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- /*
- * 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.
- */
-
- namespace ZXing.Common
- {
- /// <summary> This Binarizer implementation uses the old ZXing global histogram approach. It is suitable
- /// for low-end mobile devices which don't have enough CPU or memory to use a local thresholding
- /// algorithm. However, because it picks a global black point, it cannot handle difficult shadows
- /// and gradients.
- ///
- /// Faster mobile devices and all desktop applications should probably use HybridBinarizer instead.
- ///
- /// <author>dswitkin@google.com (Daniel Switkin)</author>
- /// <author>Sean Owen</author>
- /// </summary>
- public class GlobalHistogramBinarizer : Binarizer
- {
- private const int LUMINANCE_BITS = 5;
- private const int LUMINANCE_SHIFT = 8 - LUMINANCE_BITS;
- private const int LUMINANCE_BUCKETS = 1 << LUMINANCE_BITS;
- private static readonly byte[] EMPTY = new byte[0];
-
- private byte[] luminances;
- private readonly int[] buckets;
-
- /// <summary>
- /// Initializes a new instance of the <see cref="GlobalHistogramBinarizer"/> class.
- /// </summary>
- /// <param name="source">The source.</param>
- public GlobalHistogramBinarizer(LuminanceSource source)
- : base(source)
- {
- luminances = EMPTY;
- buckets = new int[LUMINANCE_BUCKETS];
- }
-
- /// <summary>
- /// Applies simple sharpening to the row data to improve performance of the 1D Readers.
- /// </summary>
- /// <param name="y"></param>
- /// <param name="row"></param>
- /// <returns></returns>
- public override BitArray getBlackRow(int y, BitArray row)
- {
- LuminanceSource source = LuminanceSource;
- int width = source.Width;
- if (row == null || row.Size < width)
- {
- row = new BitArray(width);
- }
- else
- {
- row.clear();
- }
-
- initArrays(width);
- byte[] localLuminances = source.getRow(y, luminances);
- int[] localBuckets = buckets;
- for (int x = 0; x < width; x++)
- {
- int pixel = localLuminances[x] & 0xff;
- localBuckets[pixel >> LUMINANCE_SHIFT]++;
- }
- int blackPoint;
- if (!estimateBlackPoint(localBuckets, out blackPoint))
- return null;
-
- int left = localLuminances[0] & 0xff;
- int center = localLuminances[1] & 0xff;
- for (int x = 1; x < width - 1; x++)
- {
- int right = localLuminances[x + 1] & 0xff;
- // A simple -1 4 -1 box filter with a weight of 2.
- int luminance = ((center << 2) - left - right) >> 1;
- row[x] = (luminance < blackPoint);
- left = center;
- center = right;
- }
- return row;
- }
-
- /// <summary>
- /// Does not sharpen the data, as this call is intended to only be used by 2D Readers.
- /// </summary>
- override public BitMatrix BlackMatrix
- {
- get
- {
- LuminanceSource source = LuminanceSource;
- byte[] localLuminances;
-
- int width = source.Width;
- int height = source.Height;
- BitMatrix matrix = new BitMatrix(width, height);
-
- // Quickly calculates the histogram by sampling four rows from the image. This proved to be
- // more robust on the blackbox tests than sampling a diagonal as we used to do.
- initArrays(width);
- int[] localBuckets = buckets;
- for (int y = 1; y < 5; y++)
- {
- int row = height * y / 5;
- localLuminances = source.getRow(row, luminances);
- int right = (width << 2) / 5;
- for (int x = width / 5; x < right; x++)
- {
- int pixel = localLuminances[x] & 0xff;
- localBuckets[pixel >> LUMINANCE_SHIFT]++;
- }
- }
- int blackPoint;
- if (!estimateBlackPoint(localBuckets, out blackPoint))
- return null;
-
- // We delay reading the entire image luminance until the black point estimation succeeds.
- // Although we end up reading four rows twice, it is consistent with our motto of
- // "fail quickly" which is necessary for continuous scanning.
- localLuminances = source.Matrix;
- for (int y = 0; y < height; y++)
- {
- int offset = y * width;
- for (int x = 0; x < width; x++)
- {
- int pixel = localLuminances[offset + x] & 0xff;
- matrix[x, y] = (pixel < blackPoint);
- }
- }
-
- return matrix;
- }
- }
-
- /// <summary>
- /// Creates a new object with the same type as this Binarizer implementation, but with pristine
- /// state. This is needed because Binarizer implementations may be stateful, e.g. keeping a cache
- /// of 1 bit data. See Effective Java for why we can't use Java's clone() method.
- /// </summary>
- /// <param name="source">The LuminanceSource this Binarizer will operate on.</param>
- /// <returns>
- /// A new concrete Binarizer implementation object.
- /// </returns>
- public override Binarizer createBinarizer(LuminanceSource source)
- {
- return new GlobalHistogramBinarizer(source);
- }
-
- private void initArrays(int luminanceSize)
- {
- if (luminances.Length < luminanceSize)
- {
- luminances = new byte[luminanceSize];
- }
- for (int x = 0; x < LUMINANCE_BUCKETS; x++)
- {
- buckets[x] = 0;
- }
- }
-
- private static bool estimateBlackPoint(int[] buckets, out int blackPoint)
- {
- blackPoint = 0;
- // Find the tallest peak in the histogram.
- int numBuckets = buckets.Length;
- int maxBucketCount = 0;
- int firstPeak = 0;
- int firstPeakSize = 0;
- for (int x = 0; x < numBuckets; x++)
- {
- if (buckets[x] > firstPeakSize)
- {
- firstPeak = x;
- firstPeakSize = buckets[x];
- }
- if (buckets[x] > maxBucketCount)
- {
- maxBucketCount = buckets[x];
- }
- }
-
- // Find the second-tallest peak which is somewhat far from the tallest peak.
- int secondPeak = 0;
- int secondPeakScore = 0;
- for (int x = 0; x < numBuckets; x++)
- {
- int distanceToBiggest = x - firstPeak;
- // Encourage more distant second peaks by multiplying by square of distance.
- int score = buckets[x] * distanceToBiggest * distanceToBiggest;
- if (score > secondPeakScore)
- {
- secondPeak = x;
- secondPeakScore = score;
- }
- }
-
- // Make sure firstPeak corresponds to the black peak.
- if (firstPeak > secondPeak)
- {
- int temp = firstPeak;
- firstPeak = secondPeak;
- secondPeak = temp;
- }
-
- // If there is too little contrast in the image to pick a meaningful black point, throw rather
- // than waste time trying to decode the image, and risk false positives.
- // TODO: It might be worth comparing the brightest and darkest pixels seen, rather than the
- // two peaks, to determine the contrast.
- if (secondPeak - firstPeak <= numBuckets >> 4)
- {
- return false;
- }
-
- // Find a valley between them that is low and closer to the white peak.
- int bestValley = secondPeak - 1;
- int bestValleyScore = -1;
- for (int x = secondPeak - 1; x > firstPeak; x--)
- {
- int fromFirst = x - firstPeak;
- int score = fromFirst*fromFirst*(secondPeak - x)*(maxBucketCount - buckets[x]);
- if (score > bestValleyScore)
- {
- bestValley = x;
- bestValleyScore = score;
- }
- }
-
- blackPoint = bestValley << LUMINANCE_SHIFT;
- return true;
- }
- }
- }
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