mnerv/freetype
1
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity
Files
5edbb7a80a158583a81de286b7fcc5392916ba38
freetype/docs/FAQ
mnerv 5edbb7a80a FreeType 1.31.1
2023-08-27 18:03:45 +02:00

800 lines
30 KiB
Plaintext
Raw Blame History

FreeType User FAQ
--------------------------------------------------------------------
Table of contents
1. How do I compile the test programs?
2. What are the test programs?
3. How do I use them?
4. How do I only compile the FreeType library?
5. The library compiles fine on my system, unfortunately it seems
the test programs won't. What can I do?
6. What is FreeType, and what is it not?
7. Can FreeType be ported to other platforms?
8. My TrueType font only works on a Mac/in Windows. Will I be able
to use it with FreeType?
9. What are the most common character mappings?
10. How do I render text with FreeType?
11. How do I render rotated/slanted text with FreeType?
12. How do I compute a glyph bitmap's dimensions?
13. Why is text rendering so slow? Does FreeType cache glyphs?
14. Do you always render the same glyphs as Windows or the Mac?
15. The program `ftlint' reports errors with one of my fonts, though
it works perfectly under Windows. What does this mean?
16. What does the `fttimer' test program benchmarks?
17. Is it possible to perform styling (like oblique, italic, bold,
underline, etc.) with FreeType?
18. When does glyph hinting takes place? Is it when the glyph is
loaded, or when it is converted to a bitmap/pixmap?
19. Which is the best, caching outlines or bitmaps?
20. Can I open OpenType and/or TrueType GX fonts with FreeType?
21. How can I access additional TrueType tables that FreeType
doesn't support?
22. When will I be able to use FreeType to display TrueType fonts in
X11, Wine, or OS/2, or even other systems?
23. What does the `free' in FreeType really means?
24. Does FreeType support threads? Is it re-entrant?
25. Does FreeType support `foreign languages'?
26. I'm using fractional char sizes, but the glyphs stay at the same
integer size. Why?
27. Hinting doesn't work at very small sizes. Why?
--------------------------------------------------------------------
1. How do I compile the test programs?
Detailed compilation steps are given in the `freetype/HOWTO.txt'
file, as well as system and compiler specific hints in the
`freetype/howto' directory. What follows is a _very_ simple
guide:
For Unix:
Do the following on the prompt while in the base directory of
the FreeType package:
% ./configure
% make
% make install
This will build and install the library (shared, if supported)
together with the test programs. Say `./configure --help' to
see the available configuring options.
This should work with any ANSI C compiler.
For other platforms:
Go to the `test' directory. Look at one of the makefiles
located in the `arch/<system>' directory, and use the
appropriate makefile from there.
For example:
make -f arch/msdos/makefile.gcc (DJGPP or emx)
wmake -f=arch\msdos\makefile.wat (Watcom DOS)
nmake -f arch\os2\makefile.icc (Visual Age OS/2)
make -f arch/amigaos/makefile.gcc (GCC Amiga)
This will compile both the library and the test programs.
--------------------------------------------------------------------
2. What are the test programs?
The test programs are simple sources that `show off' the FreeType
library. They are located in the `test' directory. They make use
of a small graphics sub-system which is able to display bitmaps
and pixmaps on a variety of platforms through the use of
system-specific `drivers'.
The test programs are:
ftdump: A simple TTF information dumper. Also prints the
memory used by each opened font file with FreeType.
Doesn't use the graphics sub-system.
ftlint: A simple TrueType bytecode verifier. It simply hints
all glyphs of one or more font files at a given size
and reports errors. Doesn't use the graphics
sub-system.
ftview: A simple font viewer. Displays all glyphs in a font
file in a window.
ftstring: Renders a simple string to the screen. Demonstrates
how to produce text with FreeType.
fttimer: A simple benchmark, used to profile the scan-line
conversion routines (and only them). Supports also
display.
ftstrpnm: A version of `ftstring' which saves a bit/pixmap to a
PNM file rather than to the screen. No graphics
sub-system needed.
ftzoom: A simple glyph viewer, useful to view, zoom and rotate
individual glyphs in a font file.
fterror: This program demonstrates how to use the ftxerr18
extension. Note that internationalized strings will
be available on some platforms only (e.g. Linux).
ftmetric: A simple metric/glyph dumper. No graphics sub-system
needed.
ftsbit: Displays data about embedded bitmaps in a TrueType
font.
ftstrtto: Renders a string to screen, using TrueType Open
features.
