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]

Ill tell you what I know, which isnt 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 isnt 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 isnt 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 cant 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

 

[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); shouldve been FromHbitmap(hbitmap);
and it works fine!

Thanks all!