content: /*
   * Copyright 2009 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.ailiwean.core.zxing.core.common;
  
  
  import com.ailiwean.core.zxing.core.Binarizer;
  import com.ailiwean.core.zxing.core.LuminanceSource;
  import com.ailiwean.core.zxing.core.NotFoundException;
  
  /**
   * This class implements a local thresholding algorithm, which while slower than the
   * GlobalHistogramBinarizer, is fairly efficient for what it does. It is designed for
   * high frequency images of barcodes with black data on white backgrounds. For this application,
   * it does a much better job than a global blackpoint with severe shadows and gradients.
   * However it tends to produce artifacts on lower frequency images and is therefore not
   * a good general purpose binarizer for uses outside ZXing.
   * <p>
   * This class extends GlobalHistogramBinarizer, using the older histogram approach for 1D readers,
   * and the newer local approach for 2D readers. 1D decoding using a per-row histogram is already
   * inherently local, and only fails for horizontal gradients. We can revisit that problem later,
   * but for now it was not a win to use local blocks for 1D.
   * <p>
   * This Binarizer is the default for the unit tests and the recommended class for library users.
   *
   * @author dswitkin@google.com (Daniel Switkin)
   */
  public final class HybridBinarizer extends GlobalHistogramBinarizer {
  
      // This class uses 5x5 blocks to compute local luminance, where each block is 8x8 pixels.
      // So this is the smallest dimension in each axis we can accept.
      private static final int BLOCK_SIZE_POWER = 3;
      private static final int BLOCK_SIZE = 1 << BLOCK_SIZE_POWER; // ...0100...00
      private static final int BLOCK_SIZE_MASK = BLOCK_SIZE - 1;   // ...0011...11
      private static final int MINIMUM_DIMENSION = BLOCK_SIZE * 5;
      private static final int MIN_DYNAMIC_RANGE = 24;
  
      private BitMatrix matrix;
  
      public HybridBinarizer(LuminanceSource source) {
          super(source);
      }
  
      /**
       * Calculates the final BitMatrix once for all requests. This could be called once from the
       * constructor instead, but there are some advantages to doing it lazily, such as making
       * profiling easier, and not doing heavy lifting when callers don't expect it.
       */
      @Override
      public BitMatrix getBlackMatrix() throws NotFoundException {
          if (bitMatrix == null) {
              synchronized (this) {
                  if (bitMatrix == null)
                      bitMatrix = creatBitMatrix();
              }
          }
          return bitMatrix;
      }
  
      @Override
      protected BitMatrix creatBitMatrix() throws NotFoundException {
          LuminanceSource source = getLuminanceSource();
          int width = source.getWidth();
          int height = source.getHeight();
          if (width >= MINIMUM_DIMENSION && height >= MINIMUM_DIMENSION) {
              byte[] luminances = source.getMatrix();
              int subWidth = width >> BLOCK_SIZE_POWER;
              if ((width & BLOCK_SIZE_MASK) != 0) {
                  subWidth++;
              }
              int subHeight = height >> BLOCK_SIZE_POWER;
              if ((height & BLOCK_SIZE_MASK) != 0) {
                  subHeight++;
              }
  
              int[][] blackPoints = calculateBlackPoints(luminances, subWidth, subHeight, width, height);
  
              BitMatrix newMatrix = new BitMatrix(width, height);
              calculateThresholdForBlock(luminances, subWidth, subHeight, width, height, blackPoints, newMatrix);
              matrix = newMatrix;
          } else {
              // If the image is too small, fall back to the global histogram approach.
              matrix = super.getBlackMatrix();
          }
          return matrix;
      }
  
      @Override
      public Binarizer createBinarizer(LuminanceSource source) {
          return new HybridBinarizer(source);
      }
  
      /**
       * For each block in the image, calculate the average black point using a 5x5 grid
       * of the blocks around it. Also handles the corner cases (fractional blocks are computed based
       * on the last pixels in the row/column which are also used in the previous block).
       */
      private static void calculateThresholdForBlock(byte[] luminances,
                                                     int subWidth,
                                                     int subHeight,
                                                     int width,
                                                     int height,
                                                     int[][] blackPoints,
                                                     BitMatrix matrix) {
          int maxYOffset = height - BLOCK_SIZE;
          int maxXOffset = width - BLOCK_SIZE;
          for (int y = 0; y < subHeight; y++) {
              int yoffset = y << BLOCK_SIZE_POWER;
              if (yoffset > maxYOffset) {
                  yoffset = maxYOffset;
              }
              int top = cap(y, subHeight - 3);
              for (int x = 0; x < subWidth; x++) {
                  int xoffset = x << BLOCK_SIZE_POWER;
                  if (xoffset > maxXOffset) {
                      xoffset = maxXOffset;
                  }
                  int left = cap(x, subWidth - 3);
                  int sum = 0;
                  for (int z = -2; z <= 2; z++) {
                      int[] blackRow = blackPoints[top + z];
                      sum += blackRow[left - 2] + blackRow[left - 1] + blackRow[left] + blackRow[left + 1] + blackRow[left + 2];
                  }
                  int average = sum / 25;
                  thresholdBlock(luminances, xoffset, yoffset, average, width, matrix);
              }
          }
      }
  
      private static int cap(int value, int max) {
          return value < 2 ? 2 : Math.min(value, max);
      }
  
