OpenGL doesn't provide direct font support,
so the application must use any of OpenGL's other features
for font rendering, such as drawing bitmaps or pixmaps,
creating texture maps containing an entire character set,
drawing character outlines, or creating 3D geometry for each
character.
Use bitmaps or pixmaps
The most straightforward method for
rendering simple fonts is to use a glBitmap() or
glDrawPixels() call for each character. The result is
simple 2D text, which is suitable for labeling GUI
controls, annotating 3D parts, etc.
glBitmap() is the fastest and simplest
of the two, and renders characters in the current color.
You can also use glDrawPixels() if required. However,
note that glDrawPixels() always draws a rectangle, so if
you desire a transparent background, it must be removed
with alpha test and/or blending.
Typically, each glBitmap() call, one
for every glyph in the font, is stored in an individual
display list, which is indexed by its ASCII character
value. Thus, a single call to glCallLists() can render an
entire string of characters.
In X Windows, the glXUseXFont() call is
available to create these display lists painlessly from a
given font.
If you're using Microsoft Windows, look
at the MSDN documentation for wglUseFontBitmaps(). It's
conceptually identical to glXUseXFonts().
For GLUT, you need to use the
glutBitmapCharacter() routine, which generates a bitmap
for the specified character from the specified GLUT
bitmap font.
Use texture mapping
In many OpenGL implementations,
rendering glBitmap() and glDrawPixels() primitives is
inherently slower than rendering an equivalent texture
mapped quad. Use texture mapped primitives to render
fonts on such devices.
The basic idea is to create a single
texture map that contains all characters in a font (or at
least all the characters that need to be rendered). To
render an individual character, draw a texture mapped
quad with texture coordinates configured to select the
desired individual character. If desired, you can use
alpha test to discard background pixels.
You can find instructions on using texture
mapped fonts here. This site
also contains image files of fonts that you can download
and read into your program to use as font texture maps.
Additional useful information on other aspects of using
OpenGL can be found at this site as well.
A library for using texture mapped fonts can be
found here. It comes with
source code.
Additional extensive information on texture
mapped text and example code, can be found here.
The NeHe web page has
a tutorial on using texture mapped fonts.
Stroked fonts
If you're using Microsoft Windows, look
up the MSDN documentation on wglUseFontOutlines(). It
contains example code for rendering stroked characters.
The glutStrokeCharacter() routine
renders a single stroked character from a specified GLUT
stroke font.
Geometric fonts
The NeHe web page has
a tutorial for rendering geometric fonts. Look for the
tutorial on outline fonts.
See the
NeHe web page for a tutorial on
using geometric fonts. Look for the tutorial on outline fonts.
See the Free Type
library.
GLTT, formerly available from http://www.moonlight3d.org/gltt/, supports geometric TrueType fonts. Click here to download GLTT
(~125KB).
Glut 3.7 has an example called progs/contrib/text3d.c
that may be informative.