NOTE: The file `ftdebug.c' isn't part of the test suite. It is
used only by the developers team to debug the engine at a
higher level. It won't work with a standard library
compile.
The tiny graphics subsystem is defined in the following files:
gmain.h/gmain.c: The subsystem's main body/interface.
gevents.h: The definition of events defined for the
subsystem
gdriver.h: The generic interface of all system-specific
drivers.
System-specific drivers are in the `test/arch/<system>'
directories, like:
test/arch/msdos/gfs_dos.c Full-Screen DOS driver
test/arch/os2/gpm_os2.c PM (Windowed) OS/2 driver
test/arch/unix/gwin_x11.c X11 Windowed driver
etc.
--------------------------------------------------------------------
3. How do I use them?
Please read the file freetype/README for a full description.
--------------------------------------------------------------------
4. How do I only compile the FreeType library?
For Unix:
Do a `configure' run as described in section 1. Then change to
the lib subdirectory and say `make' and `make install'.
For other platforms:
Go to the `lib' directory. Look at one of the makefiles located
in the `arch/<system>' directory, and use it from `lib'.
For example:
make -f arch/msdos/makefile.gcc (DJGPP or emx)
wmake -f=arch\msdos\makefile.wat (Watcom DOS)
nmake -f arch\os2\makefile.icc (Visual Age OS/2)
make -f arch/amigaos/makefile.gcc (GCC Amiga)
The library is pure ANSI C and doesn't depend on any
system-specific package. You need not gcc to compile it.
--------------------------------------------------------------------
5. The library compiles fine on my system, unfortunately it seems
the test programs won't. What can I do?
As said before, the test programs rely on a small graphics
sub-system to display the bitmaps and pixmaps produced by the
library. The following systems are currently supported:
DOS Full-Screen
Amiga Full-Screen Note that the windowed graphics
Amiga Windowed driver are very 'basic', they do
OS/2 Full-Screen not provide menus, dialog boxes,
OS/2 Windowed etc. Rather, they provide one
X11 Windowed windowed bitmap/pixmap and translate
Win32 Windowed events to the ones defined in
`test/gevents'.
If you want to add support to your system, you should write a new
graphics driver. To do that, read the file `test/gdriver.h' which
contains the driver generic interface, then browse the drivers
that are already provided.
The graphics drivers are located in the `test/arch/<system>'
directories. They're not part of the library proper.
IMPORTANT NOTE:
You can also directly link the library to your own application
and render glyphs directly into your own bitmaps with very few
efforts.
--------------------------------------------------------------------
6. What is FreeType, and what is it not?
FreeType is a rather low-level font engine. It has been designed
primarily to be the basis of several font servers, for very
different systems which all have their own metrics and graphics
models. This means that it focuses on the following points:
- Opening and loading font files in memory.
- Giving access to most important font properties and TrueType
tables.
- Providing a simple way to translate system-specific character
codes into glyph indexes, according to the TrueType `cmap'
specification.
- Loading and rendering _individual_ glyphs as nicely as possible
(either in outlines, bitmaps, or pixmaps), and performing
excellent hinting.
It isn't a high-level text rendering library, and many tasks will
have to be performed by higher level layers like:
- glyph caching (outlines and/or maps)
- text rendering
- justification, kerning
- font mapping
- rotated/slanted text
However, its API provides many functions that ease these
operations considerably:
- Glyph outlines can be copied and transformed easily with the API
functions, then rendered to bitmaps or pixmaps with FreeType's
scan-line converter, which is very fast.
