delphi - Tile/Center image in the forms background

Question

Is there a way to place an image in the form background and be able to tile it or center it ?

Also I need to place other components on top of the image.

I tried rmControls but I cannot place anything on top of the image.

Answer 1

You can paint your image in an OnPaint handler for the form. Here's a simple example of tiling:

procedure TMyForm.FormPaint(Sender: TObject);
var
  Bitmap: TBitmap;
  Left, Top: Integer;
begin
  Bitmap := TBitmap.Create;
  Try
    Bitmap.LoadFromFile('C:desktopitmap.bmp');
    Left := 0;
    while Left<Width do begin
      Top := 0;
      while Top<Height do begin
        Canvas.Draw(Left, Top, Bitmap);
        inc(Top, Bitmap.Height);
      end;
      inc(Left, Bitmap.Width);
    end;
  Finally
    Bitmap.Free;
  End;
end;

In real code you would want to cache the bitmap rather than load it every time. I'm sure you can work out how to adapt this to centre a bitmap.

The output looks like this:

However, since this is the background to the form, it's much better to do the painting in a handler for WM_ERASEBACKGROUND. That will also make sure that you won't have any flickering when you resize. Here's a more advanced version of the program that demonstrates this, together with a stretch draw option.

procedure TMyForm.FormCreate(Sender: TObject);
begin
  FBitmap := TBitmap.Create;
  FBitmap.LoadFromFile('C:desktopitmap.bmp');
end;

procedure TMyForm.RadioGroup1Click(Sender: TObject);
begin
  Invalidate;
end;

procedure TMyForm.FormResize(Sender: TObject);
begin
  //needed for stretch drawing
  Invalidate;
end;

procedure TMyForm.PaintTile(Canvas: TCanvas);
var
  Left, Top: Integer;
begin
  Left := 0;
  while Left<Width do begin
    Top := 0;
    while Top<Height do begin
      Canvas.Draw(Left, Top, FBitmap);
      inc(Top, FBitmap.Height);
    end;
    inc(Left, FBitmap.Width);
  end;
end;

procedure TMyForm.PaintStretch(Canvas: TCanvas);
begin
  Canvas.StretchDraw(ClientRect, FBitmap);
end;

procedure TMyForm.WMEraseBkgnd(var Message: TWmEraseBkgnd);
var
  Canvas: TCanvas;
begin
  Canvas := TCanvas.Create;
  Try
    Canvas.Handle := Message.DC;
    case RadioGroup1.ItemIndex of
    0:
      PaintTile(Canvas);
    1:
      PaintStretch(Canvas);
    end;
  Finally
    Canvas.Free;
  End;
  Message.Result := 1;
end;