Author: Fernando Silva
TFont doesn't give you all the capabilities of Windows fonts when you need to draw
some text.
This is because the mechanism TFont uses differs from the one Windows have.
Answer:
TFont is a descent of TGraphicsObject. This is the abstract base class for objects
which encapsulate a system graphics object: TBrush, TFont, and TPen.
Internally TFont uses the TFontData record to keep track of all changes to our font.
While TFontData structure is like this:
1 TFontData = record
2 Handle: HFont;
3 Height: Integer;
4 Pitch: TFontPitch;
5 Style: TFontStylesBase;
6 Charset: TFontCharset;
7 Name: TFontDataName;
8 end;
windows LogFont structure defines the following attributes of a font:
9 tagLOGFONTA = packed record
10 lfHeight: Longint;
11 lfWidth: Longint;
12 lfEscapement: Longint;
13 lfOrientation: Longint;
14 lfWeight: Longint;
15 lfItalic: Byte;
16 lfUnderline: Byte;
17 lfStrikeOut: Byte;
18 lfCharSet: Byte;
19 lfOutPrecision: Byte;
20 lfClipPrecision: Byte;
21 lfQuality: Byte;
22 lfPitchAndFamily: Byte;
23 lfFaceName: array[0..LF_FACESIZE - 1] of AnsiChar;
24 end;
25
26 TLogFontA = tagLOGFONTA;
27 TLogFont = TLogFontA;
you can already see the difference between both. Anyway, while trying to simplify
the process Delphi team forgot to add to TFontData : lfEscapment, lfOrientation
(with the two we can rotate a font) and lfQuality (this one give us the possibility
of drawing an antialised font).
When you call TextOut, internally when the draw text routine gets the handle for
the font to be used with the windows API ExtTextOut, TFont maps the TFont structure
to windows logfont structure (much like the way I did the DrawText routine forward).
In TFont.GetHandle we see the following:
28 (...split...)
29 lfEscapement := 0; { only straight fonts }
30 lfOrientation := 0; { no rotation }
31 (...split...)
32 lfQuality := DEFAULT_QUALITY;
33 (...split...)
so... they didn't want to code just a little more :)
I thougth about changing (or even extending) TFont class to have these two
properties available, anyway (if someone is interested I could do that) I'll just
present here a routine that can draw some text (rotated and with more quality).
34
35 procedure DrawText(ACanvas: TCanvas; AAngle: Integer; AQuality: byte; X, Y: Integer;
36 AText: string);
37 var
38 lf: TLogFont;
39
40 begin
41 with ACanvas do
42 begin
43 GetObject(Font.Handle, SizeOf(lf), @lf);
44 with lf do
45 begin
46 lfQuality := AQuality;
47 lfOrientation := AAngle * 10;
48 lfEscapement := lfOrientation;
49 end;
50 Font.Handle := CreateFontIndirect(lf);
51 TextOut(X, Y, AText);
52 end;
53 end;
AQuality can be :
DEFAULT_QUALITY
Appearance of the font does not matter.
DRAFT_QUALITY
Appearance of the font is less important than when PROOF_QUALITY is used. For GDI
raster fonts, scaling is enabled, which means that more font sizes are available,
but the quality may be lower. Bold, italic, underline, and strikeout fonts are
synthesized if necessary.
PROOF_QUALITY
Character quality of the font is more important than exact matching of the
logical-font attributes. For GDI raster fonts, scaling is disabled and the font
closest in size is chosen. Although the chosen font size may not be mapped
exactly when PROOF_QUALITY is used, the quality of the font is high and there is
no distortion of appearance. Bold, italic, underline, and strikeout fonts are
synthesized if necessary.
NONANTIALIASED_QUALITY
ANTIALIASED_QUALITY
Note: Remember that the 3 attributes just work with True Type Fonts.
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