将图形内容复制到位图

Question

我尝试将图形对象的内容复制到位图.我正在使用此代码

public static class GraphicsBitmapConverter
{
    [DllImport("gdi32.dll", SetLastError = true)]
    [return: MarshalAs(UnmanagedType.Bool)]
    private static extern bool BitBlt(IntPtr hdc, int nXDest, int nYDest, int nWidth, int nHeight, IntPtr hdcSrc, int nXSrc, int nYSrc, TernaryRasterOperations dwRop);

    public static Bitmap GraphicsToBitmap(Graphics g, Rectangle bounds)
    {
        Bitmap bmp = new Bitmap(bounds.Width, bounds.Height);

        using (Graphics bmpGrf = Graphics.FromImage(bmp))
        {
            IntPtr hdc1 = g.GetHdc();
            IntPtr hdc2 = bmpGrf.GetHdc();

            BitBlt(hdc2, 0, 0, bmp.Width, bmp.Height, hdc1, 0, 0, TernaryRasterOperations.SRCCOPY);

            g.ReleaseHdc(hdc1);
            bmpGrf.ReleaseHdc(hdc2);
        }

        return bmp;
    }
}

如果我像这样使用这种方法

Graphics g = button1.CreateGraphics();
var bmp = GraphicsBitmapConverter.GraphicsToBitmap(g, Rectangle.Truncate(g.VisibleClipBounds));

位图包含内容.但是如果我在调用方法之前绘制图形对象,则位图是空白的:

using (Bitmap bmp = new Bitmap(100, 100))
{
    using (Graphics g = Graphics.FromImage(bmp))
    {
        g.FillRectangle(Brushes.Red, 10, 10, 50, 50);
        g.FillRectangle(Brushes.Blue, 20, 20, 50, 50);
        g.FillRectangle(Brushes.Green, 0, 0, bmp.Width, bmp.Height);

        var bmp2 = GraphicsBitmapConverter.GraphicsToBitmap(g, Rectangle.Truncate(g.VisibleClipBounds));
    }
}

为什么它适用于第一种情况而不适用于后者？

Answer 1

据我了解

    Graphics g = Graphics.FromImage(bmp)

创建一个用于绘制图像的 Graphics 上下文，而

    Graphics g = button1.CreateGraphics();

为已经绘制到屏幕上的控件创建一个。您可以将动态创建的位图绘制到 PictureBox 中，然后对按钮执行相同的操作，例如

        using (Bitmap bmp = new Bitmap(100, 100)) 
            {
            using (Graphics g = Graphics.FromImage(bmp)) 
                {
                g.FillRectangle(Brushes.Green, 0, 0, bmp.Width, bmp.Height);
                g.FillRectangle(Brushes.Red, 10, 10, 50, 50);
                g.FillRectangle(Brushes.Blue, 20, 20, 50, 50);

                pictureBox1.Image = bmp;
                pictureBox1.Update(); // force an update before doing anything with it
                Graphics g2 = pictureBox1.CreateGraphics();
                var bmp2 = GraphicsBitmapConverter.GraphicsToBitmap(g2, Rectangle.Truncate(g.VisibleClipBounds)); 
                // bmp2 now has the same contents as bmp1
                pictureBox1.Image = null; // bmp is about to be Disposed so remove the reference to it
                }
            }

当然，这只是重新创建您已经拥有的位图，所以我假设您还有其他用途。