改变图片的对比度

        private void button1_Click(object sender, EventArgs e)
        {
            UpdatePicBoxEventHandle UpdatePicBox = new UpdatePicBoxEventHandle(SetContrast);

            Image img = pictureBox1.Image;
            for (double i = -255; i < 255; i++)
            {
                UpdatePicBox.BeginInvoke(img.Clone() as Bitmap, i, new AsyncCallback(UpdatePicBoxCallBack), UpdatePicBox);
            }
        }
        delegate Image UpdatePicBoxEventHandle(Image img, double contrast);
        public void UpdatePicBoxCallBack(IAsyncResult result)
        {
            if (result.IsCompleted)
            {
                UpdatePicBoxEventHandle updatepicbox = result.AsyncState as UpdatePicBoxEventHandle;
                using (Image img = updatepicbox.EndInvoke(result) as Image)
                {
                    //img.Save(Guid.NewGuid().ToString() + ".jpg");
                    pictureBox2.Image = new Bitmap(img);
                }
            }
        }
        //设置对比度
        public Image SetContrast(Image oImg, double contrast)
        {
            using (Bitmap tImg = oImg as Bitmap)
            {
                if (contrast < -255) contrast = -255;
                if (contrast > 255) contrast = 255;
                contrast = (255.0 + contrast) / 255.0;//对比度值
                contrast *= contrast;
                Color c;
                //遍历图像中的像素
                for (int i = 0; i < tImg.Width; i++)
                    for (int j = 0; j < tImg.Height; j++)
                    {
                        //获取像素点的颜色信息
                        c = tImg.GetPixel(i, j);
                        //红色分量值
                        double R = c.R / 255.0;
                        R -= 0.5;
                        R *= contrast;
                        R += 0.5;
                        R *= 255;
                        if (R < 0) R = 0;
                        if (R > 255) R = 255;
                        //绿色分量值
                        double G = c.G / 255.0;
                        G -= 0.5;
                        G *= contrast;
                        G += 0.5;
                        G *= 255;
                        if (G < 0) G = 0;
                        if (G > 255) G = 255;
                        //蓝色分量值
                        double B = c.B / 255.0;
                        B -= 0.5;
                        B *= contrast;
                        B += 0.5;
                        B *= 255;
                        if (B < 0) B = 0;
                        if (B > 255) B = 255;
                        //重新设置像素点的颜色
                        tImg.SetPixel(i, j, Color.FromArgb((byte)R, (byte)G, (byte)B));
                    }
                return tImg.Clone() as Image;
            }
        }