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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 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.
- ///
- /// </summary>
- /// <author> dswitkin@google.com (Daniel Switkin)
- /// </author>
- /// <author>www.Redivivus.in (suraj.supekar@redivivus.in) - Ported from ZXING Java Source
- /// </author>
- public sealed class HybridBinarizer : GlobalHistogramBinarizer
- {
- override public BitMatrix BlackMatrix
- {
- get
- {
- binarizeEntireImage();
- return matrix;
- }
- }
-
- // 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 const int BLOCK_SIZE_POWER = 3;
- private const int BLOCK_SIZE = 1 << BLOCK_SIZE_POWER; // ...0100...00
- private const int BLOCK_SIZE_MASK = BLOCK_SIZE - 1; // ...0011...11
- private const int MINIMUM_DIMENSION = 40;
- private const int MIN_DYNAMIC_RANGE = 24;
-
- private BitMatrix matrix = null;
-
- public HybridBinarizer(LuminanceSource source)
- : base(source)
- {
- }
-
- public override Binarizer createBinarizer(LuminanceSource source)
- {
- return new HybridBinarizer(source);
- }
-
- /// <summary>
- /// 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.
- /// </summary>
- private void binarizeEntireImage()
- {
- if (matrix == null)
- {
- LuminanceSource source = LuminanceSource;
- int width = source.Width;
- int height = source.Height;
- if (width >= MINIMUM_DIMENSION && height >= MINIMUM_DIMENSION)
- {
- byte[] luminances = source.Matrix;
-
- int subWidth = width >> BLOCK_SIZE_POWER;
- if ((width & BLOCK_SIZE_MASK) != 0)
- {
- subWidth++;
- }
- int subHeight = height >> BLOCK_SIZE_POWER;
- if ((height & BLOCK_SIZE_MASK) != 0)
- {
- subHeight++;
- }
- int[][] blackPoints = calculateBlackPoints(luminances, subWidth, subHeight, width, height);
-
- var newMatrix = new BitMatrix(width, height);
- calculateThresholdForBlock(luminances, subWidth, subHeight, width, height, blackPoints, newMatrix);
- matrix = newMatrix;
- }
- else
- {
- // If the image is too small, fall back to the global histogram approach.
- matrix = base.BlackMatrix;
- }
- }
- }
-
- /// <summary>
- /// 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 (fractional blocks are computed based
- /// on the last 8 pixels in the row/column which are also used in the previous block).
- /// </summary>
- /// <param name="luminances">The luminances.</param>
- /// <param name="subWidth">Width of the sub.</param>
- /// <param name="subHeight">Height of the sub.</param>
- /// <param name="width">The width.</param>
- /// <param name="height">The height.</param>
- /// <param name="blackPoints">The black points.</param>
- /// <param name="matrix">The matrix.</param>
- private static void calculateThresholdForBlock(byte[] luminances, int subWidth, int subHeight, int width, int height, int[][] blackPoints, BitMatrix matrix)
- {
- for (int y = 0; y < subHeight; y++)
- {
- int yoffset = y << BLOCK_SIZE_POWER;
- int maxYOffset = height - BLOCK_SIZE;
- if (yoffset > maxYOffset)
- {
- yoffset = maxYOffset;
- }
- for (int x = 0; x < subWidth; x++)
- {
- int xoffset = x << BLOCK_SIZE_POWER;
- int maxXOffset = width - BLOCK_SIZE;
- if (xoffset > maxXOffset)
- {
- xoffset = maxXOffset;
- }
- int left = cap(x, 2, subWidth - 3);
- int top = cap(y, 2, 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;
- thresholdBlock(luminances, xoffset, yoffset, average, width, matrix);
- }
- }
- }
-
- private static int cap(int value, int min, int max)
- {
- return value < min ? min : value > max ? max : value;
- }
-
- /// <summary>
- /// Applies a single threshold to an 8x8 block of pixels.