- The glyph loader places the outlines in order to ease the
process of text rendering. See the documentation file named
`glyphs.htm' (resp. `glyphs.txt') or `ftstring's source for more
details.
- The engine gives you access to several TrueType tables that can
be processed by your application to provide more powerful
rendering (e.g. kerning).
- It is possible, and rather simple, to write separately
compilable extensions to the engine to access other TrueType
tables that are not supported by the engine in this release.
This can be handy if one wants to access data in TrueType GX or
OpenType fonts (as these formats comply to the TrueType `sfnt'
file storage format, they can be opened by the FreeType engine
as a normal TrueType file).
--------------------------------------------------------------------
7. Can FreeType be ported to other platforms?
FreeType is written in pure ANSI C and should compile well on all
16, 32, and 64 bits processors. Note, however, that the 16-bit
port requires a large memory model, as some tables found in
TrueType programs could exceed 64kByte (this is really true for
CJK and Unicode fonts. Nearly all others should work fine with
it).
It doesn't support some very exotic platforms though, like a
32-bit only processor (where the only word size is 32-bit, even
for chars and shorts). However, nothing prevents you to test it
by yourself...
The memory management and file access routines have been gathered
in two components, namely `ttmemory' and `ttfile', which can be
specialized for a specific system. Their standard version, found
in the `lib' directory, uses simply the ANSI libc. However,
examples are given of alternative file components for OS/2 and
Unix in:
lib/arch/os2/ttfile.c
(accessing the low-level OS/2 file API directly)
lib/arch/unix/ttmmap.c
(using memory-mapped files for improved access)
You would certainly need to rewrite these components if you intend
to use FreeType in an embedded system.
--------------------------------------------------------------------
8. My TrueType font only works on a Mac/in Windows. Will I be able
to use it with FreeType?
Short Answer: YES, but be cautious!
If you have read section 9 or 25, you know that a font file might
have different charMaps entries, used to translate character codes
to glyph indexes. The problem of most `system-specific' TrueType
fonts (sigh) is that they only contain one single mapping, like
Apple Roman, or Windows Glyph List, making it usable only on the
platform it was `designed' for.
The test program `ftdump' can be used to display the character
encodings supported in a font file. Most fonts come with Apple
Roman and Windows Unicode.
FreeType can use every charmap found in a font file, so it is up
to your application to choose the one that fits its task best. If
you use a font which only provides an Apple Roman charcode, you'll
probably have a hard time rendering Unicode strings without an
additional translation.
Note that the tool `ttf_edit' can actually add missing cmaps to a
TrueType font file. More info on this can be found at:
http://www.truetex.com
Another possibility is to use Just van Rossum's TTX compiler
(still beta) which can convert a TrueType font into an XML
description and vice versa; it can be found at
http://www.letterror.com/ttx/
--------------------------------------------------------------------
9. What are the most common character mappings?
If you don't want to read the TrueType specification, here is some
information about the most used char maps. Each map is designed
by a `platform ID', followed by a platform-specific `encoding ID':
Examples:
0, 0 : Apple Unicode
1, 0 : Apple Roman
3, 0 : Windows Symbol
3, 1 : Windows Unicode
Windows and Apple Unicode charmaps differ only in internal storage
layout. Both can be used transparently with FreeType.
Many fonts come also with both Apple Roman and Windows Unicode.
--------------------------------------------------------------------
10. How do I render text with FreeType?
This is explained with great detail in the glyphs documentation
file available in both text (glyphs.txt) and HTML (glyphs.htm,
including schematics).
Text rendering isn't difficult. One can also look at the code for
`ftstring' or `ftstrtto' to see how it is done.
--------------------------------------------------------------------
11. How do I render rotated/slanted text with FreeType?
It is possible to transform the outlines returned by the glyph
loader, hence producing rotated or slanted text. Please read the
`glyphs' documentation file, which explains this in great detail,
as well as some other important things.
