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Image Processing for Dummies with C# and GDI+ Part 2 - Convolution Filters

, 7 Nov 2005
The second in a series of articles which will build an image processing library in C# and GDI+.
using System;
using System.Drawing;
using System.Drawing.Imaging;

namespace CSharpFilters
{
	public class ConvMatrix
	{
		public int TopLeft = 0, TopMid = 0, TopRight = 0;
		public int MidLeft = 0, Pixel = 1, MidRight = 0;
		public int BottomLeft = 0, BottomMid = 0, BottomRight = 0;
		public int Factor = 1;
		public int Offset = 0;
		public void SetAll(int nVal)
		{
			TopLeft = TopMid = TopRight = MidLeft = Pixel = MidRight = BottomLeft = BottomMid = BottomRight = nVal;
		}
	}


	public class BitmapFilter
	{
		public static bool Invert(Bitmap b)
		{
			// GDI+ still lies to us - the return format is BGR, NOT RGB.
			BitmapData bmData = b.LockBits(new Rectangle(0, 0, b.Width, b.Height), ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb);

			int stride = bmData.Stride;
			System.IntPtr Scan0 = bmData.Scan0;

			unsafe
			{
				byte * p = (byte *)(void *)Scan0;

				int nOffset = stride - b.Width*3;
				int nWidth = b.Width * 3;
	
				for(int y=0;y<b.Height;++y)
				{
					for(int x=0; x < nWidth; ++x )
					{
						p[0] = (byte)(255-p[0]);
						++p;
					}
					p += nOffset;
				}
			}

			b.UnlockBits(bmData);

			return true;
		}

		public static bool GrayScale(Bitmap b)
		{
			// GDI+ still lies to us - the return format is BGR, NOT RGB.
			BitmapData bmData = b.LockBits(new Rectangle(0, 0, b.Width, b.Height), ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb);

			int stride = bmData.Stride;
			System.IntPtr Scan0 = bmData.Scan0;

			unsafe
			{
				byte * p = (byte *)(void *)Scan0;

				int nOffset = stride - b.Width*3;

				byte red, green, blue;
	
				for(int y=0;y<b.Height;++y)
				{
					for(int x=0; x < b.Width; ++x )
					{
						blue = p[0];
						green = p[1];
						red = p[2];

						p[0] = p[1] = p[2] = (byte)(.299 * red + .587 * green + .114 * blue);

						p += 3;
					}
					p += nOffset;
				}
			}

			b.UnlockBits(bmData);

			return true;
		}

		public static bool Brightness(Bitmap b, int nBrightness)
		{
			if (nBrightness < -255 || nBrightness > 255)
				return false;

			// GDI+ still lies to us - the return format is BGR, NOT RGB.
			BitmapData bmData = b.LockBits(new Rectangle(0, 0, b.Width, b.Height), ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb);

			int stride = bmData.Stride;
			System.IntPtr Scan0 = bmData.Scan0;

			int nVal = 0;

			unsafe
			{
				byte * p = (byte *)(void *)Scan0;

				int nOffset = stride - b.Width*3;
				int nWidth = b.Width * 3;

				for(int y=0;y<b.Height;++y)
				{
					for(int x=0; x < nWidth; ++x )
					{
						nVal = (int) (p[0] + nBrightness);
		
						if (nVal < 0) nVal = 0;
						if (nVal > 255) nVal = 255;

						p[0] = (byte)nVal;

						++p;
					}
					p += nOffset;
				}
			}

			b.UnlockBits(bmData);

			return true;
		}

		public static bool Contrast(Bitmap b, sbyte nContrast)
		{
			if (nContrast < -100) return false;
			if (nContrast >  100) return false;

			double pixel = 0, contrast = (100.0+nContrast)/100.0;

			contrast *= contrast;

			int red, green, blue;
			
			// GDI+ still lies to us - the return format is BGR, NOT RGB.
			BitmapData bmData = b.LockBits(new Rectangle(0, 0, b.Width, b.Height), ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb);

			int stride = bmData.Stride;
			System.IntPtr Scan0 = bmData.Scan0;

			unsafe
			{
				byte * p = (byte *)(void *)Scan0;

				int nOffset = stride - b.Width*3;

				for(int y=0;y<b.Height;++y)
				{
					for(int x=0; x < b.Width; ++x )
					{
						blue = p[0];
						green = p[1];
						red = p[2];
				
						pixel = red/255.0;
						pixel -= 0.5;
						pixel *= contrast;
						pixel += 0.5;
						pixel *= 255;
						if (pixel < 0) pixel = 0;
						if (pixel > 255) pixel = 255;
						p[2] = (byte) pixel;

						pixel = green/255.0;
						pixel -= 0.5;
						pixel *= contrast;
						pixel += 0.5;
						pixel *= 255;
						if (pixel < 0) pixel = 0;
						if (pixel > 255) pixel = 255;
						p[1] = (byte) pixel;

