0  -1   0
-1   5  -1
 0  -1   0

public partial class FilterForm : Form
{
    public FilterForm()
    {
        InitializeComponent();

        Bitmap image = (Bitmap)Bitmap.FromFile("lena.jpg");
        InputPictureBox.Image = image;

        double[,] dImage = ToDouble2d(image);
        double[,] dMask = { { 0,-1, 0, },
                            { -1, 5, -1, },
                            { 0,-1, 0, }, };

        double[,]dConv = LinearConvolutionSpatial(dImage, dMask);

        Bitmap conv = ToBitmap2d(dConv, PixelFormat.Format32bppArgb);

        OutputPictureBox.Image = conv;
    }

    public double[,] ToDouble2d(Bitmap input)
    {
        int width = input.Width;
        int height = input.Height;

        double[,] array2d = new double[width, height];

        for (int y = 0; y < height; y++)
        {
            for (int x = 0; x < width; x++)
            {
                Color cl = input.GetPixel(x, y);

                double gray = ((cl.R * 0.3) + (cl.G * 0.59) + (cl.B * 0.11));

                array2d[x, y] = gray / 255.0;
            }
        }

        return array2d;
    }

    public Bitmap ToBitmap2d(double[,] image, PixelFormat pixelFormat)
    {
        int Width = image.GetLength(0);
        int Height = image.GetLength(1);

        Bitmap bmp = new Bitmap(Width, Height, pixelFormat);

        for (int y = 0; y < Height; y++)
        {
            for (int x = 0; x < Width; x++)
            {
                int i = (int)(image[x, y] * 255.0);

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

                Color clr = Color.FromArgb(i, i, i);

                bmp.SetPixel(x, y, clr);
            }
        }

        return bmp;
    }

    private double[,] LinearConvolutionSpatial(double[,] paddedImage, double[,] mask)
    {
        int paddedImageWidth = paddedImage.GetLength(0);
        int paddedImageHeight = paddedImage.GetLength(1);

        int maskWidth = mask.GetLength(0);
        int maskHeight = mask.GetLength(1);

        int imageWidth = paddedImageWidth - maskWidth;
        int imageHeight = paddedImageHeight - maskHeight;

        double[,] convolve = new double[imageWidth, imageHeight];

        for (int y = 0; y < imageHeight; y++)
        {
            for (int x = 0; x < imageWidth; x++)
            {
                double sum = Sum(paddedImage, mask, x, y);
                int xxx = x;
                int yyy = y;
                convolve[xxx, yyy] = sum;

                string str = string.Empty;
            }
        }

        Rescale(convolve);

        return convolve;
    }

    double Sum(double[,] paddedImage1, double[,] mask1, int startX, int startY)
    {
        double sum = 0;

        int maskWidth = mask1.GetLength(0);
        int maskHeight = mask1.GetLength(1);

        for (int y = startY; y < (startY + maskHeight); y++)
        {
            for (int x = startX; x < (startX + maskWidth); x++)
            {
                double img = paddedImage1[x, y];
                double msk = mask1[maskWidth - x + startX - 1, maskHeight - y + startY - 1];
                sum = sum + (img * msk);
            }
        }

        return sum;
    }

    void Rescale(double[,] convolve)
    {
        int imageWidth = convolve.GetLength(0);
        int imageHeight = convolve.GetLength(1);

        double minAmp = 0.0;
        double maxAmp = 0.0;

        for (int j = 0; j < imageHeight; j++)
        {
            for (int i = 0; i < imageWidth; i++)
            {
                minAmp = Math.Min(minAmp, convolve[i, j]);
                maxAmp = Math.Max(maxAmp, convolve[i, j]);
            }
        }

        double scale = 1 / (Math.Abs(minAmp) + maxAmp);

        for (int j = 0; j < imageHeight; j++)
        {
            for (int i = 0; i < imageWidth; i++)
            {
                double d = (convolve[i, j] + Math.Abs(minAmp)) * scale;
                convolve[i, j] = d;
            }
        }
    }
}