--------------------------------------------------------------------
12. How do I compute a glyph bitmap's dimensions?
You should grid-fit its bounding box, then compute its width and
height. This is explained in the `bitmaps.txt' documentation
file.
--------------------------------------------------------------------
13. Why is text rendering so slow?
Does FreeType cache glyphs?
The FreeType engine doesn't cache anything, be it outlines or
bitmaps. Hence, a program that renders text by calling the glyph
loader on each letter is slow.
Because caching cannot be performed in both an _easy_ and
_portable_ way it is left to the application. Moreover, some
graphics systems already provide some sort of caching, and it is
better to take advantage of it rather than re-implementing it.
The `how's and `when's of caching are explained in the `glyphs'
documentation file. The `bitmaps' documentation file is also a
good source of information if you intend to render individual
glyph bitmaps.
--------------------------------------------------------------------
14. Do you always render the same glyphs as Windows or the Mac?
Short answer: No for rare cases.
There are a lot of technical details, too numerous and lengthy to
be put here, that prevents the FreeType library from matching 100%
the glyphs produced by the Windows or Macintosh rasterizers.
This engine is a clean-room implementation, and most of the
reasons why its development took so much time is the largely
deficient TrueType specification published by Apple and Microsoft.
A number of key technical aspects are missing, and we found many
`undocumented' features in the TrueType bytecode, after several
months of testing and trying. We are now very satisfied with the
current quality, though still try to improve it.
The glyph metrics (bearings and advances) also match significantly
those computed by Windows, or found in the TrueType pre-calc
tables.
As hinting glyphs also makes use of several rounding operations,
we will be unable to provide a perfect clone unless we implement
the exact same computations _and_ rounding errors -- this is
very unlikely...
--------------------------------------------------------------------
15. The program `ftlint' reports errors with one of my fonts, though
it works perfectly under Windows. What does this mean?
Associated to each glyph is a small `program', written in a
specific bytecode language, which is in charge of hinting the
glyph's outline to make it perfect on screen as on paper. Some of
these programs can be broken (e.g., accessing invalid areas of
memory, performing a divide by zero, etc.), and these are the
errors that are reported by `ftlint'.
They can also mean a bug in the TrueType bytecode interpreter (or
more likely an `undocumented' feature we haven't discovered yet),
but should now be extremely rare.
Surprisingly, even largely distributed fonts can contain broken
glyph programs. For example:
- antqua.ttf, glyph 163 (MS Office 4.2):
The program tries to access point number 0x127, which is too
much.
- timesbs, arialbs, courbs (MS Office International):
The back-slanted versions of the MS core fonts produce stack
overflows in many glyphs, and other oddities. It seems their
`maximum profile' table is invalid.
- a ton of `free' fonts, apparently designed with mediocre tools.
It seems the Windows TrueType engine doesn't check its arguments
often, and let all overflows run, possibly writing to critical
portions of memory. Please, don't ask us what this could do to
NT 4 :-)
The FreeType engine performs checks on every opcode. We cannot
guarantee that it is bullet proof, of course, but it seems to work
well and catch font bugs accordingly.
We also have artificially `enlarged' some allocated tables to make
the engine work with the back-slanted fonts without compromising
security, but it's clearly a hack we would have preferred to
avoid!
--------------------------------------------------------------------
16. What does the `fttimer' test program benchmarks?
This test program is only dedicated to profiling FreeType's
scan-line converter, a component also called `rasterizer', which
is in charge of converting a vectorial outline into a bitmap, or a
pixmap.
It simply loads all glyphs of a font file (by slices of 512 at
once), then converts them. Only the conversion is actually
benchmarked.
The glyphs are rendered at size 400pt at a resolution of 96dpi
(this is about 500 pixels high!). As you'll see by running
`fttimer', the rasterizer is very fast.
--------------------------------------------------------------------
17. Is it possible to perform styling (like oblique, italic, bold,
underline, etc.) with FreeType?
