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optimization: only calculating the cdf until the maximum histogram index

pull/673/head
popow 8 years ago
parent
e6011f05f2
commit
94d85db455
4 changed files with 66 additions and 28 deletions
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  1. 79
      src/ImageSharp/Processing/Processors/Normalization/AdaptiveHistEqualizationProcessor.cs
  2. 2
      src/ImageSharp/Processing/Processors/Normalization/GlobalHistogramEqualizationProcessor.cs
  3. 4
      src/ImageSharp/Processing/Processors/Normalization/HistogramEqualizationOptions.cs
  4. 9
      src/ImageSharp/Processing/Processors/Normalization/HistogramEqualizationProcessor.cs

79
src/ImageSharp/Processing/Processors/Normalization/AdaptiveHistEqualizationProcessor.cs
View File

@ -64,14 +64,17 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
Span<int> histogram = histogramBuffer.GetSpan();
Span<int> histogramCopy = histogramBufferCopy.GetSpan();
Span<int> cdf = cdfBuffer.GetSpan();
histogram.Clear();
int maxHistIdx = 0;
// Build the histogram of grayscale values for the current grid.
for (int dy = -halfGridSize; dy < halfGridSize; dy++)
{
Span<TPixel> rowSpan = this.GetPixelRow(source, x - halfGridSize, dy, this.GridSize);
this.AddPixelsTooHistogram(rowSpan, histogram, this.LuminanceLevels);
int maxIdx = this.AddPixelsTooHistogram(rowSpan, histogram, this.LuminanceLevels);
if (maxIdx > maxHistIdx)
{
maxHistIdx = maxIdx;
}
}
for (int y = 0; y < source.Height; y++)
@ -79,12 +82,12 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
if (this.ClipHistogramEnabled)
{
// Clipping the histogram, but doing it on a copy to keep the original un-clipped values for the next iteration.
histogram.CopyTo(histogramCopy);
histogram.Slice(0, maxHistIdx).CopyTo(histogramCopy);
this.ClipHistogram(histogramCopy, this.ClipLimitPercentage, pixelsInGrid);
}
// Calculate the cumulative distribution function, which will map each input pixel in the current grid to a new value.
int cdfMin = this.ClipHistogramEnabled ? this.CalculateCdf(cdf, histogramCopy) : this.CalculateCdf(cdf, histogram);
int cdfMin = this.ClipHistogramEnabled ? this.CalculateCdf(cdf, histogramCopy, maxHistIdx) : this.CalculateCdf(cdf, histogram, maxHistIdx);
float numberOfPixelsMinusCdfMin = pixelsInGrid - cdfMin;
// Map the current pixel to the new equalized value
@ -94,11 +97,15 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
// Remove top most row from the histogram, mirroring rows which exceeds the borders.
Span<TPixel> rowSpan = this.GetPixelRow(source, x - halfGridSize, y - halfGridSize, this.GridSize);
this.RemovePixelsFromHistogram(rowSpan, histogram, this.LuminanceLevels);
maxHistIdx = this.RemovePixelsFromHistogram(rowSpan, histogram, this.LuminanceLevels, maxHistIdx);
// Add new bottom row to the histogram, mirroring rows which exceeds the borders.
rowSpan = this.GetPixelRow(source, x - halfGridSize, y + halfGridSize, this.GridSize);
this.AddPixelsTooHistogram(rowSpan, histogram, this.LuminanceLevels);
int maxIdx = this.AddPixelsTooHistogram(rowSpan, histogram, this.LuminanceLevels);
if (maxIdx > maxHistIdx)
{
maxHistIdx = maxIdx;
}
}
}
});
@ -127,34 +134,39 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
y = source.Height - diff - 1;
}
// special cases for the left and the right border where GetPixelRowSpan can not be used
// Special cases for the left and the right border where GetPixelRowSpan can not be used
if (x < 0)
{
var rowPixels = new List<TPixel>();
var rowPixels = new TPixel[gridSize];
int idx = 0;
for (int dx = x; dx < x + gridSize; dx++)
{
rowPixels.Add(source[Math.Abs(dx), y]);
rowPixels[idx] = source[Math.Abs(dx), y];
idx++;
}
return rowPixels.ToArray();
return rowPixels;
}
else if (x + gridSize > source.Width)
{
var rowPixels = new List<TPixel>();
var rowPixels = new TPixel[gridSize];
int idx = 0;
for (int dx = x; dx < x + gridSize; dx++)
{
if (dx >= source.Width)
{
int diff = dx - source.Width;
rowPixels.Add(source[dx - diff - 1, y]);
rowPixels[idx] = source[dx - diff - 1, y];
}
else
{
rowPixels.Add(source[dx, y]);
rowPixels[idx] = source[dx, y];
}
idx++;
}
return rowPixels.ToArray();
return rowPixels;
}
return source.GetPixelRowSpan(y).Slice(start: x, length: gridSize);
@ -166,13 +178,21 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
/// <param name="greyValues">The grey values to add</param>
/// <param name="histogram">The histogram</param>
/// <param name="luminanceLevels">The number of different luminance levels.</param>
private void AddPixelsTooHistogram(Span<TPixel> greyValues, Span<int> histogram, int luminanceLevels)
/// <returns>The maximum index where a value was changed.</returns>
private int AddPixelsTooHistogram(Span<TPixel> greyValues, Span<int> histogram, int luminanceLevels)
{
for (int i = 0; i < greyValues.Length; i++)
int maxIdx = 0;
for (int idx = 0; idx < greyValues.Length; idx++)
{
int luminance = this.GetLuminance(greyValues[i], luminanceLevels);
int luminance = this.GetLuminance(greyValues[idx], luminanceLevels);
histogram[luminance]++;
if (luminance > maxIdx)
{
maxIdx = luminance;
}
}
return maxIdx;
}
/// <summary>
@ -181,13 +201,30 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
/// <param name="greyValues">The grey values to remove</param>
/// <param name="histogram">The histogram</param>
/// <param name="luminanceLevels">The number of different luminance levels.</param>
private void RemovePixelsFromHistogram(Span<TPixel> greyValues, Span<int> histogram, int luminanceLevels)
/// <param name="maxHistIdx">The current maximum index of the histogram.</param>
/// <returns>The (maybe changed) maximum index of the histogram.</returns>
private int RemovePixelsFromHistogram(Span<TPixel> greyValues, Span<int> histogram, int luminanceLevels, int maxHistIdx)
{
for (int i = 0; i < greyValues.Length; i++)
for (int idx = 0; idx < greyValues.Length; idx++)
{
int luminance = this.GetLuminance(greyValues[i], luminanceLevels);
int luminance = this.GetLuminance(greyValues[idx], luminanceLevels);
histogram[luminance]--;
// If the histogram at the maximum index has changed to 0, search for the next smaller value.
if (luminance == maxHistIdx && histogram[luminance] == 0)
{
for (int j = luminance; j >= 0; j--)
{
maxHistIdx = j;
if (histogram[j] != 0)
{
break;
}
}
}
}
return maxHistIdx;
}
}
}

