获取位图的alpha分量，C＃

Question

我正在尝试获取位图的alpha分量。使用GetPixel方法只获取rgb，alpha组件全部为255（不透明）。这是我尝试过的：

System.Drawing.Color tmp;

int rows = bitmap.Width;
int cols = bitmap.Height;

string[,] alphaMatrix = new string[rows,cols];
for (int i = 0; i < rows; i++)
{
    for (int j = 0; j < cols; j++)
    {
        tmp = bitmap.GetPixel(i, j);
        alphaMatrix[i, j] = tmp.ToArgb().ToString();
    }
}
return alphaMatrix;

编辑： 最初我的图片是BitmapImage，我将其转换为System.Drawing.Bitmap。我正在使用这种方法：

private System.Drawing.Bitmap BitmapImageToBitmap(BitmapImage bitmapImage)
{
    using (MemoryStream outStream = new MemoryStream())
    {
        BitmapEncoder enc = new BmpBitmapEncoder();
        enc.Frames.Add(BitmapFrame.Create(bitmapImage));
        enc.Save(outStream);
        System.Drawing.Bitmap bitmap = new System.Drawing.Bitmap(outStream);

        return new System.Drawing.Bitmap(bitmap);
    }
}