如题，C#怎样获取灰度图的灰度值

Bitmap bmp = new Bitmap(100, 100);
using (Graphics g = Graphics.FromImage(bmp))
{
g.FillEllipse(Brushes.Red, new Rectangle(0, 0, 80, 80));
g.FillRectangle(new SolidBrush(Color.FromArgb(128, Color.Green)), new Rectangle(20, 20, 80, 80));
}
e.Graphics.DrawImage(bmp, new Point(10, 10));

//简单的
ControlPaint.DrawImageDisabled(e.Graphics, bmp, 120, 10, Color.White);

//用颜色矩阵
System.Drawing.Imaging.ImageAttributes attr = new System.Drawing.Imaging.ImageAttributes();
float[][] colorMatrixElements = {
new float[] {.33f, .33f, .33f, 0, 0}, // r = (r+g+b)/3
new float[] {.33f, .33f, .33f, 0, 0}, // g = (r+g+b)/3
new float[] {.33f, .33f, .33f, 0, 0}, // b = (r+g+b)/3
new float[] {0, 0, 0, 1, 0}, // alpha scaling factor of 1
new float[] {0, 0, 0, 0, 1}}; //
System.Drawing.Imaging.ColorMatrix matrix = new System.Drawing.Imaging.ColorMatrix(colorMatrixElements);
attr.SetColorMatrix(matrix);
e.Graphics.DrawImage(bmp, new Rectangle(10, 120, 100, 100), 0, 0, 100, 100, GraphicsUnit.Pixel, attr);

我文件循环写进每数字都表示像素值没哪位置表示图像宽我知道文件行数8729列数10458像素数91287882.我知道编码啊办呢