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NiblackThreshold.cs
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NiblackThreshold.cs
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// Accord Imaging Library
// The Accord.NET Framework
// http://accord-framework.net
//
// Copyright © Diego Catalano, 2013
// diego.catalano at live.com
//
// This library is free software; you can redistribute it and/or
// modify it under the terms of the GNU Lesser General Public
// License as published by the Free Software Foundation; either
// version 2.1 of the License, or (at your option) any later version.
//
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
// Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public
// License along with this library; if not, write to the Free Software
// Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
//
namespace Accord.Imaging.Filters
{
using System;
using System.Collections.Generic;
using System.Drawing.Imaging;
/// <summary>
/// Niblack Threshold.
/// </summary>
///
/// <remarks>
/// <para>
/// The Niblack filter is a local thresholding algorithm that separates
/// white and black pixels given the local mean and standard deviation
/// for the current window.</para>
///
/// <para>
/// This filter implementation has been contributed by Diego Catalano.</para>
///
/// <para>
/// References:
/// <list type="bullet">
/// <item><description>
/// W. Niblack, An Introduction to Digital Image Processing, pp. 115-116.
/// Prentice Hall, 1986.</description></item>
/// </list></para>
/// </remarks>
///
/// <example>
/// <code>
/// Bitmap image = ... // Lena's picture
///
/// // Create a new Niblack threshold:
/// var niblack = new NiblackThreshold();
///
/// // Compute the filter
/// Bitmap result = niblack.Apply(image);
///
/// // Show on the screen
/// ImageBox.Show(result);
/// </code>
///
/// <para>
/// The resulting image is shown below:</para>
///
/// <img src="..\images\niblack.png" />
///
/// </example>
///
/// <seealso cref="SauvolaThreshold"/>
///
public class NiblackThreshold : BaseFilter
{
private Dictionary<PixelFormat, PixelFormat> formatTranslations;
private int radius = 15;
private double k = 0.2D;
private double c = 0;
/// <summary>
/// Gets or sets the filter convolution
/// radius. Default is 15.
/// </summary>
///
public int Radius
{
get { return radius; }
set { radius = value; }
}
/// <summary>
/// Gets or sets the user-defined
/// parameter k. Default is 0.2.
/// </summary>
///
public double K
{
get { return k; }
set { k = value; }
}
/// <summary>
/// Gets or sets the mean offset C. This value should
/// be between 0 and 255. The default value is 0.
/// </summary>
///
public double C
{
get { return c; }
set { c = value; }
}
/// <summary>
/// Format translations dictionary.
/// </summary>
///
public override Dictionary<PixelFormat, PixelFormat> FormatTranslations
{
get { return formatTranslations; }
}
/// <summary>
/// Initializes a new instance of the <see cref="NiblackThreshold"/> class.
/// </summary>
///
public NiblackThreshold()
{
formatTranslations = new Dictionary<PixelFormat, PixelFormat>();
formatTranslations[PixelFormat.Format8bppIndexed] = PixelFormat.Format8bppIndexed;
formatTranslations[PixelFormat.Format24bppRgb] = PixelFormat.Format24bppRgb;
formatTranslations[PixelFormat.Format32bppRgb] = PixelFormat.Format32bppRgb;
formatTranslations[PixelFormat.Format32bppArgb] = PixelFormat.Format32bppArgb;
}
/// <summary>
/// Process the filter on the specified image.
