AlignmentPatternFinder.java
/*
* 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.NotFoundException;
import com.google.zxing.ResultPointCallback;
import com.google.zxing.common.BitMatrix;
import java.util.ArrayList;
import java.util.List;
/**
* <p>This class attempts to find alignment patterns in a QR Code. Alignment patterns look like finder
* patterns but are smaller and appear at regular intervals throughout the image.</p>
*
* <p>At the moment this only looks for the bottom-right alignment pattern.</p>
*
* <p>This is mostly a simplified copy of {@link FinderPatternFinder}. It is copied,
* pasted and stripped down here for maximum performance but does unfortunately duplicate
* some code.</p>
*
* <p>This class is thread-safe but not reentrant. Each thread must allocate its own object.</p>
*
* @author Sean Owen
*/
final class AlignmentPatternFinder {
private final BitMatrix image;
private final List<AlignmentPattern> possibleCenters;
private final int startX;
private final int startY;
private final int width;
private final int height;
private final float moduleSize;
private final int[] crossCheckStateCount;
private final ResultPointCallback resultPointCallback;
/**
* <p>Creates a finder that will look in a portion of the whole image.</p>
*
* @param image image to search
* @param startX left column from which to start searching
* @param startY top row from which to start searching
* @param width width of region to search
* @param height height of region to search
* @param moduleSize estimated module size so far
*/
AlignmentPatternFinder(BitMatrix image,
int startX,
int startY,
int width,
int height,
float moduleSize,
ResultPointCallback resultPointCallback) {
this.image = image;
this.possibleCenters = new ArrayList<>(5);
this.startX = startX;
this.startY = startY;
this.width = width;
this.height = height;
this.moduleSize = moduleSize;
this.crossCheckStateCount = new int[3];
this.resultPointCallback = resultPointCallback;
}
/**
* <p>This method attempts to find the bottom-right alignment pattern in the image. It is a bit messy since
* it's pretty performance-critical and so is written to be fast foremost.</p>
*
* @return {@link AlignmentPattern} if found
* @throws NotFoundException if not found
*/
AlignmentPattern find() throws NotFoundException {
int startX = this.startX;
int height = this.height;
int maxJ = startX + width;
int middleI = startY + (height / 2);
// We are looking for black/white/black modules in 1:1:1 ratio;
// this tracks the number of black/white/black modules seen so far
int[] stateCount = new int[3];
for (int iGen = 0; iGen < height; iGen++) {
// Search from middle outwards
int i = middleI + ((iGen & 0x01) == 0 ? (iGen + 1) / 2 : -((iGen + 1) / 2));
stateCount[0] = 0;
stateCount[1] = 0;
stateCount[2] = 0;
int j = startX;
// Burn off leading white pixels before anything else; if we start in the middle of
// a white run, it doesn't make sense to count its length, since we don't know if the
// white run continued to the left of the start point
while (j < maxJ && !image.get(j, i)) {
j++;
}
int currentState = 0;
while (j < maxJ) {
if (image.get(j, i)) {
// Black pixel
if (currentState == 1) { // Counting black pixels
stateCount[1]++;
} else { // Counting white pixels
if (currentState == 2) { // A winner?
if (foundPatternCross(stateCount)) { // Yes
AlignmentPattern confirmed = handlePossibleCenter(stateCount, i, j);
if (confirmed != null) {
return confirmed;
}
}
stateCount[0] = stateCount[2];
stateCount[1] = 1;
stateCount[2] = 0;
currentState = 1;
} else {
stateCount[++currentState]++;
}
}
} else { // White pixel
if (currentState == 1) { // Counting black pixels
currentState++;
}
stateCount[currentState]++;
}
j++;
}
if (foundPatternCross(stateCount)) {
AlignmentPattern confirmed = handlePossibleCenter(stateCount, i, maxJ);
if (confirmed != null) {
return confirmed;
}
}
}
// Hmm, nothing we saw was observed and confirmed twice. If we had
// any guess at all, return it.
if (!possibleCenters.isEmpty()) {
return possibleCenters.get(0);
}
throw NotFoundException.getNotFoundInstance();
}
/**
* Given a count of black/white/black pixels just seen and an end position,
* figures the location of the center of this black/white/black run.
