Capturing a Window that is hidden or minimized

问题:

I followed this tutorial (there's a bit more than what's listed here because in my code I get a window via mouse click) for grabbing a window as a bitmap and then rendering that bitmap in a different window.

My question:

When that window is minimized or hidden (SW_HIDE) my screen capture doesn't work, so is it possible to capture a window when it is minimized or hidden?

回答1:

The PrintWindow api works well, I use it for capturing thumbnails for hidden windows. Despite the name, it is different than WM_PRINT and WM_PRINTCLIENT, it works with pretty much every window except for Direct X / WPF windows.

I added some code (C#) but after reviewing how I used the code, I realized that the window isn't actually hidden when I capture its bitmap, its just off screen so this may not work for your case. Could you show the window off screen, do a print and then hide it again?

        public static Bitmap PrintWindow(IntPtr hwnd)
    {
        RECT rc;
        WinUserApi.GetWindowRect(hwnd, out rc);

        Bitmap bmp = new Bitmap(rc.Width, rc.Height, PixelFormat.Format32bppArgb);
        Graphics gfxBmp = Graphics.FromImage(bmp);
        IntPtr hdcBitmap = gfxBmp.GetHdc();
        bool succeeded = WinUserApi.PrintWindow(hwnd, hdcBitmap, 0);
        gfxBmp.ReleaseHdc(hdcBitmap);
        if (!succeeded)
        {
            gfxBmp.FillRectangle(new SolidBrush(Color.Gray), new Rectangle(Point.Empty, bmp.Size));
        }
        IntPtr hRgn = WinGdiApi.CreateRectRgn(0, 0, 0, 0);
        WinUserApi.GetWindowRgn(hwnd, hRgn);
        Region region = Region.FromHrgn(hRgn);
        if (!region.IsEmpty(gfxBmp))
        {
            gfxBmp.ExcludeClip(region);
            gfxBmp.Clear(Color.Transparent);
        }
        gfxBmp.Dispose();
        return bmp;
    }

回答2:

There are WM_PRINT and WM_PRINTCLIENT messages you can send to the window, which cause its contents to be rendered into the HDC of your choice.

However, these aren't perfect: while the standard Win32 controls handle these correctly, any custom controls in the app might not.

回答3:

I am trying to get the bitmap of partially hidden controls.

I used code before that did the drawing, but included windows overlapping it. So.. maybe you want to try this. The WM_PRINTCLIENT should (in my understanding) redraw all inside the control, even if it is not really visible.

const int WM_PRINT = 0x317, WM_PRINTCLIENT = 0x318, PRF_CLIENT = 4,
PRF_CHILDREN = 0x10, PRF_NON_CLIENT = 2,
COMBINED_PRINTFLAGS = PRF_CLIENT | PRF_CHILDREN | PRF_NON_CLIENT;
SendMessage(handle, WM_PRINTCLIENT, (int)hdc, COMBINED_PRINTFLAGS);
//GDIStuff.BitBlt(hdc, 0, 0, width, height, hdcControl, 0, 0, (int)GDIStuff.TernaryRasterOperations.SRCCOPY);

The before code is commented out now. It is based on the code found here: Pocket PC: Draw control to bitmap (accepted answer). It is basically the same as Tim Robinson suggests in this thread.

Also, have a look here http://www.tcx.be/blog/2004/paint-control-onto-graphics/