How to capture an arbitrary area of ​​the screen on multiple monitors (screenshot)?

Question

Delphi

Hello!
I am writing a simple screenshoter with the ability to capture an arbitrary area.
Faced a problem - it is not possible to implement the program with multiple monitors.
What is the problem:
When I try to take a screenshot on the second monitor, the area on the first one is captured (as if I had captured the area on it).
When you try to take a screenshot on the first screen - Image1 is "filled" with white (the rectangle of the selected area is white)
I will immediately give code fragments, but just in case I attached the archive with the project (+exe).
Button "Take a screenshot":

the code

procedure TForm2.Button1Click(Sender: TObject);
var
  ScreenForm: TForm1;
begin
  // создаем полупрозрачную форму оверлей
  ScreenForm := TForm1.Create(nil);
  try
    // и растягиваем её на весь экран и позиционируем
    ScreenForm.Width := Screen.DesktopWidth;
    ScreenForm.Height := Screen.DesktopHeight;
    ScreenForm.Left := Screen.DesktopLeft;  // если 0 то программа работает только с 1 экраном - основным и корректно
    ScreenForm.Top := Screen.DesktopTop;    // если 0 то программа работает только с 1 экраном - основным и корректно
 
    // дальше прячем основную форму
    self.Hide;
    Application.ShowMainForm := FALSE;
 
    //показать форму оверлей
    ScreenForm.ShowModal;
 
    Image1.Picture.BitMap := ScreenForm.Bild;
 
    ScrollBox1.HorzScrollBar.Range := Image1.Picture.Width;
    ScrollBox1.VertScrollBar.Range := Image1.Picture.Height;
    self.Show;
  finally
    ScreenForm.Free;
  end;
end;

Capturing the area (+ "correcting" the rectangle of the selected area, if you suddenly selected "from bottom to top"):

the code

procedure NormRect(var aRect: TRect);
var tmp:Integer;
begin
if  aRect.Left > aRect.Right  then
    begin
      tmp:=aRect.Left;
      aRect.Left:=aRect.Right;
      aRect.Right:=tmp;
    end;
if  aRect.Top > aRect.Bottom  then
    begin
      tmp:=aRect.Top;
      aRect.Top:=aRect.Bottom;
      aRect.Bottom:=tmp;
    end;
end;
 
function CaptureScreenRect(aRect: TRect): TBitMap;
var
 ScreenDC: HDC;
 ActHandles:HWND;
begin
  Result := TBitMap.Create;
  with Result, aRect do
  begin
    Result.Free;
    Result := TBitMap.Create;
    NormRect(aRect); // исправить координаты выделенной области
    Result.Width := aRect.Right - aRect.Left;
    Result.Height :=aRect.Bottom - aRect.Top;
    ActHandles := GetDesktopWindow;
    ScreenDC  := GetWindowDC(ActHandles);//GetDc(0); - пробовал так, но результат тот же
    try
     BitBlt(Result.Canvas.Handle, 0, 0, Result.Width, Result.Height, ScreenDC, aRect.Left, aRect.Top, SRCCOPY);
    finally
     ReleaseDC(ActHandles, ScreenDC);//ReleaseDC(0, ScreenDC); - пробовал так, но результат тот же
    end;
  end;
end;

https://drive.google.com/file/d/1VSRkaWGla_TA7_4ft...

Answer 1

The solution turned out to be very simple)) I
experimented with the coordinates (because they are negative when working on the second monitor)
and first did this
(this is written in onmouseup, but in fact r is aRect in the CaptureScreenRect function before NormRect is applied)

r.Left := downX-screen.Width;
r.Top := downY;
r.Right := X-screen.Width;
r.Bottom := Y;

in this case it works correctly, but only if my monitors are set in the settings as [2][1] (the additional monitor is on my left)
if I change the order in the Windows settings to [1][2] (as if I rearranged the additional monitor right), then this "fix" does not work.
But after redoing the fragment above, everything seems to work like this)

if Screen.DesktopLeft < 0 then
  begin
    r.Left := downX-screen.Width;
    r.Right := X-screen.Width;
  end
  else
  begin
  r.Left := downX;
    r.Right := X;
  end;
  if Screen.DesktopTop < 0 then
  begin
    r.Top := downY-screen.Height;
    r.Bottom := Y-screen.Height;
  end
  else
  begin
    r.Top := downY;
    r.Bottom := Y;
  end;

Please tell me - is this a good solution and what can be the "pitfalls" with such an implementation?
PS:
the truth was checked only by changing the monitor settings in Windows (I don’t know how to check otherwise):
[1][2]
[2][1]
[1]
[2]
[2]
[1]