OpenGL Programming Guide

Chapter 8Drawing Pixels, Bitmaps, Fonts, and Images

Bitmaps and Fonts

Fonts and Display Lists

Display lists are discussed in general terms in Chapter 4 . However, a few of the display-list management commands have special relevance for drawing strings of characters. As you read this section, keep in mind that the ideas presented here apply equally well to characters that are drawn using bitmap data as well as those drawn using geometric primitives (points, lines, and polygons). "Executing Multiple Display Lists" presents an example of a geometric font.

A font typically consists of a set of characters, where each character has an identifying number (usually the ASCII code), and a drawing method. For a standard ASCII character set, the capital letter A is number 65, B is 66, and so on. The string "DAB" would be represented by the three indices 68, 65, 66. In the simplest approach, display-list number 65 would draw an A, number 66 would draw a B, and so on. Then, to draw the string 68, 65, 66, just execute the corresponding display lists.

You can use the command glCallLists() in just this way.

void glCallLists(GLsizei n, GLenum type, const GLvoid *lists);

The first argument, n, indicates the number of characters to be drawn, type is usually GL_BYTE, and lists is an array of character codes.

Since many applications need to draw character strings in multiple fonts and sizes, this simplest approach isn't convenient. Instead, you'd like to use 65 as A no matter what font is currently active. You could force font 1 to encode A, B, and C as 1065, 1066, 1067, and font 2 as 2065, 2066, 2067, but then any numbers larger than 256 would no longer fit in an 8-bit byte. A better solution is to add an offset to every entry in the string and to choose the display list. In this case, font 1 has A, B, and C represented by 1065, 1066, and 1067, and in font 2, they might be 2065, 2066, and 2067. Then to draw characters in font 1, set the offset to 1000 and draw display lists 65, 66, and 67. To draw that same string in font 2, set the offset to 2000 and draw the same lists.

With this approach, use the command glListBase() to set the offset. For the preceding examples, it should be called with 1000 or 2000 as the (only) argument. Now what you need is a contiguous list of unused display-list numbers, which you can obtain from glGenLists().

GLuint glGenLists(GLsizei range); 

This function returns a block of range display-list identifiers. The returned lists are all marked as "used" even though they're empty, so that subsequent calls to glGenLists() never return the same lists (unless you've explicitly deleted them previously). Thus, if you use 4 as the argument and if glGenLists() returns 81, you can use display-list identifiers 81, 82, 83, and 84 for your characters. If glGenLists() can't find a block of unused identifiers of the requested length, it returns 0. (Note that the command glDeleteLists() makes it easy to delete all the lists associated with a font in a single operation.)

Most American and European fonts have a small number of characters (fewer than 256), so it's easy to represent each character with a different code that can be stored in a single byte. Asian fonts, among others, may require much larger character sets, so a byte-per-character encoding is impossible. OpenGL allows strings to be composed of one-, two-, three-, or four-byte characters through the type parameter in glCallLists(). This parameter can have any of the following values:

GL_BYTE GL_UNSIGNED_BYTE

GL_SHORT GL_UNSIGNED_SHORT

GL_INT GL_UNSIGNED_INT

GL_FLOAT GL_2_BYTES

GL_3_BYTES GL_4_BYTES

See "Executing Multiple Display Lists" for more information about these values.

OpenGL Programming Guide