      /**
       * Applies a single threshold to a block of pixels.
       */
      private static void thresholdBlock(byte[] luminances,
                                         int xoffset,
                                         int yoffset,
                                         int threshold,
                                         int stride,
                                         BitMatrix matrix) {
          for (int y = 0, offset = yoffset * stride + xoffset; y < BLOCK_SIZE; y++, offset += stride) {
              for (int x = 0; x < BLOCK_SIZE; x++) {
                  // Comparison needs to be <= so that black == 0 pixels are black even if the threshold is 0.
                  if ((luminances[offset + x] & 0xFF) <= threshold) {
                      matrix.set(xoffset + x, yoffset + y);
                  }
              }
          }
      }
  
      /**
       * Calculates a single black point for each block of pixels and saves it away.
       * See the following thread for a discussion of this algorithm:
       * http://groups.google.com/group/zxing/browse_thread/thread/d06efa2c35a7ddc0
       */
      private static int[][] calculateBlackPoints(byte[] luminances,
                                                  int subWidth,
                                                  int subHeight,
                                                  int width,
                                                  int height) {
          int maxYOffset = height - BLOCK_SIZE;
          int maxXOffset = width - BLOCK_SIZE;
          int[][] blackPoints = new int[subHeight][subWidth];
          for (int y = 0; y < subHeight; y++) {
              int yoffset = y << BLOCK_SIZE_POWER;
              if (yoffset > maxYOffset) {
                  yoffset = maxYOffset;
              }
              for (int x = 0; x < subWidth; x++) {
                  int xoffset = x << BLOCK_SIZE_POWER;
                  if (xoffset > maxXOffset) {
                      xoffset = maxXOffset;
                  }
                  int sum = 0;
                  int min = 0xFF;
                  int max = 0;
                  for (int yy = 0, offset = yoffset * width + xoffset; yy < BLOCK_SIZE; yy++, offset += width) {
                      for (int xx = 0; xx < BLOCK_SIZE; xx++) {
                          int pixel = luminances[offset + xx] & 0xFF;
                          sum += pixel;
                          // still looking for good contrast
                          if (pixel < min) {
                              min = pixel;
                          }
                          if (pixel > max) {
                              max = pixel;
                          }
                      }
                      // short-circuit min/max tests once dynamic range is met
                      if (max - min > MIN_DYNAMIC_RANGE) {
                          // finish the rest of the rows quickly
                          for (yy++, offset += width; yy < BLOCK_SIZE; yy++, offset += width) {
                              for (int xx = 0; xx < BLOCK_SIZE; xx++) {
                                  sum += luminances[offset + xx] & 0xFF;
                              }
                          }
                      }
                  }
  
                  // The default estimate is the average of the values in the block.
                  int average = sum >> (BLOCK_SIZE_POWER * 2);
                  if (max - min <= MIN_DYNAMIC_RANGE) {
                      // If variation within the block is low, assume this is a block with only light or only
                      // dark pixels. In that case we do not want to use the average, as it would divide this
                      // low contrast area into black and white pixels, essentially creating data out of noise.
                      //
                      // The default assumption is that the block is light/background. Since no estimate for
                      // the level of dark pixels exists locally, use half the min for the block.
                      average = min / 2;
  
                      if (y > 0 && x > 0) {
                          // Correct the "white background" assumption for blocks that have neighbors by comparing
                          // the pixels in this block to the previously calculated black points. This is based on
                          // the fact that dark barcode symbology is always surrounded by some amount of light
                          // background for which reasonable black point estimates were made. The bp estimated at
                          // the boundaries is used for the interior.
  
                          // The (min < bp) is arbitrary but works better than other heuristics that were tried.
                          int averageNeighborBlackPoint =
                                  (blackPoints[y - 1][x] + (2 * blackPoints[y][x - 1]) + blackPoints[y - 1][x - 1]) / 4;
                          if (min < averageNeighborBlackPoint) {
                              average = averageNeighborBlackPoint;
                          }
                      }
                  }
                  blackPoints[y][x] = average;
              }
          }
          return blackPoints;
      }
  
  }
  