裁剪位图会得到空位图

Question

我从这里（SO）获取了一个裁剪图像的代码。我在包含白色字体的黑色文本的位图上尝试过。我得到的结果是完全白色的输出，没有内容。

        // create new bitmap with desired size and same pixel format
        Bitmap croppedBitmap = new Bitmap(rect.Width, rect.Height, bitmap.PixelFormat);

        // create Graphics "wrapper" to draw into our new bitmap
        // "using" guarantees a call to gfx.Dispose()
        using (Graphics gfx = Graphics.FromImage(croppedBitmap))
        {
            // draw the wanted part of the original bitmap into the new bitmap
            gfx.DrawImage(bitmap, 0, 0, rect, GraphicsUnit.Pixel);
        }

        return croppedBitmap;

有什么猜测吗？

PS如果我在油漆中裁剪，它当然可以工作

编辑

如果我裁剪我的照片，例如它可以工作......

附录

代码：

矩形：

        Rectangle 1: 8 50, 95, 80, 30 // invoice number
        Rectangle 2: 625, 778, 475, 22 // Total amount

CropImage()：

    public static Bitmap CropImage(Bitmap bitmap, Rectangle rect)
    {
        Bitmap croppedBitmap = new Bitmap(rect.Width, rect.Height, bitmap.PixelFormat);

        using (Graphics gfx = Graphics.FromImage(croppedBitmap))
        {
            gfx.DrawImage(bitmap, 0, 0, rect, GraphicsUnit.Pixel);
        }

        return croppedBitmap;
    }

图像： （敏感数据被隐藏，我只留下了我想要裁剪的部分） http://img33.imageshack.us/img33/5703/modelx.png

Answer 1

固定代码：

    Rectangle rect = new Rectangle(625, 778, 475, 22);

    Bitmap bitmap = Bitmap.FromFile(@"C:\m.png") as Bitmap;

    Bitmap croppedBitmap = new Bitmap(bitmap, rect.Width, rect.Height);
    croppedBitmap.SetResolution(bitmap.HorizontalResolution, bitmap.VerticalResolution);

    using (Graphics gfx = Graphics.FromImage(croppedBitmap))
    {
        gfx.DrawImage(bitmap, 0, 0, rect, GraphicsUnit.Pixel);
    }

    croppedBitmap.Save(@"C:\m-1.png", System.Drawing.Imaging.ImageFormat.Png);