						pixel = blue/255.0;
						pixel -= 0.5;
						pixel *= contrast;
						pixel += 0.5;
						pixel *= 255;
						if (pixel < 0) pixel = 0;
						if (pixel > 255) pixel = 255;
						p[0] = (byte) pixel;					

						p += 3;
					}
					p += nOffset;
				}
			}

			b.UnlockBits(bmData);

			return true;
		}
	
		public static bool Gamma(Bitmap b, double red, double green, double blue)
		{
			if (red < .2 || red > 5) return false;
			if (green < .2 || green > 5) return false;
			if (blue < .2 || blue > 5) return false;

			byte [] redGamma = new byte [256];
			byte [] greenGamma = new byte [256];
			byte [] blueGamma = new byte [256];

			for (int i = 0; i< 256; ++i)
			{
				redGamma[i] = (byte)Math.Min(255, (int)(( 255.0 * Math.Pow(i/255.0, 1.0/red)) + 0.5));
				greenGamma[i] = (byte)Math.Min(255, (int)(( 255.0 * Math.Pow(i/255.0, 1.0/green)) + 0.5));
				blueGamma[i] = (byte)Math.Min(255, (int)(( 255.0 * Math.Pow(i/255.0, 1.0/blue)) + 0.5));
			}

			// GDI+ still lies to us - the return format is BGR, NOT RGB.
			BitmapData bmData = b.LockBits(new Rectangle(0, 0, b.Width, b.Height), ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb);

			int stride = bmData.Stride;
			System.IntPtr Scan0 = bmData.Scan0;

			unsafe
			{
				byte * p = (byte *)(void *)Scan0;

				int nOffset = stride - b.Width*3;

				for(int y=0;y<b.Height;++y)
				{
					for(int x=0; x < b.Width; ++x )
					{
						p[2] = redGamma[ p[2] ];
						p[1] = greenGamma[ p[1] ];
						p[0] = blueGamma[ p[0] ];

						p += 3;
					}
					p += nOffset;
				}
			}

			b.UnlockBits(bmData);

			return true;
		}

		public static bool Color(Bitmap b, int red, int green, int blue)
		{
			if (red < -255 || red > 255) return false;
			if (green < -255 || green > 255) return false;
			if (blue < -255 || blue > 255) return false;

			// GDI+ still lies to us - the return format is BGR, NOT RGB.
			BitmapData bmData = b.LockBits(new Rectangle(0, 0, b.Width, b.Height), ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb);

			int stride = bmData.Stride;
			System.IntPtr Scan0 = bmData.Scan0;

			unsafe
			{
				byte * p = (byte *)(void *)Scan0;

				int nOffset = stride - b.Width*3;
				int nPixel;

				for(int y=0;y<b.Height;++y)
				{
					for(int x=0; x < b.Width; ++x )
					{
						nPixel = p[2] + red;
						nPixel = Math.Max(nPixel, 0);
						p[2] = (byte)Math.Min(255, nPixel);

						nPixel = p[1] + green;
						nPixel = Math.Max(nPixel, 0);
						p[1] = (byte)Math.Min(255, nPixel);

						nPixel = p[0] + blue;
						nPixel = Math.Max(nPixel, 0);
						p[0] = (byte)Math.Min(255, nPixel);

						p += 3;
					}
					p += nOffset;
				}
			}

			b.UnlockBits(bmData);

			return true;
		}

		public static bool Conv3x3(Bitmap b, ConvMatrix m)
		{
			// Avoid divide by zero errors
			if (0 == m.Factor) return false;

			Bitmap bSrc = (Bitmap)b.Clone(); 

			// GDI+ still lies to us - the return format is BGR, NOT RGB.
			BitmapData bmData = b.LockBits(new Rectangle(0, 0, b.Width, b.Height), ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb);
			BitmapData bmSrc = bSrc.LockBits(new Rectangle(0, 0, bSrc.Width, bSrc.Height), ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb);

			int stride = bmData.Stride;
			int stride2 = stride * 2;
			System.IntPtr Scan0 = bmData.Scan0;
			System.IntPtr SrcScan0 = bmSrc.Scan0;

			unsafe
			{
				byte * p = (byte *)(void *)Scan0;
				byte * pSrc = (byte *)(void *)SrcScan0;

				int nOffset = stride + 6 - b.Width*3;
				int nWidth = b.Width - 2;
				int nHeight = b.Height - 2;

				int nPixel;

				for(int y=0;y < nHeight;++y)
				{
					for(int x=0; x < nWidth; ++x )
					{
						nPixel = ( ( ( (pSrc[2] * m.TopLeft) + (pSrc[5] * m.TopMid) + (pSrc[8] * m.TopRight) +
							(pSrc[2 + stride] * m.MidLeft) + (pSrc[5 + stride] * m.Pixel) + (pSrc[8 + stride] * m.MidRight) +
							(pSrc[2 + stride2] * m.BottomLeft) + (pSrc[5 + stride2] * m.BottomMid) + (pSrc[8 + stride2] * m.BottomRight)) / m.Factor) + m.Offset); 