Actually, these refer to very different things:
- Italic and Bold styles usually mean many variations from the
`Regular' font. This is why you normally need a proper font
file for each of these. For example, the MS core font Times
comes in the following TrueType files:
TIMES.TTF Times New Roman Regular
TIMESI.TTF Times New Roman Italic
TIMESB.TTF Times New Roman Bold
TIMESBI.TTF Times New Roman Bold Italic
(sometimes named TIMESZ.TTF)
With FreeType, you simply need the required font file to use it.
- Oblique style refers to a transformation that is applied to a
regular font in order to make it `slanted', likes italics do.
However, an italic font very frequently contains small but
important variations that cannot be produced by this method and
make the font more appealing.
Slanting can easily be done with a transformation under
FreeType, with the exact same process as rendering rotated text.
Please read the `glyphs' documentation file where it is
explained in details.
Usually, Windows or the Macintosh produce oblique versions of a
regular font if the corresponding italic TrueType file isn't
available. They also stretch regular fonts horizontally if the
bold one isn't available. All of this can be done with trivial
transformations.
- Underlining and stroking are not really part of the glyphs.
They're simply lines that are printed on the glyph after it has
been rendered. Each TrueType file provides, in its OS/2 table
(which is accessible through the face object properties in
FreeType), several values that define the position and width of
those lines, in notional font units.
If you want to use them, you'll have to scale these values to
your current instance/point size, then draw the lines yourself.
--------------------------------------------------------------------
18. When does glyph hinting takes place? Is it when the glyph is
loaded, or when it is converted to a bitmap/pixmap?
The glyph loader returns fitted outlines by default (it can be
asked to return a non-fitted one, or simply the original outline
in notional coordinates too).
This is important to have a glyph's correct metrics, even if it is
not to be rendered immediately, like when caching outlines for
rotated text.
--------------------------------------------------------------------
19. Which is the best, caching outlines or bitmaps?
It depends on your application, and what it does with text.
Usually, if all you need is render some simple text at specific
point sizes, then simply cache the bitmaps or pixmaps.
However, if you want to do more advanced things, like rotated
text, which require sub-pixel placement to look good, you should
then only cache the outlines, and transform/place them as needed
before sending them to the scan-line converter.
It's always possible to produce a bitmap from an outline, and the
scan-converter is very fast. It's up to you then...
--------------------------------------------------------------------
20. Can I open OpenType and/or TrueType GX fonts with FreeType?
TrueType GX fonts are normal TrueType fonts with enhanced tables
and capabilities. They can always be opened by a normal TrueType
engine (like Windows, the Mac, or FreeType), but their improved
features won't be available.
On the contrary, OpenType fonts may vary. While some may contain
legal TrueType glyphs, an `otf' file may only contain glyphs
encoded in the Type 2 format. You won't be able to produce any
glyphs from such a font file without a dedicated font engine (like
the ones promised in NT 5 and Java 2D).
FreeType is a TrueType glyph engine and doesn't support Type 2
fonts. Supporting them would require a very different philosophy
and a different core engine (even though they could share an API).
Note that you can write extensions to the engine to access
supplemental tables defined in these formats (see next question).
Note that FreeType version 2 (which is in alpha stage currently)
already includes a PostScript Type 1 interpreter...
--------------------------------------------------------------------
21. How can I access additional TrueType tables that FreeType
doesn't support?
You can write an `engine extension'. This is a separately
compilable component which can enhance the base library's
functionalities, without recompiling it. Some important
extensions are provided in the `lib/extend' directory.
You'll need of course to know the TrueType specification
precisely, as well as some of the conventions used to access
tables and to manage memory. Read the documentation files for
more details, or contact the developers at:
devel@freetype.org
--------------------------------------------------------------------
22. When will I be able to use FreeType to display TrueType fonts in
X11, Wine or OS/2, or even other systems?
Actually, an OS/2 replacement for TRUETYPE.DLL based on FreeType
is part of this package (to be found in contrib/ftos2). Thanks go
to Michal Mecasek (mike@mendelu.cz) for his excellent work!
