/*
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* Copyright 2009 ZXing authors
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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import GlobalHistogramBinarizer from './GlobalHistogramBinarizer';
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import BitMatrix from './BitMatrix';
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/**
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* This class implements a local thresholding algorithm, which while slower than the
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* GlobalHistogramBinarizer, is fairly efficient for what it does. It is designed for
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* high frequency images of barcodes with black data on white backgrounds. For this application,
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* it does a much better job than a global blackpoint with severe shadows and gradients.
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* However it tends to produce artifacts on lower frequency images and is therefore not
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* a good general purpose binarizer for uses outside ZXing.
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*
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* This class extends GlobalHistogramBinarizer, using the older histogram approach for 1D readers,
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* and the newer local approach for 2D readers. 1D decoding using a per-row histogram is already
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* inherently local, and only fails for horizontal gradients. We can revisit that problem later,
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* but for now it was not a win to use local blocks for 1D.
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*
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* This Binarizer is the default for the unit tests and the recommended class for library users.
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*
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* @author dswitkin@google.com (Daniel Switkin)
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*/
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export default class HybridBinarizer extends GlobalHistogramBinarizer {
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constructor(source) {
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super(source);
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this.matrix = null;
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}
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/**
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* Calculates the final BitMatrix once for all requests. This could be called once from the
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* constructor instead, but there are some advantages to doing it lazily, such as making
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* profiling easier, and not doing heavy lifting when callers don't expect it.
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*/
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/*@Override*/
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getBlackMatrix() {
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if (this.matrix !== null) {
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return this.matrix;
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}
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const source = this.getLuminanceSource();
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const width = source.getWidth();
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const height = source.getHeight();
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if (width >= HybridBinarizer.MINIMUM_DIMENSION && height >= HybridBinarizer.MINIMUM_DIMENSION) {
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const luminances = source.getMatrix();
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let subWidth = width >> HybridBinarizer.BLOCK_SIZE_POWER;
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if ((width & HybridBinarizer.BLOCK_SIZE_MASK) !== 0) {
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subWidth++;
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}
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let subHeight = height >> HybridBinarizer.BLOCK_SIZE_POWER;
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if ((height & HybridBinarizer.BLOCK_SIZE_MASK) !== 0) {
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subHeight++;
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}
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const blackPoints = HybridBinarizer.calculateBlackPoints(luminances, subWidth, subHeight, width, height);
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const newMatrix = new BitMatrix(width, height);
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HybridBinarizer.calculateThresholdForBlock(luminances, subWidth, subHeight, width, height, blackPoints, newMatrix);
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this.matrix = newMatrix;
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}
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else {
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// If the image is too small, fall back to the global histogram approach.
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this.matrix = super.getBlackMatrix();
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}
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return this.matrix;
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}
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/*@Override*/
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createBinarizer(source) {
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return new HybridBinarizer(source);
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}
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/**
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* For each block in the image, calculate the average black point using a 5x5 grid
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* of the blocks around it. Also handles the corner cases (fractional blocks are computed based
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* on the last pixels in the row/column which are also used in the previous block).
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*/
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static calculateThresholdForBlock(luminances, subWidth /*int*/, subHeight /*int*/, width /*int*/, height /*int*/, blackPoints, matrix) {
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const maxYOffset = height - HybridBinarizer.BLOCK_SIZE;
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const maxXOffset = width - HybridBinarizer.BLOCK_SIZE;
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for (let y = 0; y < subHeight; y++) {
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let yoffset = y << HybridBinarizer.BLOCK_SIZE_POWER;
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if (yoffset > maxYOffset) {
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yoffset = maxYOffset;
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}
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const top = HybridBinarizer.cap(y, 2, subHeight - 3);
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for (let x = 0; x < subWidth; x++) {
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let xoffset = x << HybridBinarizer.BLOCK_SIZE_POWER;
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if (xoffset > maxXOffset) {
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xoffset = maxXOffset;
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}
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const left = HybridBinarizer.cap(x, 2, subWidth - 3);
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let sum = 0;
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for (let z = -2; z <= 2; z++) {
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const blackRow = blackPoints[top + z];
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sum += blackRow[left - 2] + blackRow[left - 1] + blackRow[left] + blackRow[left + 1] + blackRow[left + 2];
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}
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const average = sum / 25;
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HybridBinarizer.thresholdBlock(luminances, xoffset, yoffset, average, width, matrix);
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}
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}
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}
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static cap(value /*int*/, min /*int*/, max /*int*/) {
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return value < min ? min : value > max ? max : value;
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}
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/**
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* Applies a single threshold to a block of pixels.
