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?
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.
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