brightness-nizer

private void button4_Click(object sender, EventArgs e) {
    objBitmap4 = new Bitmap(objBitmap1);
    int a = Convert.ToInt16(textBox1.Text);

    for (int x = 0; x < objBitmap1.Width; x++)
    {
        for (int y = 0; y < objBitmap1.Height; y++)
        {
            Color w = objBitmap1.GetPixel(x, y);
            int xg = (int)((w.R + w.G + w.B) / 3);
            int xb = xg + a;

            xb = xb > 255 ? 255 : xb;
            xb = xb < 0 ? 0 : xb;

            Color new_w = Color.FromArgb(xb, xb, xb);
            objBitmap4.SetPixel(x, y, new_w);
        }
    }
    pictureBox4.Image = objBitmap4;
}