- /// </summary>
- /// <param name="luminances">The luminances.</param>
- /// <param name="xoffset">The xoffset.</param>
- /// <param name="yoffset">The yoffset.</param>
- /// <param name="threshold">The threshold.</param>
- /// <param name="stride">The stride.</param>
- /// <param name="matrix">The matrix.</param>
- private static void thresholdBlock(byte[] luminances, int xoffset, int yoffset, int threshold, int stride, BitMatrix matrix)
- {
- int offset = (yoffset * stride) + xoffset;
- for (int y = 0; y < BLOCK_SIZE; y++, offset += stride)
- {
- for (int x = 0; x < BLOCK_SIZE; x++)
- {
- int pixel = luminances[offset + x] & 0xff;
- // Comparison needs to be <= so that black == 0 pixels are black even if the threshold is 0.
- matrix[xoffset + x, yoffset + y] = (pixel <= threshold);
- }
- }
- }
-
- /// <summary>
- /// Calculates a single black point for each 8x8 block of pixels and saves it away.
- /// See the following thread for a discussion of this algorithm:
- /// http://groups.google.com/group/zxing/browse_thread/thread/d06efa2c35a7ddc0
- /// </summary>
- /// <param name="luminances">The luminances.</param>
- /// <param name="subWidth">Width of the sub.</param>
- /// <param name="subHeight">Height of the sub.</param>
- /// <param name="width">The width.</param>
- /// <param name="height">The height.</param>
- /// <returns></returns>
- private static int[][] calculateBlackPoints(byte[] luminances, int subWidth, int subHeight, int width, int height)
- {
- int[][] blackPoints = new int[subHeight][];
- for (int i = 0; i < subHeight; i++)
- {
- blackPoints[i] = new int[subWidth];
- }
-
- for (int y = 0; y < subHeight; y++)
- {
- int yoffset = y << BLOCK_SIZE_POWER;
- int maxYOffset = height - BLOCK_SIZE;
- if (yoffset > maxYOffset)
- {
- yoffset = maxYOffset;
- }
- for (int x = 0; x < subWidth; x++)
- {
- int xoffset = x << BLOCK_SIZE_POWER;
- int maxXOffset = width - BLOCK_SIZE;
- if (xoffset > maxXOffset)
- {
- xoffset = maxXOffset;
- }
- int sum = 0;
- int min = 0xFF;
- int max = 0;
- for (int yy = 0, offset = yoffset * width + xoffset; yy < BLOCK_SIZE; yy++, offset += width)
- {
- for (int xx = 0; xx < BLOCK_SIZE; xx++)
- {
- int pixel = luminances[offset + xx] & 0xFF;
- // still looking for good contrast
- sum += pixel;
- if (pixel < min)
- {
- min = pixel;
- }
- if (pixel > max)
- {
- max = pixel;
- }
- }
- // short-circuit min/max tests once dynamic range is met
- if (max - min > MIN_DYNAMIC_RANGE)
- {
- // finish the rest of the rows quickly
- for (yy++, offset += width; yy < BLOCK_SIZE; yy++, offset += width)
- {
- for (int xx = 0; xx < BLOCK_SIZE; xx++)
- {
- sum += luminances[offset + xx] & 0xFF;
- }
- }
- }
- }
-
- // The default estimate is the average of the values in the block.
- int average = sum >> (BLOCK_SIZE_POWER * 2);
- if (max - min <= MIN_DYNAMIC_RANGE)
- {
- // If variation within the block is low, assume this is a block with only light or only
- // dark pixels. In that case we do not want to use the average, as it would divide this
- // low contrast area into black and white pixels, essentially creating data out of noise.
- //
- // The default assumption is that the block is light/background. Since no estimate for
- // the level of dark pixels exists locally, use half the min for the block.
- average = min >> 1;
-
- if (y > 0 && x > 0)
- {
- // Correct the "white background" assumption for blocks that have neighbors by comparing
- // the pixels in this block to the previously calculated black points. This is based on
- // the fact that dark barcode symbology is always surrounded by some amount of light
- // background for which reasonable black point estimates were made. The bp estimated at
- // the boundaries is used for the interior.
-
- // The (min < bp) is arbitrary but works better than other heuristics that were tried.
- int averageNeighborBlackPoint = (blackPoints[y - 1][x] + (2 * blackPoints[y][x - 1]) +
- blackPoints[y - 1][x - 1]) >> 2;
- if (min < averageNeighborBlackPoint)
- {
- average = averageNeighborBlackPoint;
- }
- }
- }
- blackPoints[y][x] = average;
- }
- }
- return blackPoints;
- }
- }
- }
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