Copy Graphics content to bitmap

Question

I try to copy the contents of a graphics object to a bitmap. I'm using this code

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;
    }
}

If I use the methode like this

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

the bitmap contains the content. But if I draw on the graphics object before calling the methodes the bitmap is blank:

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));
    }
}

Why does it work in the first case and not in the latter?

Firewater

Are you checking the return value of BitBlt()? Maybe it is failing and could lead to clues. – Edmonton

the code is incomplete – Aphelion

Answer 1

As I understand it

    Graphics g = Graphics.FromImage(bmp)

creates a Graphics context for drawing into the image, whereas

    Graphics g = button1.CreateGraphics();

creates one for a control that's already been drawn to the screen. You could draw your dynamically created Bitmap to a PictureBox & then do the same thing as for your button, something like

        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
                }
            }

Of course this just re-creates a Bitmap that you already have so I assume you have some other use in mind.

Odrick

Answer 2

Did you try to flush you graphics?

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);

        g.Flush(); // !!

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

Chronister

Answer 3

You do it the hard way. Obviously since the same code succeeds and fails at different calls, the graphics use a different and temporary space to store information until ask to finalize with create graphics - else it should be read in both cases.

Try using

gr.Save(); 

for graphics, perhaps this will work prior to create.

However when I want to get drawing in progress I just clone the graphics image using:

public void CopyBackgroundLayer(ref Bitmap bitmap)
{
    Rectangle rect = new Rectangle(0, 0, bitmap.Width, bitmap.Height);
    bitmap = new Bitmap(buffer.Clone(rect, Primary.BackGround.PixelFormat));
}

So meanwhile I add stuff to a graphic object I call this where clones the image the graphic object draws (buffer at the example) to get the bitmap up to any point before adding more stuff.

Cringle