/// </summary>
///
/// <param name="sourceData">Source image data.</param>
/// <param name="destinationData">Destination image data.</param>
///
protected override unsafe void ProcessFilter(UnmanagedImage sourceData, UnmanagedImage destinationData)
{
int width = sourceData.Width;
int height = sourceData.Height;
int size = radius * 2;
int pixelSize = System.Drawing.Image.GetPixelFormatSize(sourceData.PixelFormat) / 8;
int srcStride = sourceData.Stride;
int dstStride = destinationData.Stride;
int srcOffset = srcStride - width * pixelSize;
int dstOffset = dstStride - width * pixelSize;
byte* src = (byte*)sourceData.ImageData.ToPointer();
byte* dst = (byte*)destinationData.ImageData.ToPointer();
// do the processing job
if (sourceData.PixelFormat == PixelFormat.Format8bppIndexed)
{
// for each line
for (int y = 0; y < height; y++)
{
// for each pixel
for (int x = 0; x < width; x++, src++, dst++)
{
long sum = 0;
int count = 0;
for (int i = 0; i < size; i++)
{
int ir = i - radius;
int t = y + ir;
if (t < 0)
continue;
if (t >= height)
break;
for (int j = 0; j < size; j++)
{
int jr = j - radius;
t = x + jr;
if (t < 0)
continue;
if (t >= width)
continue;
sum += src[ir * srcStride + jr];
count++;
}
}
double mean = sum / (double)count;
double variance = 0;
for (int i = 0; i < size; i++)
{
int ir = i - radius;
int t = y + ir;
if (t < 0)
continue;
if (t >= height)
break;
for (int j = 0; j < size; j++)
{
int jr = j - radius;
t = x + jr;
if (t < 0)
continue;
if (t >= width)
continue;
byte val = src[ir * srcStride + jr];
variance += (val - mean) * (val - mean);
}
}
variance /= count - 1;
double cut = mean + k * Math.Sqrt(variance) - c;
*dst = (*src > cut) ? (byte)255 : (byte)0;
}
src += srcOffset;
dst += dstOffset;
}
}
else
{
// for each line
for (int y = 0; y < height; y++)
{
// for each pixel
for (int x = 0; x < width; x++, src += pixelSize, dst += pixelSize)
{
long sumR = 0;
long sumG = 0;
long sumB = 0;
int count = 0;
for (int i = 0; i < size; i++)
{
int ir = i - radius;
int t = y + ir;
if (t < 0)
continue;
if (t >= height)
break;
for (int j = 0; j < size; j++)
{
int jr = j - radius;
t = x + jr;
if (t < 0)
continue;
if (t >= width)
continue;
byte* p = &src[ir * srcStride + jr * pixelSize];
count++;
sumR += p[RGB.R];
sumG += p[RGB.G];
sumB += p[RGB.B];
}
}
double meanR = sumR / (double)count;
double meanG = sumG / (double)count;
double meanB = sumB / (double)count;
double varR = 0;
double varG = 0;
double varB = 0;
for (int i = 0; i < size; i++)
{
int ir = i - radius;
int t = y + ir;
if (t < 0)
continue;
if (t >= height)
break;
// for each kernel column
for (int j = 0; j < size; j++)
{
int jr = j - radius;
t = x + jr;
if (t < 0)
continue;
if (t >= width)
continue;
byte* p = &src[ir * srcStride + jr * pixelSize];
varR += (p[RGB.R] - meanR) * (p[RGB.R] - meanR);
varG += (p[RGB.G] - meanG) * (p[RGB.G] - meanG);
varB += (p[RGB.B] - meanB) * (p[RGB.B] - meanB);
}
}
varR /= count - 1;
varG /= count - 1;
varB /= count - 1;
double cutR = (meanR + k * Math.Sqrt(varR) - c);
double cutG = (meanG + k * Math.Sqrt(varG) - c);
double cutB = (meanB + k * Math.Sqrt(varB) - c);
dst[RGB.R] = (src[RGB.R] > cutR) ? (byte)255 : (byte)0;
dst[RGB.G] = (src[RGB.G] > cutG) ? (byte)255 : (byte)0;
dst[RGB.B] = (src[RGB.B] > cutB) ? (byte)255 : (byte)0;
// take care of alpha channel
if (pixelSize == 4)
dst[RGB.A] = src[RGB.A];
}
src += srcOffset;
dst += dstOffset;
}
}
}
}
}