Author: Lou Adler

I currently use DC := GetDC(GetDesktopWindow) to get a screen capture. What do I 
need to do do to capture the desktop when multiple monitors are present instead of 
just the primary monitor?

Answer:

Solve 1:

On multi-monitor systems the desktop is one big bitmap. On my dual-monitor system 
the following captures both desktops into one big bitmap. If you just wanted the 
second monitor you could adjust the left setting of the TRect to be captured. Keep 
in mind, however, that some people might arrange monitors other than side by side 
with monitor 1 on the left and monitor 2 on the right.
1   
2   procedure GrabScreen(const SourceRect: TRect; Bitmap: TBitmap);
3   var
4     ScreenCanvas: TCanvas;
5   begin
6     ScreenCanvas := TCanvas.Create;
7     try
8       ScreenCanvas.Handle := GetDC(0);
9       try
10        Bitmap.Width := SourceRect.Right - SourceRect.Left;
11        Bitmap.Height := SourceRect.Bottom - SourceRect.Top;
12        Bitmap.Canvas.CopyRect(Rect(0, 0, Bitmap.Width, Bitmap.Height), ScreenCanvas,
13          SourceRect);
14      finally
15        ReleaseDC(0, ScreenCanvas.Handle);
16        ScreenCanvas.Handle := 0;
17      end;
18    finally
19      ScreenCanvas.Free;
20    end;
21  end;
22  
23  procedure TForm1.Button1Click(Sender: TObject);
24  begin
25    GrabScreen(Rect(0, 0, Screen.Monitors[0].Width + Screen.Monitors[1].Width,
26      Screen.Height),
27      Image1.Picture.Bitmap);
28    Caption := Format('Width %d Height %d', [Image1.Width, Image1.Height]);
29  end;



Solve 2:

Referring to Answer 1: Finally, I went through and used the debugger to check out 
all of the Screen variable fields at runtime and found what I needed:

30  { ... }
31  DC := GetDC(GetDesktopWindow);
32  BMPImage := TBitmap.Create;
33  try
34    BMPImage.Width := Screen.DesktopWidth;
35    BMPImage.Height := Screen.DesktopHeight;
36    BitBlt(BMPImage.Canvas.Handle, 0, 0, BMPImage.Width, BMPImage.Height,
37      DC, Screen.DesktopLeft, Screen.DesktopTop, SRCCOPY);
38  finally
39    ReleaseDC(GetDesktopWindow, DC);
40  end;
41  { ... }