*/
private static float centerFromEnd(int[] stateCount, int end) {
return (end - stateCount[2]) - stateCount[1] / 2.0f;
}
/**
* @param stateCount count of black/white/black pixels just read
* @return true iff the proportions of the counts is close enough to the 1/1/1 ratios
* used by alignment patterns to be considered a match
*/
private boolean foundPatternCross(int[] stateCount) {
float moduleSize = this.moduleSize;
float maxVariance = moduleSize / 2.0f;
for (int i = 0; i < 3; i++) {
if (Math.abs(moduleSize - stateCount[i]) >= maxVariance) {
return false;
}
}
return true;
}
/**
* <p>After a horizontal scan finds a potential alignment pattern, this method
* "cross-checks" by scanning down vertically through the center of the possible
* alignment pattern to see if the same proportion is detected.</p>
*
* @param startI row where an alignment pattern was detected
* @param centerJ center of the section that appears to cross an alignment 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 alignment pattern, or {@link Float#NaN} if not found
*/
private float crossCheckVertical(int startI, int centerJ, int maxCount,
int originalStateCountTotal) {
BitMatrix image = this.image;
int maxI = image.getHeight();
int[] stateCount = crossCheckStateCount;
stateCount[0] = 0;
stateCount[1] = 0;
stateCount[2] = 0;
// Start counting up from center
int i = startI;
while (i >= 0 && image.get(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.get(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.get(centerJ, i) && stateCount[1] <= maxCount) {
stateCount[1]++;
i++;
}
if (i == maxI || stateCount[1] > maxCount) {
return Float.NaN;
}
while (i < maxI && !image.get(centerJ, i) && stateCount[2] <= maxCount) {
stateCount[2]++;
i++;
}
if (stateCount[2] > maxCount) {
return Float.NaN;
}
int stateCountTotal = stateCount[0] + stateCount[1] + stateCount[2];
if (5 * Math.abs(stateCountTotal - originalStateCountTotal) >= 2 * originalStateCountTotal) {
return Float.NaN;
}
return foundPatternCross(stateCount) ? centerFromEnd(stateCount, i) : Float.NaN;
}
/**
* <p>This is called when a horizontal scan finds a possible alignment pattern. It will
* cross check with a vertical scan, and if successful, will see if this pattern had been
* found on a previous horizontal scan. If so, we consider it confirmed and conclude we have
* found the alignment pattern.</p>
*
* @param stateCount reading state module counts from horizontal scan
* @param i row where alignment pattern may be found
* @param j end of possible alignment pattern in row
* @return {@link AlignmentPattern} if we have found the same pattern twice, or null if not
*/
private AlignmentPattern handlePossibleCenter(int[] stateCount, int i, int j) {
int stateCountTotal = stateCount[0] + stateCount[1] + stateCount[2];
float centerJ = centerFromEnd(stateCount, j);
float centerI = crossCheckVertical(i, (int) centerJ, 2 * stateCount[1], stateCountTotal);
if (!Float.isNaN(centerI)) {
float estimatedModuleSize = (stateCount[0] + stateCount[1] + stateCount[2]) / 3.0f;
for (AlignmentPattern center : possibleCenters) {
// Look for about the same center and module size:
if (center.aboutEquals(estimatedModuleSize, centerI, centerJ)) {
return center.combineEstimate(centerI, centerJ, estimatedModuleSize);
}
}
// Hadn't found this before; save it
AlignmentPattern point = new AlignmentPattern(centerJ, centerI, estimatedModuleSize);
possibleCenters.add(point);
if (resultPointCallback != null) {
resultPointCallback.foundPossibleResultPoint(point);
}
}
return null;
}
}