To display text diagonally (or by any other degree), you need to create a font.
The font may be created using the function CreateFontIndirect.
The parameter's record member .lfOrientation specifies the angle in 0.1 degrees,
e.g. 450 equals 45 degrees.
When the font handle is no longer needed, it should be deleted with DeleteObject().
The following function writes a sort of watermark on a DC and uses the API function
TextOut for this:
procedure Draw_Watermark_on_DC (const aDC : hDC; const x,y : integer);
var
plf : TLOGFONT;
hfnt, hfntPrev : HFONT;
const
txt1 : PChar = 'Created with the demo version of'#0;
txt2 : PChar = ' pasDOC'#0;
WaterMarkWidth = 300;
WaterMarkHeight= 300;
begin
// Specify a font typeface name and weight.
ZeroMemory (@plf, sizeof(plf));
lstrcpy(plf.lfFaceName, 'Arial');
plf.lfHeight := 30;
plf.lfEscapement := 0;
plf.lfOrientation:= 450;
plf.lfWeight := FW_NORMAL;
plf.lfCharset:= ANSI_CHARSET;
plf.lfOutPrecision := OUT_TT_PRECIS;
plf.lfQuality := PROOF_QUALITY;
hfnt := CreateFontIndirect(plf);
// Draw the rotated string
SetBkMode(aDC, TRANSPARENT);
hfntPrev := SelectObject(aDC, hfnt);
Windows.TextOut(aDC, x, y + WaterMarkHeight - 25, txt1, strlen(txt1));
Windows.TextOut(aDC, x+plf.lfHeight * 3, y + WaterMarkHeight - 25, txt2, strlen(txt2));
SelectObject(aDC, hfntPrev);
DeleteObject(hfnt);
end;