/*
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* Copyright 2012 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 "ZXByteArray.h"
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#import "ZXHybridBinarizer.h"
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#import "ZXIntArray.h"
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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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const int ZX_BLOCK_SIZE_POWER = 3;
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const int ZX_BLOCK_SIZE = 1 << ZX_BLOCK_SIZE_POWER; // ...0100...00
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const int ZX_BLOCK_SIZE_MASK = ZX_BLOCK_SIZE - 1; // ...0011...11
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const int ZX_MINIMUM_DIMENSION = ZX_BLOCK_SIZE * 5;
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const int ZX_MIN_DYNAMIC_RANGE = 24;
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@interface ZXHybridBinarizer ()
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@property (nonatomic, strong) ZXBitMatrix *matrix;
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@end
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@implementation ZXHybridBinarizer
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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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- (ZXBitMatrix *)blackMatrixWithError:(NSError **)error {
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if (self.matrix != nil) {
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return self.matrix;
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}
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ZXLuminanceSource *source = [self luminanceSource];
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int width = source.width;
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int height = source.height;
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if (width >= ZX_MINIMUM_DIMENSION && height >= ZX_MINIMUM_DIMENSION) {
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ZXByteArray *luminances = source.matrix;
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int subWidth = width >> ZX_BLOCK_SIZE_POWER;
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if ((width & ZX_BLOCK_SIZE_MASK) != 0) {
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subWidth++;
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}
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int subHeight = height >> ZX_BLOCK_SIZE_POWER;
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if ((height & ZX_BLOCK_SIZE_MASK) != 0) {
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subHeight++;
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}
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int **blackPoints = [self calculateBlackPoints:luminances.array subWidth:subWidth subHeight:subHeight width:width height:height];
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ZXBitMatrix *newMatrix = [[ZXBitMatrix alloc] initWithWidth:width height:height];
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[self calculateThresholdForBlock:luminances.array subWidth:subWidth subHeight:subHeight width:width height:height blackPoints:blackPoints matrix:newMatrix];
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self.matrix = newMatrix;
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for (int i = 0; i < subHeight; i++) {
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free(blackPoints[i]);
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}
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free(blackPoints);
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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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self.matrix = [super blackMatrixWithError:error];
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}
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return self.matrix;
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}
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- (ZXBinarizer *)createBinarizer:(ZXLuminanceSource *)source {
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return [[ZXHybridBinarizer alloc] initWithSource: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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- (void)calculateThresholdForBlock:(int8_t *)luminances
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subWidth:(int)subWidth
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subHeight:(int)subHeight
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width:(int)width
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height:(int)height
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blackPoints:(int **)blackPoints
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matrix:(ZXBitMatrix *)matrix {
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for (int y = 0; y < subHeight; y++) {
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int yoffset = y << ZX_BLOCK_SIZE_POWER;
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int maxYOffset = height - ZX_BLOCK_SIZE;
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if (yoffset > maxYOffset) {
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yoffset = maxYOffset;
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}
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for (int x = 0; x < subWidth; x++) {
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int xoffset = x << ZX_BLOCK_SIZE_POWER;
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int maxXOffset = width - ZX_BLOCK_SIZE;
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if (xoffset > maxXOffset) {
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xoffset = maxXOffset;
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}
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int left = [self cap:x min:2 max:subWidth - 3];
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int top = [self cap:y min:2 max:subHeight - 3];
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int sum = 0;
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for (int z = -2; z <= 2; z++) {
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int *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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int average = sum / 25;
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[self thresholdBlock:luminances xoffset:xoffset yoffset:yoffset threshold:average stride:width matrix:matrix];
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}
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}
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}
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- (int)cap:(int)value min:(int)min max:(int)max {
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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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- (void)thresholdBlock:(int8_t *)luminances
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xoffset:(int)xoffset
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yoffset:(int)yoffset
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threshold:(int)threshold
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stride:(int)stride
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matrix:(ZXBitMatrix *)matrix {
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for (int y = 0, offset = yoffset * stride + xoffset; y < ZX_BLOCK_SIZE; y++, offset += stride) {
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for (int x = 0; x < ZX_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 setX:xoffset + x y: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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- (int **)calculateBlackPoints:(int8_t *)luminances
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subWidth:(int)subWidth
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subHeight:(int)subHeight
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width:(int)width
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height:(int)height {
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int **blackPoints = (int **)malloc(subHeight * sizeof(int *));
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for (int y = 0; y < subHeight; y++) {
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blackPoints[y] = (int *)malloc(subWidth * sizeof(int));
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int yoffset = y << ZX_BLOCK_SIZE_POWER;
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int maxYOffset = height - ZX_BLOCK_SIZE;
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if (yoffset > maxYOffset) {
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yoffset = maxYOffset;
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}
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for (int x = 0; x < subWidth; x++) {
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int xoffset = x << ZX_BLOCK_SIZE_POWER;
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int maxXOffset = width - ZX_BLOCK_SIZE;
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if (xoffset > maxXOffset) {
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xoffset = maxXOffset;
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}
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int sum = 0;
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int min = 0xFF;
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int max = 0;
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for (int yy = 0, offset = yoffset * width + xoffset; yy < ZX_BLOCK_SIZE; yy++, offset += width) {
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for (int xx = 0; xx < ZX_BLOCK_SIZE; xx++) {
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int 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 > ZX_MIN_DYNAMIC_RANGE) {
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// finish the rest of the rows quickly
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for (yy++, offset += width; yy < ZX_BLOCK_SIZE; yy++, offset += width) {
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for (int xx = 0; xx < ZX_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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int average = sum >> (ZX_BLOCK_SIZE_POWER * 2);
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if (max - min <= ZX_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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int averageNeighborBlackPoint =
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(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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@end
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