c# 图像转化成灰度图

 /// <summary>
        /// 将源图像灰度化，但是没有转化为8位灰度图像。
        /// http://www.bobpowell.net/grayscale.htm
        /// </summary>
        /// <param name="original"> 源图像。 </param>
        /// <returns> 灰度RGB图像。 </returns>
        public static Bitmap MakeGrayScale(Bitmap original)
        {
            //create a blank bitmap the same size as original
            Bitmap newBitmap = new Bitmap(original.Width, original.Height, PixelFormat.Format24bppRgb);

            //get a graphics object from the new image
            Graphics g = Graphics.FromImage(newBitmap);

            //create the grayscale ColorMatrix
            ColorMatrix colorMatrix = new ColorMatrix(
                new float[][] 
        {
            new float[] { .3f, .3f, .3f, 0, 0 },
            new float[] { .59f, .59f, .59f, 0, 0 },
            new float[] { .11f, .11f, .11f, 0, 0 },
            new float[] { 0, 0, 0, 1, 0 },
            new float[] { 0, 0, 0, 0, 1 }
        });
            /* * * * * * * * * * * * * * * * * * * * * * * * * * * * *
            ┌                          ┐
            │  0.3   0.3   0.3   0   0 │
            │ 0.59  0.59  0.59   0   0 │
            │ 0.11  0.11  0.11   0   0 │
            │    0     0     0   1   0 │
            │    0     0     0   0   1 │
            └                          ┘
             * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
            //create some image attributes
            ImageAttributes attributes = new ImageAttributes();

            //set the color matrix attribute
            attributes.SetColorMatrix(colorMatrix);

            //draw the original image on the new image
            //using the grayscale color matrix
            g.DrawImage(original, new Rectangle(0, 0, original.Width, original.Height),
               0, 0, original.Width, original.Height, GraphicsUnit.Pixel, attributes);

            //dispose the Graphics object
            g.Dispose();
            return newBitmap;
        }






        /// <summary>
/// 将源图像灰度化，并转化为8位灰度图像。
/// </summary>
/// <param name="original"> 源图像。 </param>
/// <returns> 8位灰度图像。 </returns>
public static Bitmap RgbToGrayScale(Bitmap original)
{
     if (original != null)
     {
          // 将源图像内存区域锁定
          Rectangle rect = new Rectangle(0, 0, original.Width, original.Height);
          BitmapData bmpData = original.LockBits(rect, ImageLockMode.ReadOnly,
               original.PixelFormat);

          // 获取图像参数
          int width = bmpData.Width;
          int height = bmpData.Height;
          int stride = bmpData.Stride;  // 扫描线的宽度
          int offset = stride - width * 3;  // 显示宽度与扫描线宽度的间隙
          IntPtr ptr = bmpData.Scan0;   // 获取bmpData的内存起始位置
          int scanBytes = stride * height;  // 用stride宽度，表示这是内存区域的大小

          // 分别设置两个位置指针，指向源数组和目标数组
          int posScan = 0, posDst = 0;
          byte[] rgbValues = new byte[scanBytes];  // 为目标数组分配内存
          Marshal.Copy(ptr, rgbValues, 0, scanBytes);  // 将图像数据拷贝到rgbValues中
          // 分配灰度数组
          byte[] grayValues = new byte[width * height]; // 不含未用空间。
          // 计算灰度数组
          for (int i = 0; i < height; i++)
          {
               for (int j = 0; j < width; j++)
               {
                    double temp = rgbValues[posScan++] * 0.11 +
                        rgbValues[posScan++] * 0.59 + 
                        rgbValues[posScan++] * 0.3;
                    grayValues[posDst++] = (byte)temp;
               }
               // 跳过图像数据每行未用空间的字节，length = stride - width * bytePerPixel
               posScan += offset;
          }

          // 内存解锁
          Marshal.Copy(rgbValues, 0, ptr, scanBytes);
          original.UnlockBits(bmpData);  // 解锁内存区域

          // 构建8位灰度位图
          Bitmap retBitmap = BuiltGrayBitmap(grayValues, width, height);
          return retBitmap;
     }
     else
     {
          return null;
     }
}







