/*
* 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 th.co.yellowpages.zxing.common;
import th.co.yellowpages.zxing.Binarizer;
import th.co.yellowpages.zxing.LuminanceSource;
import th.co.yellowpages.zxing.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.
*
* 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.
*
* 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 MINIMUM_DIMENSION = 40;
private BitMatrix matrix = null;
public HybridBinarizer(LuminanceSource source) {
super(source);
}
public BitMatrix getBlackMatrix() throws NotFoundException {
binarizeEntireImage();
return matrix;
}
public Binarizer createBinarizer(LuminanceSource source) {
return new HybridBinarizer(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.
private void binarizeEntireImage() throws NotFoundException {
if (matrix == null) {
LuminanceSource source = getLuminanceSource();
if (source.getWidth() >= MINIMUM_DIMENSION && source.getHeight() >= MINIMUM_DIMENSION) {
byte[] luminances = source.getMatrix();
int width = source.getWidth();
int height = source.getHeight();
int subWidth = width >> 3;
int subHeight = height >> 3;
int[][] blackPoints = calculateBlackPoints(luminances, subWidth, subHeight, width);
matrix = new BitMatrix(width, height);
calculateThresholdForBlock(luminances, subWidth, subHeight, width, blackPoints, matrix);
} else {
// If the image is too small, fall back to the global histogram approach.
matrix = super.getBlackMatrix();
}
}
}
// For each 8x8 block in the image, calculate the average black point using a 5x5 grid
// of the blocks around it. Also handles the corner cases, but will ignore up to 7 pixels
// on the right edge and 7 pixels at the bottom of the image if the overall dimensions are not
// multiples of eight. In practice, leaving those pixels white does not seem to be a problem.
private static void calculateThresholdForBlock(byte[] luminances, int subWidth, int subHeight,
int stride, int[][] blackPoints, BitMatrix matrix) {
for (int y = 0; y < subHeight; y++) {
for (int x = 0; x < subWidth; x++) {
int left = (x > 1) ? x : 2;
left = (left < subWidth - 2) ? left : subWidth - 3;
int top = (y > 1) ? y : 2;
top = (top < subHeight - 2) ? top : subHeight - 3;
int sum = 0;
for (int z = -2; z <= 2; z++) {
int[] blackRow = blackPoints[top + z];
sum += blackRow[left - 2];
sum += blackRow[left - 1];
sum += blackRow[left];
sum += blackRow[left + 1];
sum += blackRow[left + 2];
}
int average = sum / 25;
threshold8x8Block(luminances, x << 3, y << 3, average, stride, matrix);
}
}
}
// Applies a single threshold to an 8x8 block of pixels.
private static void threshold8x8Block(byte[] luminances, int xoffset, int yoffset, int threshold,
int stride, BitMatrix matrix) {
for (int y = 0; y < 8; y++) {
int offset = (yoffset + y) * stride + xoffset;
for (int x = 0; x < 8; x++) {
int pixel = luminances[offset + x] & 0xff;
if (pixel < threshold) {
matrix.set(xoffset + x, yoffset + y);
}
}
}
}
// Calculates a single black point for each 8x8 block of pixels and saves it away.
private static int[][] calculateBlackPoints(byte[] luminances, int subWidth, int subHeight,
int stride) {
int[][] blackPoints = new int[subHeight][subWidth];
for (int y = 0; y < subHeight; y++) {
for (int x = 0; x < subWidth; x++) {
int sum = 0;
int min = 255;
int max = 0;
for (int yy = 0; yy < 8; yy++) {
int offset = ((y << 3) + yy) * stride + (x << 3);
for (int xx = 0; xx < 8; xx++) {
int pixel = luminances[offset + xx] & 0xff;
sum += pixel;
if (pixel < min) {
min = pixel;
}
if (pixel > max) {
max = pixel;
}
}
}
// If the contrast is inadequate, use half the minimum, so that this block will be
// treated as part of the white background, but won't drag down neighboring blocks
// too much.
int average = (max - min > 24) ? (sum >> 6) : (min >> 1);
blackPoints[y][x] = average;
}
}
return blackPoints;
}
}
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