【C#/WPF】调节图像的对比度（Contrast）

关于对比度：
调节对比度直观感受是，高对比度的图像明暗关系更明显，色彩更鲜艳；低对比度的图像表面像是蒙上一层灰，色彩不鲜艳。

需求：
制作一个面板，一个滑动条，拖动滑动条可以修改目标图片的对比度。

资料参考：

• https://softwarebydefault.com/2013/04/20/image-contrast/

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界面滑动条两端的值是-30~30，默认处于中间位置0。已知目标图像的Bitmap数据。

1. 修改Bitmap的对比度。
2. 将修改之后的Bitmap重新赋值给界面Image控件显示。

        /// <summary>
        /// 调节对比度
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void Contrast_ValueChanged(object sender, RoutedPropertyChangedEventArgs<double> e)
        {
            // 滑动条是值是-30~30
            // originalBitmap是目标图像的Bitmap
            int threshold = (int)e.NewValue;
            Bitmap newBitmap = BitmapHelper.Contrast(originalBitmap, threshold);

            // 重新给Image控件赋值新图像
            image.Source = SystemUtils.ConvertBitmapToBitmapImage(newBitmap);
        }

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下面调节图像对比度的工具方法：

    
    /// <summary>
    /// 代码来自：https://softwarebydefault.com/2013/04/20/image-contrast/
    /// </summary>
    public static class BitmapHelper
    {
        /// <summary>
        /// 调节图像的对比度
        /// </summary>
        /// <param name="sourceBitmap"></param>
        /// <param name="threshold">阈值，通过该参数控制调节</param>
        /// <returns></returns>
        public static Bitmap Contrast(this Bitmap sourceBitmap, int threshold)
        {
            BitmapData sourceData = sourceBitmap.LockBits(new Rectangle(0, 0,
                                        sourceBitmap.Width, sourceBitmap.Height),
                                        ImageLockMode.ReadOnly, System.Drawing.Imaging.PixelFormat.Format32bppArgb);

            byte[] pixelBuffer = new byte[sourceData.Stride * sourceData.Height];

            Marshal.Copy(sourceData.Scan0, pixelBuffer, 0, pixelBuffer.Length);

            sourceBitmap.UnlockBits(sourceData);

            double contrastLevel = Math.Pow((100.0 + threshold) / 100.0, 2);

            double blue = 0;
            double green = 0;
            double red = 0;

            for (int k = 0; k + 4 < pixelBuffer.Length; k += 4)
            {
                blue = ((((pixelBuffer[k] / 255.0) - 0.5) *
                            contrastLevel) + 0.5) * 255.0;

                green = ((((pixelBuffer[k + 1] / 255.0) - 0.5) *
                            contrastLevel) + 0.5) * 255.0;

                red = ((((pixelBuffer[k + 2] / 255.0) - 0.5) *
                            contrastLevel) + 0.5) * 255.0;

                if (blue > 255)
                { blue = 255; }
                else if (blue < 0)
                { blue = 0; }

                if (green > 255)
                { green = 255; }
                else if (green < 0)
                { green = 0; }

                if (red > 255)
                { red = 255; }
                else if (red < 0)
                { red = 0; }

                pixelBuffer[k] = (byte)blue;
                pixelBuffer[k + 1] = (byte)green;
                pixelBuffer[k + 2] = (byte)red;
            }

            Bitmap resultBitmap = new Bitmap(sourceBitmap.Width, sourceBitmap.Height);

            BitmapData resultData = resultBitmap.LockBits(new Rectangle(0, 0,
                                        resultBitmap.Width, resultBitmap.Height),
                                        ImageLockMode.WriteOnly, PixelFormat.Format32bppArgb);

            Marshal.Copy(pixelBuffer, 0, resultData.Scan0, pixelBuffer.Length);
            resultBitmap.UnlockBits(resultData);

            return resultBitmap;
        }
    }

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下面是将Bitmap转换为BitmapImage的工具方法，以供WPF的Image控件使用图像：

    public static class SystemUtils
    {
        /// <summary>
        /// 转换类型：Bitmap --> BitmapImage
        /// <summary>
        /// <returns></returns>
        public static BitmapImage ConvertBitmapToBitmapImage(Bitmap bitmap)
        {
            using (MemoryStream stream = new MemoryStream())
            {
                bitmap.Save(stream, ImageFormat.Png);

                stream.Position = 0;
                BitmapImage bi = new BitmapImage();
                bi.BeginInit();
                // According to MSDN, "The default OnDemand cache option retains access to the stream until the image is needed."
                // Force the bitmap to load right now so we can dispose the stream.
                bi.CacheOption = BitmapCacheOption.OnLoad;
                bi.StreamSource = stream;
                bi.EndInit();
                bi.Freeze();

                return bi;
            }
        }
    }

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测试效果如下：

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另外，关于图像的HSL（色相、饱和度、明度）的调节，可参考在下的另一篇博文：

• http://www.cnblogs.com/guxin/p/csharp-wpf-adjust-image-hue-saturation-lightness-by-imagemagick.html

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参考资料：

https://softwarebydefault.com/2013/04/20/image-contrast/