PDFlib 8 Windows COM/.NET Tutorial

6.2 Font Metrics and Text Variations
6.2.1 Font and Glyph Metrics
Text position. PDFlib maintains the text position independently from the current
point for drawing graphics. While the former can be queried via the textx/texty parameters,
the latter can be queried via currentx/currenty.
Glyph metrics. PDFlib uses the glyph and font metrics system used by PostScript and
PDF which shall be briefly discussed here.
The font size which must be specified by PDFlib users is the minimum distance between
adjacent text lines which is required to avoid overlapping character parts. The
font size is generally larger than individual characters in a font, since it spans ascender
and descender, plus possibly additional space between lines.
The leading (line spacing) specifies the vertical distance between the baselines of adjacent
lines of text. By default it is set to the value of the font size. The capheight is the
height of capital letters such as T or H in most Latin fonts. The xheight is the height of
lowercase letters such as x in most Latin fonts. The ascender is the height of lowercase
letters such as f or d in most Latin fonts. The descender is the distance from the baseline
to the bottom of lowercase letters such as j or p in most Latin fonts. The descender is
usually negative. The values of xheight, capheight, ascender, and descender are measured
as a fraction of the font size, and must be multiplied with the required font size before
being used.
The gaplen property is only available in TrueType and OpenType fonts (it will be estimated
for other font formats). The gaplen value is read from the font file, and specifies
the difference between the recommended distance between baselines and the sum of
ascender and descender.
PDFlib may have to estimate one or more of these values since they are not guaranteed
to be present in the font or metrics file. In order to find out whether real or estimated
values are used you can call info_font( ) to query the xheight with the option faked.
The character metrics for a specific font can be queried from PDFlib as follows:
font = p.load_font("Times-Roman", "unicode", "");
capheight = p.info_font(font, "capheight", "");
ascender = p.info_font(font, "ascender", "");
Fig. 6.1 Font and character metrics
font size
capheight
ascender
baseline
descender
152 Chapter 6: Text Output (Edition for COM, .NET, and REALbasic)