mirror of https://github.com/SixLabors/ImageSharp
popow
8 years ago
4 changed files with 350 additions and 147 deletions
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// Copyright (c) Six Labors and contributors.
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// Licensed under the Apache License, Version 2.0.
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using System; |
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using System.Numerics; |
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using SixLabors.ImageSharp.Advanced; |
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using SixLabors.ImageSharp.Memory; |
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using SixLabors.ImageSharp.PixelFormats; |
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using SixLabors.Memory; |
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using SixLabors.Primitives; |
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namespace SixLabors.ImageSharp.Processing.Processors.Normalization |
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{ |
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/// <summary>
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/// Applies an adaptive histogram equalization to the image.
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/// </summary>
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/// <typeparam name="TPixel">The pixel format.</typeparam>
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internal class AdaptiveHistEqualizationSWProcessor<TPixel> : HistogramEqualizationProcessor<TPixel> |
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where TPixel : struct, IPixel<TPixel> |
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{ |
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/// <summary>
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/// Initializes a new instance of the <see cref="AdaptiveHistEqualizationSWProcessor{TPixel}"/> class.
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/// </summary>
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/// <param name="luminanceLevels">The number of different luminance levels. Typical values are 256 for 8-bit grayscale images
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/// or 65536 for 16-bit grayscale images.</param>
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/// <param name="clipHistogram">Indicating whether to clip the histogram bins at a specific value.</param>
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/// <param name="clipLimitPercentage">Histogram clip limit in percent of the total pixels in the grid. Histogram bins which exceed this limit, will be capped at this value.</param>
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/// <param name="gridSize">The grid size of the adaptive histogram equalization. Minimum value is 4.</param>
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public AdaptiveHistEqualizationSWProcessor(int luminanceLevels, bool clipHistogram, float clipLimitPercentage, int gridSize) |
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: base(luminanceLevels, clipHistogram, clipLimitPercentage) |
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{ |
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Guard.MustBeGreaterThanOrEqualTo(gridSize, 4, nameof(gridSize)); |
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this.GridSize = gridSize; |
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} |
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/// <summary>
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/// Gets the size of the grid for the adaptive histogram equalization.
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/// </summary>
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public int GridSize { get; } |
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/// <inheritdoc/>
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protected override void OnFrameApply(ImageFrame<TPixel> source, Rectangle sourceRectangle, Configuration configuration) |
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{ |
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MemoryAllocator memoryAllocator = configuration.MemoryAllocator; |
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int numberOfPixels = source.Width * source.Height; |
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Span<TPixel> pixels = source.GetPixelSpan(); |
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int pixelsInGrid = this.GridSize * this.GridSize; |
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int halfGridSize = this.GridSize / 2; |
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using (Buffer2D<TPixel> targetPixels = configuration.MemoryAllocator.Allocate2D<TPixel>(source.Width, source.Height)) |
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{ |
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ParallelFor.WithConfiguration( |
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0, |
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source.Width, |
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configuration, |
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x => |
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{ |
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using (System.Buffers.IMemoryOwner<int> histogramBuffer = memoryAllocator.Allocate<int>(this.LuminanceLevels, AllocationOptions.Clean)) |
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using (System.Buffers.IMemoryOwner<int> histogramBufferCopy = memoryAllocator.Allocate<int>(this.LuminanceLevels, AllocationOptions.Clean)) |
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using (System.Buffers.IMemoryOwner<int> cdfBuffer = memoryAllocator.Allocate<int>(this.LuminanceLevels, AllocationOptions.Clean)) |
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{ |
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Span<int> histogram = histogramBuffer.GetSpan(); |
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Span<int> histogramCopy = histogramBufferCopy.GetSpan(); |
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Span<int> cdf = cdfBuffer.GetSpan(); |
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int maxHistIdx = 0; |
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// Build the histogram of grayscale values for the current grid.
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for (int dy = -halfGridSize; dy < halfGridSize; dy++) |
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{ |
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Span<TPixel> rowSpan = this.GetPixelRow(source, (int)x - halfGridSize, dy, this.GridSize); |
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int maxIdx = this.AddPixelsToHistogram(rowSpan, histogram, this.LuminanceLevels); |
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if (maxIdx > maxHistIdx) |
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{ |
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maxHistIdx = maxIdx; |
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} |
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} |
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for (int y = 0; y < source.Height; y++) |
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{ |
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if (this.ClipHistogramEnabled) |
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{ |
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// Clipping the histogram, but doing it on a copy to keep the original un-clipped values for the next iteration.
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histogram.Slice(0, maxHistIdx).CopyTo(histogramCopy); |
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this.ClipHistogram(histogramCopy, this.ClipLimitPercentage, pixelsInGrid); |
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} |
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// Calculate the cumulative distribution function, which will map each input pixel in the current grid to a new value.