/// <summary>
/// 用灰度数组新建一个8位灰度图像。
/// http://www.cnblogs.com/spadeq/archive/2009/03/17/1414428.html
/// </summary>
/// <param name="rawValues"> 灰度数组(length = width * height)。 </param>
/// <param name="width"> 图像宽度。 </param>
/// <param name="height"> 图像高度。 </param>
/// <returns> 新建的8位灰度位图。 </returns>
private static Bitmap BuiltGrayBitmap(byte[] rawValues, int width, int height)
{
    // 新建一个8位灰度位图，并锁定内存区域操作
    Bitmap bitmap = new Bitmap(width, height, PixelFormat.Format8bppIndexed);
    BitmapData bmpData = bitmap.LockBits(new Rectangle(0, 0, width, height),
         ImageLockMode.WriteOnly, PixelFormat.Format8bppIndexed);

    // 计算图像参数
    int offset = bmpData.Stride - bmpData.Width;        // 计算每行未用空间字节数
    IntPtr ptr = bmpData.Scan0;                         // 获取首地址
    int scanBytes = bmpData.Stride * bmpData.Height;    // 图像字节数 = 扫描字节数 * 高度
    byte[] grayValues = new byte[scanBytes];            // 为图像数据分配内存

    // 为图像数据赋值
    int posSrc = 0, posScan = 0;                        // rawValues和grayValues的索引
    for (int i = 0; i < height; i++)
    {
        for (int j = 0; j < width; j++)
        {
            grayValues[posScan++] = rawValues[posSrc++];
        }
        // 跳过图像数据每行未用空间的字节，length = stride - width * bytePerPixel
        posScan += offset;
    }

    // 内存解锁
    Marshal.Copy(grayValues, 0, ptr, scanBytes);

    bitmap.UnlockBits(bmpData);  // 解锁内存区域

    // 修改生成位图的索引表，从伪彩修改为灰度
    ColorPalette palette;
    // 获取一个Format8bppIndexed格式图像的Palette对象
    using (Bitmap bmp = new Bitmap(1, 1, PixelFormat.Format8bppIndexed))
    {
        palette = bmp.Palette;
    }
    for (int i = 0; i < 256; i++)
    {
        palette.Entries[i] = Color.FromArgb(i, i, i);
    }
    // 修改生成位图的索引表
    bitmap.Palette = palette;

    return bitmap;
}

速度最快
public static unsafe void ConvertTo8BitBitmap(Bitmap img, string destFile)
        {
            var bit = new Bitmap(img.Width, img.Height, PixelFormat.Format8bppIndexed);
            BitmapData data = img.LockBits(new Rectangle(0, 0, img.Width, img.Height), ImageLockMode.ReadOnly,
                                           PixelFormat.Format24bppRgb);
            var bp = (byte*)data.Scan0.ToPointer();
            BitmapData data2 = bit.LockBits(new Rectangle(0, 0, bit.Width, bit.Height), ImageLockMode.ReadWrite,
                                            PixelFormat.Format8bppIndexed);
            var bp2 = (byte*)data2.Scan0.ToPointer();
            for (int i = 0; i != data.Height; i++)
            {
                for (int j = 0; j != data.Width; j++)
                {
                    //0.3R+0.59G+0.11B
                    float value = 0.11F * bp[i * data.Stride + j * 3] + 0.59F * bp[i * data.Stride + j * 3 + 1] +
                                  0.3F * bp[i * data.Stride + j * 3 + 2];
                    bp2[i * data2.Stride + j] = (byte)value;
                }
            }
            img.UnlockBits(data);
            bit.UnlockBits(data2);
            ColorPalette palette = bit.Palette;
            for (int i = 0; i != palette.Entries.Length; i++)
            {
                palette.Entries[i] = Color.FromArgb(i, i, i);
            }
            bit.Palette = palette;
            bit.Save(destFile, ImageFormat.Bmp);
            img.Dispose();
            bit.Dispose();
        }
bp[i * data.Stride + j * 3]