Using Fonts and Strings

Using Fonts and Strings

The simplest approach to text and font handling in GLX is using the glXUseXFont() function together with display lists. This section shows you how to use the function by providing an example program. Note that this information is relevant regardless of whether you use widgets or program in Xlib.

The advantage of glXUseXFont() is that bitmaps for X glyphs in the font match exactly what OpenGL draws. This solves the problem of font matching between X and OpenGL display areas in your application.

To use display lists to display X bitmap fonts, your code should do the following:

Use X calls to load information about the font you want to use.
Generate a series of display lists using glXUseXFont(), one for each glyph in the font.
The glXUseXFont() function automatically generates display lists (one per glyph) for a contiguous range of glyphs in a font.
To display a string, use glListBase() to set the display list base to the base for your character series. Then pass the string as an argument to glCallLists().
Each glyph display list contains a glBitmap() call to render the glyph and update the current raster position based on the glyph's width.

The example code fragment provided in Example 3-4 prints the string "The quick brown fox jumps over a lazy dog" in Times Medium. It also prints the entire character set, from ASCII 32 to 127.

Note: You can also use the glc library, which sits atop of OpenGL, for fonts and strings. The library is not specific to GLX and lets you do more than glXUseXFont().

Example 3-4 : Font and Text Handling

#include <GL/gl.h>
#include <GL/glu.h>
#include <GL/glx.h>
#include <X11/Xlib.h>
#include <X11/Xutil.h>

GLuint base;

void makeRasterFont(Display *dpy)
{
    XFontStruct *fontInfo;
    Font id;
    unsigned int first, last;
    fontInfo = XLoadQueryFont(dpy, 
        "-adobe-times-medium-r-normal--17-120-100-100-p-88-iso8859-1");
    

if (fontInfo == NULL) {
        printf ("no font found\n");
        exit (0);
    }

    id = fontInfo->fid;
    first = fontInfo->min_char_or_byte2;
    last = fontInfo->max_char_or_byte2;

    base = glGenLists(last+1);
    if (base == 0) {
        printf ("out of display lists\n");
    exit (0);
    }
    glXUseXFont(id, first, last-first+1, base+first);
}

void printString(char *s)
{
    glListBase(base);
    glCallLists(strlen(s), GL_UNSIGNED_BYTE, (unsigned char *)s);
}

void display(void)
{
    GLfloat white[3] = { 1.0, 1.0, 1.0 };
    long i, j;
    char teststring[33];

    glClear(GL_COLOR_BUFFER_BIT);
    glColor3fv(white);
    for (i = 32; i < 127; i += 32) {
        glRasterPos2i(20, 200 - 18*i/32);
        for (j = 0; j < 32; j++)
            teststring[j] = i+j;
        teststring[32] = 0;
        printString(teststring);
    }
    glRasterPos2i(20, 100);
    printString("The quick brown fox jumps");
    glRasterPos2i(20, 82);
    printString("over a lazy dog.");
    glFlush ();
}