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*/
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static thresholdBlock(luminances, xoffset /*int*/, yoffset /*int*/, threshold /*int*/, stride /*int*/, matrix) {
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for (let y = 0, offset = yoffset * stride + xoffset; y < HybridBinarizer.BLOCK_SIZE; y++, offset += stride) {
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for (let x = 0; x < HybridBinarizer.BLOCK_SIZE; x++) {
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// Comparison needs to be <= so that black == 0 pixels are black even if the threshold is 0.
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if ((luminances[offset + x] & 0xFF) <= threshold) {
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matrix.set(xoffset + x, yoffset + y);
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}
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}
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}
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}
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/**
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* Calculates a single black point for each block of pixels and saves it away.
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* See the following thread for a discussion of this algorithm:
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* http://groups.google.com/group/zxing/browse_thread/thread/d06efa2c35a7ddc0
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*/
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static calculateBlackPoints(luminances, subWidth /*int*/, subHeight /*int*/, width /*int*/, height /*int*/) {
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const maxYOffset = height - HybridBinarizer.BLOCK_SIZE;
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const maxXOffset = width - HybridBinarizer.BLOCK_SIZE;
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// tslint:disable-next-line:whitespace
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const blackPoints = new Array(subHeight); // subWidth
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for (let y = 0; y < subHeight; y++) {
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blackPoints[y] = new Int32Array(subWidth);
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let yoffset = y << HybridBinarizer.BLOCK_SIZE_POWER;
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if (yoffset > maxYOffset) {
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yoffset = maxYOffset;
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}
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for (let x = 0; x < subWidth; x++) {
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let xoffset = x << HybridBinarizer.BLOCK_SIZE_POWER;
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if (xoffset > maxXOffset) {
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xoffset = maxXOffset;
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}
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let sum = 0;
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let min = 0xFF;
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let max = 0;
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for (let yy = 0, offset = yoffset * width + xoffset; yy < HybridBinarizer.BLOCK_SIZE; yy++, offset += width) {
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for (let xx = 0; xx < HybridBinarizer.BLOCK_SIZE; xx++) {
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const pixel = luminances[offset + xx] & 0xFF;
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sum += pixel;
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// still looking for good contrast
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if (pixel < min) {
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min = pixel;
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}
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if (pixel > max) {
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max = pixel;
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}
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}
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// short-circuit min/max tests once dynamic range is met
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if (max - min > HybridBinarizer.MIN_DYNAMIC_RANGE) {
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// finish the rest of the rows quickly
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for (yy++, offset += width; yy < HybridBinarizer.BLOCK_SIZE; yy++, offset += width) {
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for (let xx = 0; xx < HybridBinarizer.BLOCK_SIZE; xx++) {
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sum += luminances[offset + xx] & 0xFF;
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}
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}
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}
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}
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// The default estimate is the average of the values in the block.
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let average = sum >> (HybridBinarizer.BLOCK_SIZE_POWER * 2);
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if (max - min <= HybridBinarizer.MIN_DYNAMIC_RANGE) {
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// If variation within the block is low, assume this is a block with only light or only
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// dark pixels. In that case we do not want to use the average, as it would divide this
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// low contrast area into black and white pixels, essentially creating data out of noise.
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//
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// The default assumption is that the block is light/background. Since no estimate for
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// the level of dark pixels exists locally, use half the min for the block.
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average = min / 2;
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if (y > 0 && x > 0) {
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// Correct the "white background" assumption for blocks that have neighbors by comparing
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// the pixels in this block to the previously calculated black points. This is based on
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// the fact that dark barcode symbology is always surrounded by some amount of light
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// background for which reasonable black point estimates were made. The bp estimated at
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// the boundaries is used for the interior.
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// The (min < bp) is arbitrary but works better than other heuristics that were tried.
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const averageNeighborBlackPoint = (blackPoints[y - 1][x] + (2 * blackPoints[y][x - 1]) + blackPoints[y - 1][x - 1]) / 4;
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if (min < averageNeighborBlackPoint) {
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average = averageNeighborBlackPoint;
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}
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}
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}
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blackPoints[y][x] = average;
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}
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}
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return blackPoints;
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}
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}
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// This class uses 5x5 blocks to compute local luminance, where each block is 8x8 pixels.
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// So this is the smallest dimension in each axis we can accept.
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HybridBinarizer.BLOCK_SIZE_POWER = 3;
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HybridBinarizer.BLOCK_SIZE = 1 << HybridBinarizer.BLOCK_SIZE_POWER; // ...0100...00
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HybridBinarizer.BLOCK_SIZE_MASK = HybridBinarizer.BLOCK_SIZE - 1; // ...0011...11
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HybridBinarizer.MINIMUM_DIMENSION = HybridBinarizer.BLOCK_SIZE * 5;
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HybridBinarizer.MIN_DYNAMIC_RANGE = 24;
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