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int cdfMin = this.ClipHistogramEnabled ? this.CalculateCdf(cdf, histogramCopy, maxHistIdx) : this.CalculateCdf(cdf, histogram, maxHistIdx); |
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float numberOfPixelsMinusCdfMin = pixelsInGrid - cdfMin; |
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// Map the current pixel to the new equalized value
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int luminance = this.GetLuminance(source[x, y], this.LuminanceLevels); |
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float luminanceEqualized = cdf[luminance] / numberOfPixelsMinusCdfMin; |
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targetPixels[x, y].PackFromVector4(new Vector4(luminanceEqualized)); |
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// Remove top most row from the histogram, mirroring rows which exceeds the borders.
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Span<TPixel> rowSpan = this.GetPixelRow(source, x - halfGridSize, y - halfGridSize, this.GridSize); |
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maxHistIdx = this.RemovePixelsFromHistogram(rowSpan, histogram, this.LuminanceLevels, maxHistIdx); |
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// Add new bottom row to the histogram, mirroring rows which exceeds the borders.
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rowSpan = this.GetPixelRow(source, x - halfGridSize, y + halfGridSize, this.GridSize); |
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int maxIdx = this.AddPixelsToHistogram(rowSpan, histogram, this.LuminanceLevels); |
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if (maxIdx > maxHistIdx) |
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{ |
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maxHistIdx = maxIdx; |
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} |
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} |
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} |
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}); |
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Buffer2D<TPixel>.SwapOrCopyContent(source.PixelBuffer, targetPixels); |
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} |
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} |
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/// <summary>
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/// Get the a pixel row at a given position with a length of the grid size. Mirrors pixels which exceeds the edges.
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/// </summary>
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/// <param name="source">The source image.</param>
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/// <param name="x">The x position.</param>
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/// <param name="y">The y position.</param>
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/// <param name="gridSize">The grid size.</param>
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/// <returns>A pixel row of the length of the grid size.</returns>
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private Span<TPixel> GetPixelRow(ImageFrame<TPixel> source, int x, int y, int gridSize) |
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{ |
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if (y < 0) |
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{ |
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y = Math.Abs(y); |
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} |
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else if (y >= source.Height) |
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{ |
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int diff = y - source.Height; |
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y = source.Height - diff - 1; |
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} |
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// Special cases for the left and the right border where GetPixelRowSpan can not be used
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if (x < 0) |
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{ |
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var rowPixels = new TPixel[gridSize]; |
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int idx = 0; |
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for (int dx = x; dx < x + gridSize; dx++) |
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{ |
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rowPixels[idx] = source[Math.Abs(dx), y]; |
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idx++; |
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} |
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return rowPixels; |
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} |
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else if (x + gridSize > source.Width) |
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{ |
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var rowPixels = new TPixel[gridSize]; |
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int idx = 0; |
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for (int dx = x; dx < x + gridSize; dx++) |
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{ |
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if (dx >= source.Width) |
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{ |
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int diff = dx - source.Width; |
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rowPixels[idx] = source[dx - diff - 1, y]; |
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} |
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else |
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{ |
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rowPixels[idx] = source[dx, y]; |
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} |
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idx++; |
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} |
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return rowPixels; |
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} |
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return source.GetPixelRowSpan(y).Slice(start: x, length: gridSize); |
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} |
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/// <summary>
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/// Adds a row of grey values to the histogram.
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/// </summary>
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/// <param name="greyValues">The grey values to add</param>
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/// <param name="histogram">The histogram</param>
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/// <param name="luminanceLevels">The number of different luminance levels.</param>
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/// <returns>The maximum index where a value was changed.</returns>
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private int AddPixelsToHistogram(Span<TPixel> greyValues, Span<int> histogram, int luminanceLevels) |
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{ |
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int maxIdx = 0; |
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for (int idx = 0; idx < greyValues.Length; idx++) |
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{ |
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int luminance = this.GetLuminance(greyValues[idx], luminanceLevels); |
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histogram[luminance]++; |
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if (luminance > maxIdx) |
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{ |
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maxIdx = luminance; |
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} |
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} |
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return maxIdx; |
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} |
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/// <summary>
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/// Removes a row of grey values from the histogram.
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/// </summary>
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/// <param name="greyValues">The grey values to remove</param>
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/// <param name="histogram">The histogram</param>
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/// <param name="luminanceLevels">The number of different luminance levels.</param>
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/// <param name="maxHistIdx">The current maximum index of the histogram.</param>
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/// <returns>The (maybe changed) maximum index of the histogram.</returns>
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private int RemovePixelsFromHistogram(Span<TPixel> greyValues, Span<int> histogram, int luminanceLevels, int maxHistIdx) |
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{ |
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for (int idx = 0; idx < greyValues.Length; idx++) |
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{ |
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int luminance = this.GetLuminance(greyValues[idx], luminanceLevels); |
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histogram[luminance]--; |
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// If the histogram at the maximum index has changed to 0, search for the next smaller value.
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if (luminance == maxHistIdx && histogram[luminance] == 0) |
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{ |
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for (int j = luminance; j >= 0; j--) |
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{ |
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maxHistIdx = j; |
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if (histogram[j] != 0) |
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{ |
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break; |
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} |
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} |
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} |
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} |
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return maxHistIdx; |
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} |
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} |
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} |
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