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Asked: 2026-05-11T18:07:12+00:00

I want to put a sharpen filter on an image. I have found a

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I want to put a sharpen filter on an image. I have found a web with short tutorial.
I tried to do it in C# so here is my code. Anyway, I tried to find out why it is not working. I do not know if I am doing something wrong, if yes, please advise me what to do to make it work as it should be. Thanks

        public static Bitmap sharpen(Bitmap image)
    {
        Bitmap sharpenImage = new Bitmap(image.Width, image.Height);

        int filterWidth = 3;
        int filterHeight = 3;
        int w = image.Width;
        int h = image.Height;

        double[,] filter = new double[filterWidth, filterHeight];

        filter[0, 0] = filter[0, 1] = filter[0, 2] = filter[1, 0] = filter[1, 2] = filter[2, 0] = filter[2, 1] = filter[2, 2] = -1;
        filter[1, 1] = 9;

        double factor = 1.0;
        double bias = 0.0;

        Color[,] result = new Color[image.Width, image.Height];

        for (int x = 0; x < w; ++x)
        {
            for (int y = 0; y < h; ++y)
            {
                double red = 0.0, green = 0.0, blue = 0.0;
                Color imageColor = image.GetPixel(x, y);

                for (int filterX = 0; filterX < filterWidth; filterX++)
                {
                    for (int filterY = 0; filterY < filterHeight; filterY++)
                    {
                        int imageX = (x - filterWidth / 2 + filterX + w) % w;
                        int imageY = (y - filterHeight / 2 + filterY + h) % h;
                        red += imageColor.R * filter[filterX, filterY];
                        green += imageColor.G * filter[filterX, filterY];
                        blue += imageColor.B * filter[filterX, filterY];
                    }
                    int r = Math.Min(Math.Max((int)(factor * red + bias), 0), 255);
                    int g = Math.Min(Math.Max((int)(factor * green + bias), 0), 255);
                    int b = Math.Min(Math.Max((int)(factor * blue + bias), 0), 255);

                    result[x, y] = Color.FromArgb(r, g, b);
                }
            }
        }
        for (int i = 0; i < w; ++i)
        {
            for (int j = 0; j < h; ++j)
            {
                sharpenImage.SetPixel(i, j, result[i, j]);
            }
        }
        return sharpenImage;
    }
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1 Answer

  1. Editorial Team
    public static Bitmap sharpen(Bitmap image)
{
    Bitmap sharpenImage = new Bitmap(image.Width, image.Height);

    int filterWidth = 3;
    int filterHeight = 3;
    int w = image.Width;
    int h = image.Height;

    double[,] filter = new double[filterWidth, filterHeight];

    filter[0, 0] = filter[0, 1] = filter[0, 2] = filter[1, 0] = filter[1, 2] = filter[2, 0] = filter[2, 1] = filter[2, 2] = -1;
    filter[1, 1] = 9;

    double factor = 1.0;
    double bias = 0.0;

    Color[,] result = new Color[image.Width, image.Height];

    for (int x = 0; x < w; ++x)
    {
        for (int y = 0; y < h; ++y)
        {
            double red = 0.0, green = 0.0, blue = 0.0;

//=====[REMOVE LINES]========================================================
// Color must be read per filter entry, not per image pixel.
            Color imageColor = image.GetPixel(x, y);
//===========================================================================

            for (int filterX = 0; filterX < filterWidth; filterX++)
            {
                for (int filterY = 0; filterY < filterHeight; filterY++)
                {
                    int imageX = (x - filterWidth / 2 + filterX + w) % w;
                    int imageY = (y - filterHeight / 2 + filterY + h) % h;

//=====[INSERT LINES]========================================================
// Get the color here - once per fiter entry and image pixel.
                    Color imageColor = image.GetPixel(imageX, imageY);
//===========================================================================

                    red += imageColor.R * filter[filterX, filterY];
                    green += imageColor.G * filter[filterX, filterY];
                    blue += imageColor.B * filter[filterX, filterY];
                }
                int r = Math.Min(Math.Max((int)(factor * red + bias), 0), 255);
                int g = Math.Min(Math.Max((int)(factor * green + bias), 0), 255);
                int b = Math.Min(Math.Max((int)(factor * blue + bias), 0), 255);

                result[x, y] = Color.FromArgb(r, g, b);
            }
        }
    }
    for (int i = 0; i < w; ++i)
    {
        for (int j = 0; j < h; ++j)
        {
            sharpenImage.SetPixel(i, j, result[i, j]);
        }
    }
    return sharpenImage;
}
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