Capture a hidden window without making it top level?

[thomas10001]

I need to capture the image of a hidden window. Can this be done without making it the top level window?

 

 

(I have already code to capture an image of the top level window. But that involves a lot of hiding and showing windows and creates problem with flickering and that the whole area is not redrawn and I do not always capture what I want.)

[snarfblam]

I'll tell you what I know, which isn't much.

 

In Windows generally the image for a window is only that which is stored on the screen, which means that if all or part of a window isn't visible, the image for the concealed portion does not exist. There must be a way, however, to have a form draw itself somewhere other than the screen because Microsoft offers an alt-tab replacement program which shows the image of the window you are tabbing to, even if it isn't visible (or completely visible).

[jo0ls]

Everyone uses bitblt, but there is the printWindow API too (which works for off-screen and hidden forms). One thing that you can't do though is create a form that is bigger than the available working screen space.

 

   Private Declare Function PrintWindow Lib "user32.dll" (ByVal _
              hwnd As IntPtr, ByVal hdcBlt As IntPtr, ByVal nFlags As _
              UInt32) As Boolean
   Dim WithEvents t As New Timer

   Dim screenCapture As Bitmap
   Dim otherForm As New Form

   Private Sub CaptureScreen()
       Me.Text = "capturing, " & Me.Location.X & "," & Me.Location.Y
       screenCapture = New Bitmap(Me.Width, Me.Height)
       Dim g As Graphics = Graphics.FromImage(screenCapture)
       Dim hdc As IntPtr = g.GetHdc
       Form1.PrintWindow(Me.Handle, hdc, Nothing)
       g.ReleaseHdc(hdc)
       g.Flush()
       g.Dispose()

   End Sub

   Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
       ' uncomment to test capturing various hidden forms:

       '1- hidden off screen -> works!
       'Me.Location = New Point(-500, -500)

       '2- Minimizes - works but just takes a capture of the small minimized form...
       'Me.WindowState = FormWindowState.Minimized

       '3- big form laid over the top
       otherForm.Size = New Size(4000, 4000)
       otherForm.Show()

       t.Interval = 1000
       t.Start()
   End Sub



   Private Sub t_Tick(ByVal sender As Object, ByVal e As System.EventArgs) Handles t.Tick
       CaptureScreen()
       If IO.File.Exists("C:\\ScreenCapBlah.bmp") Then
           IO.File.Delete("C:\\ScreenCapBlah.bmp")
       End If
       screenCapture.Save("c:\\ScreenCapBlah.bmp")
       Me.Text = ""

       '1 - move the form back
       ' Me.Location = New Point(0, 0)

       '2- maximize again
       ' Me.WindowState = FormWindowState.Maximized

       '3 - hide otherForm
       otherForm.Hide()
       t.Stop()
   End Sub

Edited December 7, 2005 by jo0ls

[thomas10001]

PrintWindow

 

It seems that I should use PrintWindow

 

I found the following code that I modified a bit.

 

public static Bitmap captureHiddenWindow(IntPtr hwnd)

{

Bitmap bitmap = null;

 

// Takes a snapshot of the window hwnd, stored in the memory device context hdcMem

IntPtr hdc = PlatformInvokeGDI32.GetWindowDC(hwnd);

if (hdc != IntPtr.Zero)

{

IntPtr hdcMem = PlatformInvokeGDI32.CreateCompatibleDC(hdc);

if (hdcMem != IntPtr.Zero)

{

Rectangle rc;

PlatformInvokeGDI32.GetWindowRect(hwnd,out rc);

IntPtr hbitmap = PlatformInvokeGDI32.CreateCompatibleBitmap(hdc, rc.Width, rc.Height);

if (hbitmap != IntPtr.Zero)

{

PlatformInvokeGDI32.SelectObject(hdcMem, hbitmap);

Win32.PrintWindow(hwnd, hdcMem, 0);

bitmap = System.Drawing.Image.FromHbitmap(hdcMem);

PlatformInvokeGDI32.DeleteObject(hbitmap);

}

PlatformInvokeGDI32.DeleteObject(hdcMem);

}

PlatformInvokeGDI32.ReleaseDC(hwnd, hdc);

}

return bitmap;

}

 

 

 

I added the bitmap = System.Drawing.Image.FromHbitmap(hdcMem);

because I need a bitmap returned. I got an exception when I tried it due

to (I guess) hdcMem was negative. What do I do wrong here?

 

Is there another way to use the dc that I am not aware of?

[thomas10001]

I just found out that I did mix up the parameters

FromHbitmap(hdcMem); should've been FromHbitmap(hbitmap);

and it works fine!

 

Thanks all!