Font::TTFMetrics(3) User Contributed Perl Documentation Font::TTFMetrics(3)NAMEFont::TTFMetrics - A parser for the TTF file.
SYNOPSIS
use Font::TTFMetrics;
my $metrics = Font::TTFMetrics->new("somefont.ttf");
my $ascent = $metrics->get_ascent();
DESCRIPTION
"Font::TTFMetrics" encapsulates the font metrics of a true type font
file. A true type font file contains several tables which need to be
parsed before any useful information could be gathered about the font.
There is the excellent module for parsing TTF font in CPAN by Martin
Hosken, "Font::TTF". But in my opinion the use of "Font::TTF" requires
intimate knowledge of TTF font format. This module was written to
support the use of TTF in "Pastel" 2D graphics library in Perl. Three
factors prompted me to write this module: first, I required a fast
module to access TTF file. Second, all the access required was read-
only. Last, I wanted a user friendly, higher level API to access TTF
file.
Each font file actually contains several informations the most
important information is how a particular character will display on
screen. The shape of a character (glyph) is determined by a series of
points. The points are generally lines or points on curved path. For
details see the TTF specification. Remember, the points actually
determines the outline of the curve.TTF file stores the glyph shape in
the "glyf" table of the font. The first glyph described in this table
will be always a particular glyph, called "missing-glyph" which is
shown in case the font file doesnot contains the glyph that a software
wants.
Each character in computer is actually a number. You can find what
number corresponds to the character, you can call "ord()" on the
character. This value is called the ordinal value of the character. If
you just use common english typically the number of any character falls
between 32-126, commonly called as ASCII. If you use some more extra
character not commonly found in key-board like "degree" then your
character code will fall between 0-255, commonly called LATIN-1
character set. Unicode is a way to use charaters with ordinal values
beyond 255. The good thing about it is that the UTF8 encoding in perl
works silently in the backdrop and you can intermix characters with any
ordinal value. This ofcourse does not mean that you will be able to use
character with any ordinal values for display. The font file must
contains the corresponding glyph.
The way to extract the glyph for a character is done by looking into
"cmap" table of the font. This table contains the character ordinal
number and a correspoding index. This index is used to look into the
"glyf" table to extract the shape of the character. Thar means if you
just substitute another index for a particular ordinal number you can
actually display a different character, a mechanism known as "glyph
substitution". As you can guess there is one more way to display a
particular character instead of what if should display in a more font
specific manner. If you just add a particular offset to a glyph ordinal
value and provide the index for this added value in the "cmap" table,
you can generate a completely different glyph. This mechanism works for
a particular type of fonts supplied by Microsoft called symbol fonts.
Example of these are symbol.ttf and wingding. Both these fonts does not
supply any glyphs corresponding to LATIN-1 character sets but with
ordinal values in the range of 61472-61695. But notice if you fire up
your word-processor and change the font to symbol and type any
character on the key board you get a display. For example, if you type
A (ordinal value 65) what you get is greek capital alpha. This works
this way: as soon as the word-processor find that you are using a
symbol font (you can call "is_symbol()" method to find that) it just
adds 61440 to any character you type and then queries the "cmap" table
for the glyph.
One more important aspect of using a TTF file is to find the width of a
string. The easiest way to find this to query "htmx" table, which
contains advanced width of each character, add up all the advance
widths of the individual characters in the string and then go look into
"kern" table, which contains the kerning value for pair of glyphs add
deduct these values from the total width. You need to deduct also the
left-side bearing of the first character and the right-side bearing of
the last character from the total width.
User of this module should keep in mind that all the values returned
from this modules are in font-units and should be converted to pixel
unit by:
fIUnits * pointsize * resolution /(72 * units_per_em)
An example from the true type specification at
<http://www.microsoft.com/typography/otspec/TTCH01.htm>:
A font-feature of 550 units when used with 18 pt on screen (typically
72 dpi resolution) will be
550 * 18 * 72 / ( 72 * 2048 ) = 4.83 pixels long.
Note that the "units_per_em" value is 2048 which is typical for a TTF
file. This value can be obtained by calling "get_units_per_em()" call.
This module also takes full advantage of the unicode support of Perl.
Any strings that you pass to any function call in this module can have
unicode built into into it. That means a string like:
"Something \x{70ff}" is perfectly valid.
CONSTRUCTORnew()
Creates and returns a "Font::TTFMetrics" object.
Usage : my $metrics = Font::TTFMetrics->new($file);
Args : $file - TTF filename.
Returns : A Font::TTFMetrics object.
METHODSis_symbol()
Returns true if the font is a Symbol font from Microsoft. Remember that
Wingding is also a symbol font.
Usage : $metrics->is_symbol();
Args : Nothing.
Returns : True if the font is a Symbol font, false otherwise.
char_width()
Returns the advance width of a single character, in font units.
Usage : $font->char_width('a');
Args : A single perl character. Can be even a unicode.
Returns : A scalar value. The width of the character in font units.
string_width()
Given a string the function returns the width of the string in font
units. The function at present only calculates the advanced width of
the each character and deducts the calculated kerning from the whole
length. If some one has any better idea then let me know.
Usage : $font->string_width("Some string");
Args : A perl string. Can be embedded unicode.
Returns : A scalar indicating the width of the whole string in font units.
get_leading()
"Leading" is the gap between two lines. The value is present in the
"OS/2" table of the font.
Usage : $metrics->get_leading();
Args : None.
Returns : A scalar Integer.
get_units_per_em()
Get "units_per_em" of the font. This value is present in the "head"
table of the font and for TTF is generally 2048.
Usage : $metrics->get_units_per_em();
Args : None.
Returns : A scalar integer.
get_ascent()
"Ascent" is the distance between the baseline to the top of the glyph.
Usage : $metrics->get_ascent();
Args : None.
Returns : A scalar integer.
get_descent()
"Descent" is the negative distance from the baseline to the lowest
point of the glyph.
Usage : $metrics->get_descent();
Args : None.
Returns : A scalar integer.
is_bold()
Returns true if the font is a bold variation of the font. That means if
you call this function of arial.ttf, it returns false. If you call this
function on arialb.ttf it returns true.
Usage : $metrics->is_bold()
Args : None.
Returns : True if the font is a bold font, returns false otherwise.
is_italic()
Returns true if the font is italic version of the font. Thar means if
you call this function on arialbi.ttf or ariali.ttf it returns true.
Usage : $metrics->is_italic()
Args : None
Returns : True if the font italic, false otherwise
get_font_family()
Returns the family name of the font.
Usage : $metrics->get_font_family()
Args : None
Returns : A scalar
get_subfamily()
Reuturns the style variation of the font in text. Note that depending
on this description might actully be pretty confusing. Call "is_bold()"
and/or "is_italic()" to detemine the style. For example a "demi"
version of the font is not "bold" by text. But in display this in
actually bold variation. In this case "is_bold()" will return true.
Usage : $metrics->get_subfamily()
Args : None
Returns : A scalar.
is_fixed_pitch()
Returns true for a fixed-pitched font like courier.
Usage : $metrics->is_fixed_pitch()
Args : None
Returns : True for a fixed-pitched font, false otherwise
SEE ALSO
Font::TTF, Pastel::Font::TTF.
COPYRIGHTS
Copyright (c) 2003 by Malay <curiouser@ccmb.res.in>. All rights
reserved.
This program is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
perl v5.14.0 2003-06-12 Font::TTFMetrics(3)
