c# - How to repair dimmed image of book page

Question

I have 1000 Image of book page, Its text is dimmed as this piece
Now I tried to repair it to be more clearly to read, I use this code

 private Bitmap repairImage(Bitmap bmp)
    {
        Color cc=Color.FromArgb(255, 251, 251, 251);
        for (int x = 0; x < bmp.Width; x++)
        {
            for (int y = 0; y < bmp.Height; y++)
            {
                if (bmp.GetPixel(x, y).R>238)
                {
                    bmp.SetPixel(x, y, Color.White);
                }
                else
                {
                    bmp.SetPixel(x, y, Color.Black);
                }
            }
        }
       return bmp;
    }

Due to the image dimensions is 1168 x 1807 it took a lot of time to finish repair, it exactly loops 2110576 cycles.
Is there any another way to solve this problem? Thanks.

Answer 1

The best way I can think of it to use the built-in ColorMatrix class to change the Gamma and the Contrast of the image.

Here is a result for a Gamma = 6.27 and a Contrast = +1.04:

Here is the code I used:

using System.Drawing.Imaging;
..

public static Bitmap ApplyGamma(Bitmap bmp0, float gamma, float contrast)
{

    Bitmap bmp1 = new Bitmap(bmp0.Width, bmp0.Height);
    using (Graphics g = Graphics.FromImage(bmp1))
    {
        ColorMatrix colorMatrix = new ColorMatrix(new float[][] 
                {
                    new float[] {contrast, 0, 0, 0, 0},
                    new float[] {0,contrast, 0, 0, 0},
                    new float[] {0, 0, contrast, 0, 0},
                    new float[] {0, 0, 0, 1, 0},
                    new float[] {0, 0, 0, 0, 1}
                });


        ImageAttributes attributes = new ImageAttributes();
        attributes.SetColorMatrix(colorMatrix, ColorMatrixFlag.Default,
                                               ColorAdjustType.Bitmap);
        attributes.SetGamma(gamma, ColorAdjustType.Bitmap);
        g.DrawImage(bmp0, new Rectangle(0, 0, bmp0.Width, bmp0.Height),
                    0, 0, bmp0.Width, bmp0.Height, GraphicsUnit.Pixel, attributes);
    }
    return bmp1;
}

The function uses two variables and two TrackBars along with two Labels:

float gamma = 1f ;
float contrast = 1f;

private void trackBar1_Scroll(object sender, EventArgs e)
{
    gamma = 1f * trackBar1.Value / 100f;
    label1.Text = gamma.ToString("#0.00");
    pictureBox1.Image = ApplyGamma(originalImage, gamma, contrast);
}


private void trackBar2_Scroll(object sender, EventArgs e)
{
    contrast = 1f * trackBar2.Value / 1000f;
    label2.Text = contrast.ToString("#0.00");
    pictureBox1.Image = ApplyGamma(originalImage, gamma, contrast);
}

Note that I am leaking the Bitmaps; it is just for testing ;-)