How to capture assigned area with mouse button event?

if you want to capture on any desktop area by clicking mouse left button in its client area then add two TImage, TTimer, TLabel, TCheckBox and TButton components to the form.

Here is code;
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unit Unit1;

interface

uses Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
     AppEvnts, Buttons, Jpeg, StdCtrls, ExtCtrls;  

type
  TForm1 = class(TForm)
    Image1: TImage;
    Image2: TImage;
    Button1: TButton;
    Label1: TLabel;
    Timer1: TTimer;
    CheckBox1: TCheckBox;
    procedure FormDestroy(Sender: TObject);
    procedure Timer1Timer(Sender: TObject);
    procedure FormClose(Sender: TObject; var Action: TCloseAction);
    procedure ApplicationEvents1Message(var Msg: tagMSG;
      var Handled: Boolean);
    procedure FormCreate(Sender: TObject);
    procedure Button1Click(Sender: TObject);
  private
     MHookGo : Boolean;
  public
  end;

var
  Form1: TForm1;
  MHook: THandle;

implementation

{$R *.DFM}

function JournalProc(Code, wParam: Integer; var EventW: TEventMsg): Integer; stdcall;
label go, jump;
var
  s, tm: string;
  JPGFile: TJPEGImage;
  a1, p: integer;
  Ad, s1, s2, s3: string;
begin
  Result := CallNextHookEx(MHook, Code, wParam, Longint(@EventW));

  if Code < 0 then Exit;
  if Code = HC_SYSMODALON then Exit;
  if Code = HC_ACTION then
  begin
    s := '';
    if EventW.message = WM_LBUTTONDOWN then // mouse left button pressed
    begin
      JPGFile := TJPEGImage.Create;
      s := 'Mouse coordinates:   X: '
            + IntToStr(EventW.paramL)
            + '  Y: ' + IntToStr(EventW.paramH);

      Form1.image2.Picture := Form1.Image1.Picture;
      if Form1.CheckBox1.Checked = True then
      begin
        JPGFile.Assign(Form1.Image2.Picture.Bitmap);
        { as a file name will use current time.
          Therefore, For file name, converting : character to . character }
        tm := DateTimeToStr(Now);
        go:
        Ad := tm;
        a1 := Length(Ad);
        p := Pos(':', Ad);
        if p = 0 then
        begin
          goto jump;
        end
          else
            begin
             s1 := Copy(Ad, 1, p);
             s2 := Copy(Ad, (p + 1), (a1 - p));
            end;
        s1 := Copy(Ad, 1, p - 1);
        s3 := s1 + '.' + s2;
        tm :=  s3;
        goto go;
        jump:
        JPGFile.SaveToFile(s3 + '.jpg'); // saved into the same folder
      end;
    end;
    if s <> '' then Form1.Label1.Caption := s;
  end;
end;

procedure TForm1.ApplicationEvents1Message(var Msg: tagMSG;
  var Handled: Boolean);
begin
  // if Ctrl-Alt-Del or Ctrl-Esc pressed then journal hook will be disable
  Handled := False;
  if (Msg.message = WM_CANCELJOURNAL) and MHookGo then
      MHook := SetWindowsHookEx(WH_JOURNALRECORD, @JournalProc, 0, 0);
end;

procedure TForm1.Timer1Timer(Sender: TObject);
var Asn1, Asn2, Asn3: TRect;
    aWidth, aHeight: Integer;
    aMusX, aMusY: Real;
    Cv: TCanvas;
    Crs: TPoint;
begin
  if not IsIconic(Application.Handle) then
  begin
    GetCursorPos(Crs);
          Asn3 := Rect(Form1.Left,Form1.Top, Form1.Left
                          + Form1.Width, Form1.Top
                          + Form1.Height);

    if not PtInRect(Asn3, Crs) then
    begin
      aWidth := Image1.Width;
      aHeight := Image1.Height;
      Asn2 := Rect(0, 0, aWidth, aHeight);

      // captured area proportions - you can change it
      aMusX := aWidth / (1 * 2);
      aMusY := aHeight / (1 * 2);

      Asn1 := Rect(Crs.x, Crs.y, Crs.x, Crs.y);
      InflateRect(Asn1, Round(aMusX), Round(aMusY));

      if Asn1.Left < 0 then OffsetRect(Asn1, - Asn1.Left, 0);
      if Asn1.Top < 0 then OffsetRect(Asn1, 0, - Asn1.Top);
      if Asn1.Right > Screen.Width then
         OffsetRect(Asn1, - (Asn1.Right - Screen.Width), 0);
      if Asn1.Bottom > Screen.Height then
         OffsetRect(Asn1, 0, - (Asn1.Bottom - Screen.Height));

      Cv := TCanvas.Create;
      try
        Cv.Handle := GetDC(GetDesktopWindow);
        Image1.Canvas.CopyRect(Asn2, Cv, Asn1);
      finally
        Cv.Free;
      end;
      Application.ProcessMessages;
    end;
  end;
end;

procedure TForm1.FormClose(Sender: TObject; var Action: TCloseAction);
begin
  // unhook closing
  if MHookGo then UnhookWindowsHookEx(MHook);
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  Label1.Caption := 'Mouse coordinates:';
  CheckBox1.Caption := 'Start for Capture area';
  if MHookGo then Exit; // Mouse Journaled absent
  MHook := SetWindowsHookEx(WH_JOURNALRECORD, @JournalProc, hInstance, 0);
  // Hook starting
  if MHook > 0 then MHookGo := True
     else Exit; // Journal Hook absent
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  // disable hook
  MHookGo := False;
  UnhookWindowsHookEx(MHook);
  MHook := 0;
  Label1.Caption := '';
end;

end.