How to mirror text horizontally or vertically on a TPaintBox

Problem/Question/Abstract:

How to mirror text horizontally or vertically on a TPaintBox

Answer:

This is actually not quite straightforward. The best way to do that is to first paint the text onto an off-screen bitmap and then paint that bitmap on screen using some weird coordinate manipulations. Drop a TPaintbox on the screen and connect a method to its OnPaint handler. Change the handler to the code below to see how this works:

procedure TForm1.PaintBox1Paint(Sender: TObject);
const
  test = 'Hello world';
var
  bmp: TBitmap;
  cv: TCanvas;
  ext: TSize;
  r: TRect;
begin
  cv := (Sender as TPaintbox).canvas;
  ext := cv.TextExtent(test);
  bmp := TBitmap.Create;
  try
    bmp.Width := ext.cx;
    bmp.Height := ext.cy;
    bmp.Canvas.Brush := cv.Brush;
    bmp.Canvas.Font := cv.Font;
    bmp.Canvas.FillRect(bmp.canvas.cliprect);
    bmp.Canvas.TextOut(0, 0, test);
    {draw text in normal orientation}
    cv.Draw(0, 0, bmp);
    r := Rect(ext.cx, 0, 0, ext.cy);
    OffsetRect(r, 0, ext.cy);
    {draw text horizontally mirrored}
    cv.CopyRect(r, bmp.canvas, bmp.canvas.ClipRect);
    r := Rect(0, ext.cy, ext.cx, 0);
    OffsetRect(r, 0, 2 * ext.cy);
    {draw text vertically mirrored}
    cv.CopyRect(r, bmp.canvas, bmp.canvas.ClipRect);
  finally
    bmp.Free
  end;
end;

The key here is to set up the target rectangle for CopyRect with left and right or top and bottom switched. Be warned, there is a slight potential that this code will cause acute indigestion for some video drivers!