Tracking the Origin Point

When drawing lines, track the point where the line starts with the Origin field. Origin must be set to the point where the mouse-down event occurs, so the mouse-up event handler can use Origin to place the beginning of the line, as in this code:

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procedure TForm1.FormMouseDown(Sender: TObject; Button: TMouseButton;
Shift: TShiftState; X, Y: Integer);
begin
Drawing := True;
Canvas.MoveTo(X, Y);
Origin := Point(X, Y);{ record where the line starts }
end;
procedure TForm1.FormMouseUp(Sender: TObject; Button: TMouseButton;
Shift: TShiftState; X, Y: Integer);
begin
Canvas.MoveTo(Origin.X, Origin.Y);{ move pen to starting point }
Canvas.LineTo(X, Y);
Drawing := False;
end;

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void __fastcall TForm1::FormMouseDown(TObject *Sender, TMouseButton Button,
  TShiftState Shift, int X, int Y)
{
  Drawing = true;            // set the Drawing flag
  Canvas->MoveTo(X, Y);      // set pen position
  Origin = Point(X, Y);      // record where the line starts
}
void __fastcall TForm1::FormMouseUp(TObject *Sender, TMouseButton Button,
  TShiftState Shift, int X, int Y)
{
  Canvas->MoveTo(Origin.x, Origin.y);  // move pen to starting point
  Canvas->LineTo(X, Y);                // draw line from PenPos to (X, Y)
  Drawing = false;                     // clear the Drawing flag
}

Those changes get the application to draw the final line again, but they do not draw any intermediate actions—the application does not yet support "rubber banding."