						if (nPixel < 0) nPixel = 0;
						if (nPixel > 255) nPixel = 255;

						p[5 + stride]= (byte)nPixel;

						nPixel = ( ( ( (pSrc[1] * m.TopLeft) + (pSrc[4] * m.TopMid) + (pSrc[7] * m.TopRight) +
							(pSrc[1 + stride] * m.MidLeft) + (pSrc[4 + stride] * m.Pixel) + (pSrc[7 + stride] * m.MidRight) +
							(pSrc[1 + stride2] * m.BottomLeft) + (pSrc[4 + stride2] * m.BottomMid) + (pSrc[7 + stride2] * m.BottomRight)) / m.Factor) + m.Offset); 

						if (nPixel < 0) nPixel = 0;
						if (nPixel > 255) nPixel = 255;
							
						p[4 + stride] = (byte)nPixel;

						nPixel = ( ( ( (pSrc[0] * m.TopLeft) + (pSrc[3] * m.TopMid) + (pSrc[6] * m.TopRight) +
							(pSrc[0 + stride] * m.MidLeft) + (pSrc[3 + stride] * m.Pixel) + (pSrc[6 + stride] * m.MidRight) +
							(pSrc[0 + stride2] * m.BottomLeft) + (pSrc[3 + stride2] * m.BottomMid) + (pSrc[6 + stride2] * m.BottomRight)) / m.Factor) + m.Offset); 

						if (nPixel < 0) nPixel = 0;
						if (nPixel > 255) nPixel = 255;

						p[3 + stride] = (byte)nPixel;

						p += 3;
						pSrc += 3;
					}

					p += nOffset;
					pSrc += nOffset;
				}
			}

			b.UnlockBits(bmData);
			bSrc.UnlockBits(bmSrc);

			return true;
		}
		public static bool Smooth(Bitmap b, int nWeight /* default to 1 */)
		{
			ConvMatrix m = new ConvMatrix();
			m.SetAll(1);
			m.Pixel = nWeight;
			m.Factor = nWeight + 8;

			return  BitmapFilter.Conv3x3(b, m);
		}

		public static bool GaussianBlur(Bitmap b, int nWeight /* default to 4*/)
		{
			ConvMatrix m = new ConvMatrix();
			m.SetAll(1);
			m.Pixel = nWeight;
			m.TopMid = m.MidLeft = m.MidRight = m.BottomMid = 2;
			m.Factor = nWeight + 12;

			return  BitmapFilter.Conv3x3(b, m);
		}
		public static bool MeanRemoval(Bitmap b, int nWeight /* default to 9*/ )
		{
			ConvMatrix m = new ConvMatrix();
			m.SetAll(-1);
			m.Pixel = nWeight;
			m.Factor = nWeight - 8;

			return BitmapFilter.Conv3x3(b, m);
		}
		public static bool Sharpen(Bitmap b, int nWeight /* default to 11*/ )
		{
			ConvMatrix m = new ConvMatrix();
			m.SetAll(0);
			m.Pixel = nWeight;
			m.TopMid = m.MidLeft = m.MidRight = m.BottomMid = -2;
			m.Factor = nWeight - 8;

			return  BitmapFilter.Conv3x3(b, m);
		}
		public static bool EmbossLaplacian(Bitmap b)
		{
			ConvMatrix m = new ConvMatrix();
			m.SetAll(-1);
			m.TopMid = m.MidLeft = m.MidRight = m.BottomMid = 0;
			m.Pixel = 4;
			m.Offset = 127;

			return  BitmapFilter.Conv3x3(b, m);
		}	
		public static bool EdgeDetectQuick(Bitmap b)
		{
			ConvMatrix m = new ConvMatrix();
			m.TopLeft = m.TopMid = m.TopRight = -1;
			m.MidLeft = m.Pixel = m.MidRight = 0;
			m.BottomLeft = m.BottomMid = m.BottomRight = 1;
		
			m.Offset = 127;

			return  BitmapFilter.Conv3x3(b, m);
		}
	}
}

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This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

About the Author

Christian Graus
Software Developer (Senior)
Australia Australia
Programming computers ( self taught ) since about 1984 when I bought my first Apple ][. Was working on a GUI library to interface Win32 to Python, and writing graphics filters in my spare time, and then building n-tiered apps using asp, atl and asp.net in my job at Dytech. After 4 years there, I've started working from home, at first for Code Project and now for a vet telemedicine company. I owned part of a company that sells client education software in the vet market, but we sold that and I worked for the owners for five years before leaving to get away from the travel, and spend more time with my family. I now work for a company here in Hobart, doing all sorts of Microsoft based stuff in C++ and C#, with a lot of T-SQL in the mix.

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