2
src/ImageSharp/Processing/Processors/Normalization/GlobalHistogramEqualizationProcessor.cs
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@ -55,7 +55,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
// Calculate the cumulative distribution function, which will map each input pixel to a new value.
Span<int> cdf = cdfBuffer.GetSpan();
int cdfMin = this.CalculateCdf(cdf, histogram);
int cdfMin = this.CalculateCdf(cdf, histogram, histogram.Length - 1);
// Apply the cdf to each pixel of the image
float numberOfPixelsMinusCdfMin = numberOfPixels - cdfMin;

4
src/ImageSharp/Processing/Processors/Normalization/HistogramEqualizationOptions.cs
View File

@ -20,9 +20,9 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
public int LuminanceLevels { get; set; } = 256;
/// <summary>
/// Gets or sets a value indicating whether to clip the histogram bins at a specific value. Defaults to true.
/// Gets or sets a value indicating whether to clip the histogram bins at a specific value. Defaults to false.
/// </summary>
public bool ClipHistogram { get; set; } = true;
public bool ClipHistogram { get; set; } = false;
/// <summary>
/// Gets or sets the histogram clip limit in percent of the total pixels in the grid. Histogram bins which exceed this limit, will be capped at this value.

9
src/ImageSharp/Processing/Processors/Normalization/HistogramEqualizationProcessor.cs
View File

@ -50,12 +50,13 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
/// </summary>
/// <param name="cdf">The array holding the cdf.</param>
/// <param name="histogram">The histogram of the input image.</param>
/// <param name="maxIdx">Index of the maximum of the histogram.</param>
/// <returns>The first none zero value of the cdf.</returns>
protected int CalculateCdf(Span<int> cdf, Span<int> histogram)
protected int CalculateCdf(Span<int> cdf, Span<int> histogram, int maxIdx)
{
// Calculate the cumulative histogram
int histSum = 0;
for (int i = 0; i < histogram.Length; i++)
for (int i = 0; i <= maxIdx; i++)
{
histSum += histogram[i];
cdf[i] = histSum;
@ -63,7 +64,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
// Get the first none zero value of the cumulative histogram
int cdfMin = 0;
for (int i = 0; i < histogram.Length; i++)
for (int i = 0; i <= maxIdx; i++)
{
if (cdf[i] != 0)
{
@ -73,7 +74,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
}
// Creating the lookup table: subtracting cdf min, so we do not need to do that inside the for loop
for (int i = 0; i < histogram.Length; i++)
for (int i = 0; i <= maxIdx; i++)
{
cdf[i] = Math.Max(0, cdf[i] - cdfMin);
}

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