At least three X11 TrueType font servers are available; two of
them (xfsft and xtt) are based on FreeType, whereas the third
(xfstt) uses a separately developed TrueType engine.
More information about these servers is available at the FreeType
homepage (http://www.freetype.org).
Don't hesitate to contact us if you plan to use or port the engine
to exotic platforms, we're always interested in helping out.
--------------------------------------------------------------------
23. What does the `free' in FreeType really means?
The previous releases of FreeType (alphas and beta) were placed
under the LGPL. FreeType 1.0, and later releases, come with an an
alternate license, inspired from the BSD, Artistic, and IJG's
(Independent JPEG Group) ones. In short:
- You are encouraged to use and distribute this program in all
kinds of products, including commercial ones.
- You don't have to pay us anything for it (royalty-free).
- You may not pretend you wrote it, or part of it that you may use
in one of your product, and you have to state clearly that you
use the FreeType code if you distribute products based on it, or
parts of it (credits clause).
- This source code is provided AS IS, with no warranty whatsoever,
and we cannot promise support for it.
The exact and legal terms are in the file `license.txt'.
Enjoy ;-)
--------------------------------------------------------------------
24. Does FreeType support threads? Is it re-entrant?
Short answer: Basically yes, but not fully tested.
We have changed some code since FreeType 1.0 in order to support
multi-threaded environments. However, these have not been tested
yet. Apparently, the thread-safe and reentrant builds now work
well perfectly once compiled; however, no serious concurrency
testing has been performed (of course, a serious lock analysis was
done in order to modify the source).
Right now, the mutex management functions are all gathered in the
component `ttmutex', which has to be specialized for your own
system if a thread build will be necessary.
--------------------------------------------------------------------
25. Does FreeType support `foreign languages'?
Short Answer: YES, it does!
From a TrueType font file point of view, there are several parts
to the file, one of them being the `glyphs', i.e., picture
representations of the symbols.
Another part is the mapping table, also called `charMap'.
For example, glyph #1 could be letter `A', and glyph #2 could be
letter `Z'. Glyphs can be stored in any order in a font file.
The mapping tables contain at least one char-map entry. For
example, you could have an ASCII-map that maps character code 0x41
to glyph #1, and code 0x5A to glyph #2, etc. FreeType provides a
`charMap' object class to access and use this information easily.
There are several character encodings recognized and defined by
the TrueType specification, like Latin-1, Unicode, Apple Scripts,
WGL, etc., but a font file might only contain one or two of them.
When using a more `exotic' character encoding like EBCDIC (this is
IBM mainframe stuff!), you would need to translate it to one of
the available formats (or to add a charmap table to the font).
Cf. section 8.
--------------------------------------------------------------------
26. I'm using fractional char sizes, but the glyphs stay at the same
integer size. Why?
Because hinting only works well with integer pixel sizes. There
is a flag that can be set in a TrueType font file to force integer
pixel size scaling, even if you ask for fractional sizes with an
API function like TT_Set_Instance_CharSize(). Needless to say,
nearly all fonts have the flag set. So this is normal.
Also, notice that when giving a charsize in fractional points,
this will be converted to integer pixels with the formula:
pixel_size = char_size * y_resolution / 72
For example, using a resolution of 96 dpi, we would have:
9.5 pts => 12.666 pixels => 13 pixels
10.0 pts => 13.333 pixels => 13 pixels, i.e. same size
10.5 pts => 14 pixels => 14 pixels
11.0 pts => 14.666 pixels => 15 pixels
11.5 pts => 15.333 pixels => 15 pixels, i.e. same size
If you want to control the exact size in pixels of your glyphs,
simply use a resolution of 72dpi, where char_size = pixel_size.
--------------------------------------------------------------------
27. Hinting doesn't work at very small sizes. Why?
This is normal. There are very good technical reasons why hinting
doesn't work well at sizes under 7 ppem, usually meaning the
appearance of ugly spikes and glyph distortions. This is why the
engine disables hinting by default at such low sizes.
--- end of FAQ ---
Reference in New Issue View Git Blame Copy Permalink