Tracking Movement

The problem with this example as the OnMouseMove event handler is currently written is that it draws the line to the current mouse position from the last mouse position, not from the original position. You can correct this by moving the drawing position to the origin point, then drawing to the current point:

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procedure TForm1.FormMouseMove(Sender: TObject;Button: TMouseButton;
Shift: TShiftState; X, Y: Integer);
begin
if Drawing then
begin
Canvas.MoveTo(Origin.X, Origin.Y);{ move pen to starting point }
Canvas.LineTo(X, Y);
end;
end;

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void __fastcall TForm1::FormMouseMove(TObject *Sender, TMouseButton Button,
  TShiftState Shift, int X, int Y)
{
  if (Drawing)
  {
    Canvas->MoveTo(Origin.x, Origin.y);  // move pen to starting point
    Canvas->LineTo(X, Y);
  }
}

The above tracks the current mouse position, but the intermediate lines do not go away, so you can hardly see the final line. The example needs to erase each line before drawing the next one, by keeping track of where the previous one was. The MovePt field allows you to do this.

MovePt must be set to the endpoint of each intermediate line, so you can use MovePt and Origin to erase that line the next time a line is drawn:

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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);
MovePt := Point(X, Y);{ keep track of where this move was }
end;
procedure TForm1.FormMouseMove(Sender: TObject;Button: TMouseButton;
Shift: TShiftState; X, Y: Integer);
begin
if Drawing then
begin
Canvas.Pen.Mode := pmNotXor;{ use XOR mode to draw/erase }
Canvas.MoveTo(Origin.X, Origin.Y);{ move pen back to origin }
Canvas.LineTo(MovePt.X, MovePt.Y);{ erase the old line }
Canvas.MoveTo(Origin.X, Origin.Y);{ start at origin again }
Canvas.LineTo(X, Y);{ draw the new line }
end;
MovePt := Point(X, Y);{ record point for next move }
Canvas.Pen.Mode := pmCopy;
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
  MovePt = Point(X, Y);      // record last endpoint
}
void __fastcall TForm1::FormMouseMove(TObject *Sender, TMouseButton Button,
  TShiftState Shift, int X, int Y)
{
  if (Drawing)
  {
    Canvas->Pen->Mode = pmNotXor;        // use XOR mode to draw/erase
    Canvas->MoveTo(Origin.x, Origin.y);  // move pen to starting point
    Canvas->LineTo(MovePt.x, MovePt.y);  // erase old line
    Canvas->MoveTo(Origin.x, Origin.y);  // move pen to starting point again
    Canvas->LineTo(X, Y);                // draw new line
  }
  MovePt = Point(X, Y);      // record new endpoint
  Canvas->Pen->Mode = pmCopy;
}

Now you get a "rubber band" effect when you draw the line. By changing the pen's mode to pmNotXor, you have it combine your line with the background pixels. When you go to erase the line, you're actually setting the pixels back to the way they were. By changing the pen mode back to pmCopy (its default value) after drawing the lines, you ensure that the pen is ready to do its final drawing when you release the mouse button.