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procedure TForm1.FormMouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
var
  MyFirstPoint, MyLastPoint: TPoint;
begin
  if Shift = [ssShift,ssLeft] then begin
    MyX:= x;
    MyY:= y;
    FormPaint(self);
  end;
end;

procedure TForm1.FormPaint(Sender: TObject);
begin
  Form1.Canvas.LineTo(MyX,MyY);
end;					

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with Form1.Canvas do begin
  Color := clMaroon;  // Farbe setzen
  MoveTo(10, 10);     // Startpunkt
  LineTo(MyX,MyY);    // Erste Linie
  LineTo(200, 10);    // Zweite Line
  LineTo(10, 10);     // wieder zurück zum Anfang
end;					

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procedure TForm1.Image1MouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
var MyX,MyY:Integer;
begin
    if Shift = [ssShift,ssLeft] then
    begin
        MyX:= x;
        MyY:= y;

        with Image1.Canvas do
        begin
            Pen.Color:=clWhite;                         // Stiftfarbe weiss
            Brush.Color:=ClWhite;                       // Füllfarbe weiss
            Rectangle(0,0,Image1.Width,Image1.Height);  // Image weiß machen
            Pen.Color:=clBlack;                         // Stiftfarbe schwarz
            MoveTo(10, 10);     // Startpunkt
            LineTo(MyX,MyY);    // Erste Linie
            LineTo(200, 10);    // Zweite Line
            LineTo(10, 10);     // wieder zurück zum Anfang
        end;
    end;
end;					

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unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, ExtCtrls, Math, StdCtrls;

type
  TForm1 = class(TForm)
    Button1: TButton;
    Button2: TButton;
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
  private
    { Private-Deklarationen }
  public
//    procedure DrawLine(APoint1, APoint2: TPoint; ACanvas: TCanvas);
    { Public-Deklarationen }
  end;

var
  Form1: TForm1;
{  BeginX, BeginY, EndX, EndY: Integer;
  MyX, MyY: Integer;}

implementation

{$R *.dfm}
{procedure TForm1.DrawLine(APoint1, APoint2: TPoint; ACanvas: TCanvas);
var
  Lpixel, LMaxAxisLength: integer;
  LRatio: Real;
begin
  LMaxAxisLength := Max(abs(APoint1.X - APoint2.X), abs(APoint1.Y - APoint2.Y));
  for Lpixel := 0 to LMaxAxisLength do
  begin
    LRatio := Lpixel / LMaxAxisLength;
    ACanvas.Pixels[APoint1.X + Round((APoint2.X - APoint1.X) * LRatio),
      APoint1.Y + Round((APoint2.Y - APoint1.Y) * LRatio)] :=
      ACanvas.Pen.Color;
  end;
end;}

procedure TForm1.Button1Click(Sender: TObject);
begin
  with Form1.Canvas do begin
    MoveTo(10, 10);     // Startpunkt
    LineTo(10,200);    // Erste Linie
    LineTo(200, 10);    // Zweite Line
    LineTo(10, 10);     // wieder zurück zum Anfang
  end;
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
  Form1.Repaint;
end;

end.					

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procedure TForm1.FormMouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
  if Shift = [ssShift,ssLeft] then begin
    MyX:= x;
    MyY:= y;
    Form1.Paint;
  end;
end;

procedure TForm1.FormPaint(Sender: TObject);
begin
    Form1.Canvas.LineTo(MyX,MyY);
end;					

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procedure TForm1.FormPaint(Sender: TObject);
begin
  Canvas.MoveTo(10, 10);
  Canvas.LineTo(100, 100);
  Canvas.LineTo(10, 190);
  Canvas.LineTo(10, 10);
end;					

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procedure TForm1.FormPaint(Sender: TObject);
begin
  Form1.Canvas.MoveTo(10,10);
  Form1.Canvas.LineTo(100,100);
end;