265 lines
11 KiB
PHP
265 lines
11 KiB
PHP
<?php
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/*
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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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namespace Zxing\Common;
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use Zxing\Binarizer;
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use Zxing\LuminanceSource;
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use Zxing\NotFoundException;
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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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final class HybridBinarizer extends GlobalHistogramBinarizer
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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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private static $BLOCK_SIZE_POWER = 3;
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private static $BLOCK_SIZE = 8; // ...0100...00
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private static $BLOCK_SIZE_MASK = 7; // ...0011...11
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private static $MINIMUM_DIMENSION = 40;
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private static $MIN_DYNAMIC_RANGE = 24;
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private $matrix;
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public function __construct($source)
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{
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parent::__construct($source);
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self::$BLOCK_SIZE_POWER = 3;
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self::$BLOCK_SIZE = 1 << self::$BLOCK_SIZE_POWER; // ...0100...00
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self::$BLOCK_SIZE_MASK = self::$BLOCK_SIZE - 1; // ...0011...11
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self::$MINIMUM_DIMENSION = self::$BLOCK_SIZE * 5;
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self::$MIN_DYNAMIC_RANGE = 24;
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}
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/**
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* Calculates the final BitMatrix once for all requests. This could be called once from the
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* constructor instead, but there are some advantages to doing it lazily, such as making
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* profiling easier, and not doing heavy lifting when callers don't expect it.
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*/
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public function getBlackMatrix()
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{
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if ($this->matrix !== null) {
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return $this->matrix;
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}
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$source = $this->getLuminanceSource();
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$width = $source->getWidth();
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$height = $source->getHeight();
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if ($width >= self::$MINIMUM_DIMENSION && $height >= self::$MINIMUM_DIMENSION) {
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$luminances = $source->getMatrix();
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$subWidth = $width >> self::$BLOCK_SIZE_POWER;
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if (($width & self::$BLOCK_SIZE_MASK) != 0) {
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$subWidth++;
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}
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$subHeight = $height >> self::$BLOCK_SIZE_POWER;
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if (($height & self::$BLOCK_SIZE_MASK) != 0) {
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$subHeight++;
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}
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$blackPoints = self::calculateBlackPoints($luminances, $subWidth, $subHeight, $width, $height);
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$newMatrix = new BitMatrix($width, $height);
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self::calculateThresholdForBlock($luminances, $subWidth, $subHeight, $width, $height, $blackPoints, $newMatrix);
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$this->matrix = $newMatrix;
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} else {
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// If the image is too small, fall back to the global histogram approach.
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$this->matrix = parent::getBlackMatrix();
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}
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return $this->matrix;
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}
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/**
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* Calculates a single black point for each block of pixels and saves it away.
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* See the following thread for a discussion of this algorithm:
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* http://groups.google.com/group/zxing/browse_thread/thread/d06efa2c35a7ddc0
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*/
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private static function calculateBlackPoints(
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$luminances,
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$subWidth,
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$subHeight,
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$width,
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$height
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) {
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$blackPoints = fill_array(0, $subHeight, 0);
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foreach ($blackPoints as $key => $point) {
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$blackPoints[$key] = fill_array(0, $subWidth, 0);
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}
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for ($y = 0; $y < $subHeight; $y++) {
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$yoffset = ($y << self::$BLOCK_SIZE_POWER);
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$maxYOffset = $height - self::$BLOCK_SIZE;
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if ($yoffset > $maxYOffset) {
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$yoffset = $maxYOffset;
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}
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for ($x = 0; $x < $subWidth; $x++) {
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$xoffset = ($x << self::$BLOCK_SIZE_POWER);
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$maxXOffset = $width - self::$BLOCK_SIZE;
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if ($xoffset > $maxXOffset) {
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$xoffset = $maxXOffset;
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}
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$sum = 0;
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$min = 0xFF;
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$max = 0;
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for ($yy = 0, $offset = $yoffset * $width + $xoffset; $yy < self::$BLOCK_SIZE; $yy++, $offset += $width) {
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for ($xx = 0; $xx < self::$BLOCK_SIZE; $xx++) {
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$pixel = ((int)($luminances[(int)($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 - $min > self::$MIN_DYNAMIC_RANGE) {
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// finish the rest of the rows quickly
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for ($yy++, $offset += $width; $yy < self::$BLOCK_SIZE; $yy++, $offset += $width) {
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for ($xx = 0; $xx < self::$BLOCK_SIZE; $xx++) {
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$sum += ($luminances[$offset + $xx] & 0xFF);
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}
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}
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}
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}
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// The default estimate is the average of the values in the block.
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$average = ($sum >> (self::$BLOCK_SIZE_POWER * 2));
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if ($max - $min <= self::$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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$average = (int)($min / 2);
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if ($y > 0 && $x > 0) {
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// Correct the "white background" assumption for blocks that have neighbors by comparing
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// the pixels in this block to the previously calculated black points. This is based on
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// the fact that dark barcode symbology is always surrounded by some amount of light
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// background for which reasonable black point estimates were made. The bp estimated at
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// the boundaries is used for the interior.
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// The (min < bp) is arbitrary but works better than other heuristics that were tried.
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$averageNeighborBlackPoint =
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(int)(($blackPoints[$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] = (int)($average);
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}
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}
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return $blackPoints;
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}
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/**
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* For each block in the image, calculate the average black point using a 5x5 grid
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* of the blocks around it. Also handles the corner cases (fractional blocks are computed based
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* on the last pixels in the row/column which are also used in the previous block).
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*/
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private static function calculateThresholdForBlock(
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$luminances,
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$subWidth,
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$subHeight,
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$width,
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$height,
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$blackPoints,
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$matrix
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) {
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for ($y = 0; $y < $subHeight; $y++) {
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$yoffset = ($y << self::$BLOCK_SIZE_POWER);
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$maxYOffset = $height - self::$BLOCK_SIZE;
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if ($yoffset > $maxYOffset) {
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$yoffset = $maxYOffset;
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}
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for ($x = 0; $x < $subWidth; $x++) {
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$xoffset = ($x << self::$BLOCK_SIZE_POWER);
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$maxXOffset = $width - self::$BLOCK_SIZE;
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if ($xoffset > $maxXOffset) {
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$xoffset = $maxXOffset;
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}
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$left = self::cap($x, 2, $subWidth - 3);
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$top = self::cap($y, 2, $subHeight - 3);
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$sum = 0;
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for ($z = -2; $z <= 2; $z++) {
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$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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$average = (int)($sum / 25);
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self::thresholdBlock($luminances, $xoffset, $yoffset, $average, $width, $matrix);
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}
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}
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}
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private static function cap($value, $min, $max)
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{
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if ($value < $min) {
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return $min;
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} elseif ($value > $max) {
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return $max;
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} else {
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return $value;
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}
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}
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/**
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* Applies a single threshold to a block of pixels.
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*/
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private static function thresholdBlock(
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$luminances,
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$xoffset,
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$yoffset,
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$threshold,
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$stride,
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$matrix
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) {
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for ($y = 0, $offset = $yoffset * $stride + $xoffset; $y < self::$BLOCK_SIZE; $y++, $offset += $stride) {
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for ($x = 0; $x < self::$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 + $y);
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}
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}
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}
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}
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public function createBinarizer($source)
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{
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return new HybridBinarizer($source);
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}
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}
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