2 * Copyright 2009 ZXing authors
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
8 * http://www.apache.org/licenses/LICENSE-2.0
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
17 package com.google.zxing.common;
19 import com.google.zxing.Binarizer;
20 import com.google.zxing.ReaderException;
21 import com.google.zxing.LuminanceSource;
24 * This class implements a local thresholding algorithm, which while slower than the
25 * GlobalHistogramBinarizer, is fairly efficient for what it does. It is designed for
26 * high frequency images of barcodes with black data on white backgrounds. For this application,
27 * it does a much better job than a global blackpoint with severe shadows and gradients.
28 * However it tends to produce artifacts on lower frequency images and is therefore not
29 * a good general purpose binarizer for uses outside ZXing.
31 * @author dswitkin@google.com (Daniel Switkin)
33 public final class LocalBlockBinarizer extends Binarizer {
35 private BitMatrix matrix = null;
37 public LocalBlockBinarizer(LuminanceSource source) {
41 public BitArray getBlackRow(int y, BitArray row) throws ReaderException {
42 binarizeEntireImage();
43 return matrix.getRow(y, row);
46 public BitMatrix getBlackMatrix() throws ReaderException {
47 binarizeEntireImage();
51 public Binarizer createBinarizer(LuminanceSource source) {
52 return new LocalBlockBinarizer(source);
55 // Calculates the final BitMatrix once for all requests. This could be called once from the
56 // constructor instead, but there are some advantages to doing it lazily, such as making
57 // profiling easier, and not doing heavy lifting when callers don't expect it.
58 private void binarizeEntireImage() {
60 LuminanceSource source = getLuminanceSource();
61 byte[] luminances = source.getMatrix();
62 int width = source.getWidth();
63 int height = source.getHeight();
64 sharpenRow(luminances, width, height);
66 int subWidth = width >> 3;
67 int subHeight = height >> 3;
68 int[][] blackPoints = calculateBlackPoints(luminances, subWidth, subHeight, width);
70 matrix = new BitMatrix(width, height);
71 calculateThresholdForBlock(luminances, subWidth, subHeight, width, blackPoints, matrix);
75 // For each 8x8 block in the image, calculate the average black point using a 5x5 grid
76 // of the blocks around it. Also handles the corner cases, but will ignore up to 7 pixels
77 // on the right edge and 7 pixels at the bottom of the image if the overall dimsions are not
78 // multiples of eight. In practice, leaving those pixels white does not seem to be a problem.
79 private static void calculateThresholdForBlock(byte[] luminances, int subWidth, int subHeight,
80 int stride, int[][] blackPoints, BitMatrix matrix) {
81 for (int y = 0; y < subHeight; y++) {
82 for (int x = 0; x < subWidth; x++) {
84 int left = (x > 1) ? x : 2;
85 left = (left < subWidth - 2) ? left : subWidth - 3;
86 int top = (y > 1) ? y : 2;
87 top = (top < subHeight - 2) ? top : subHeight - 3;
88 for (int z = -2; z <= 2; z++) {
89 sum += blackPoints[top + z][left - 2];
90 sum += blackPoints[top + z][left - 1];
91 sum += blackPoints[top + z][left];
92 sum += blackPoints[top + z][left + 1];
93 sum += blackPoints[top + z][left + 2];
95 int average = sum / 25;
96 threshold8x8Block(luminances, x * 8, y * 8, average, stride, matrix);
101 // Applies a single threshold to an 8x8 block of pixels.
102 private static void threshold8x8Block(byte[] luminances, int xoffset, int yoffset, int threshold,
103 int stride, BitMatrix matrix) {
104 for (int y = 0; y < 8; y++) {
105 int offset = (yoffset + y) * stride + xoffset;
106 for (int x = 0; x < 8; x++) {
107 int pixel = luminances[offset + x] & 0xff;
108 if (pixel < threshold) {
109 matrix.set(xoffset + x, yoffset + y);
115 // Calculates a single black point for each 8x8 block of pixels and saves it away.
116 private static int[][] calculateBlackPoints(byte[] luminances, int subWidth, int subHeight,
118 int[][] blackPoints = new int[subHeight][subWidth];
119 for (int y = 0; y < subHeight; y++) {
120 for (int x = 0; x < subWidth; x++) {
124 for (int yy = 0; yy < 8; yy++) {
125 int offset = (y * 8 + yy) * stride + (x * 8);
126 for (int xx = 0; xx < 8; xx++) {
127 int pixel = luminances[offset + xx] & 0xff;
138 // If the contrast is inadequate, use half the minimum, so that this block will be
139 // treated as part of the white background, but won't drag down neighboring blocks
141 int average = (max - min > 24) ? (sum >> 6) : (min >> 1);
142 blackPoints[y][x] = average;
148 // Applies a simple -1 4 -1 box filter with a weight of 2 to each row.
149 private static void sharpenRow(byte[] luminances, int width, int height) {
150 for (int y = 0; y < height; y++) {
151 int offset = y * width;
152 int left = luminances[offset] & 0xff;
153 int center = luminances[offset + 1] & 0xff;
154 for (int x = 1; x < width - 1; x++) {
155 int right = luminances[offset + x + 1] & 0xff;
156 luminances[x] = (byte)(((center << 2) - left - right) >> 1);