascending.

*/ private static class ClosestToAverageComparator implements Comparator { private final float averageModuleSize; private ClosestToAverageComparator(float averageModuleSize) { this.averageModuleSize = averageModuleSize; } public int compare(Object center1, Object center2) { return Math.abs(((FinderPattern) center1).getEstimatedModuleSize() - averageModuleSize) < Math.abs(((FinderPattern) center2).getEstimatedModuleSize() - averageModuleSize) ? -1 : 1; } } }