path: root/libraries/zxing/src/com/google/zxing/qrcode/detector/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.ResultPoint;
import com.google.zxing.ResultPointCallback;
import com.google.zxing.common.BitMatrix;

import java.util.Vector;

/**
 * <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.
 *
 * @author Sean Owen
 */
final class AlignmentPatternFinder {

  private final BitMatrix image;
  private final Vector 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 Vector(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 >> 1);
    // 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) >> 1 : -((iGen + 1) >> 1));
      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[currentState]++;
          } 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 (AlignmentPattern) possibleCenters.elementAt(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 (float) (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 = (float) (stateCount[0] + stateCount[1] + stateCount[2]) / 3.0f;
      int max = possibleCenters.size();
      for (int index = 0; index < max; index++) {
        AlignmentPattern center = (AlignmentPattern) possibleCenters.elementAt(index);
        // Look for about the same center and module size:
        if (center.aboutEquals(estimatedModuleSize, centerI, centerJ)) {
          return new AlignmentPattern(centerJ, centerI, estimatedModuleSize);
        }
      }
      // Hadn't found this before; save it
      ResultPoint point = new AlignmentPattern(centerJ, centerI, estimatedModuleSize);
      possibleCenters.addElement(point);
      if (resultPointCallback != null) {
        resultPointCallback.foundPossibleResultPoint(point);
      }
    }
    return null;
  }

}