OpenGL String (printw)

A Technical Blog article. View original blog here.[^]

There are many ways for displaying text in OpenGL. In this post, I'll use the glutBitmapCharacter function to draw characters, and then will extend it to a more generic function (printw), which will make drawing a string in OpenGL as simple as using printf.

First, I will start by defining the font to be used by the glutBitmapCharacter function. I will choose a 24-point proportional spaced Times Roman font as shown below:

//  A pointer to a font style..
//  Fonts supported by GLUT are: GLUT_BITMAP_8_BY_13,
//  GLUT_BITMAP_9_BY_15, GLUT_BITMAP_TIMES_ROMAN_10,
//  GLUT_BITMAP_TIMES_ROMAN_24, GLUT_BITMAP_HELVETICA_10,
//  GLUT_BITMAP_HELVETICA_12, and GLUT_BITMAP_HELVETICA_18.
GLvoid *font_style = GLUT_BITMAP_TIMES_ROMAN_24;

To draw a character, simply define the raster position using glRasterPos3f (x, y, z), then draw it using glutBitmapCharacter(font_style, character). To extend this to drawing a string at certain coordinates, loop over each character in the string and draw it with glutBitmapCharacter. The additional logic that goes below is in making this as pretty as printf.

This can be done by using C’s va_list type, which is C’s approach to defining functions with variables number of arguments.

In the function prototype, place ellipsis (…) as the last argument
Define a variable argument list: va_list args;
Call va_start on the args list and the first real argument in the function prototype preceding the ellipsis: va_start(args, format);
Use _vscprintf to get the number of characters that would be generated if the string pointed to by the list of arguments was printed using the specified format
Allocate memory for a string with the specified number of characters
Call vsprintf_s to build the string we want from the list of arguments
Call va_end to end the use of the variables argument list
Draw our beautified string
Free allocated memory

Function implementation with full comments are shown below:

//-------------------------------------------------------------------------
//  Draws a string at the specified coordinates.
//-------------------------------------------------------------------------
void printw (float x, float y, float z, char* format, ...)
{
    va_list args;   //  Variable argument list
    int len;        // String length
    int i;          //  Iterator
    char * text;    // Text

    //  Initialize a variable argument list
    va_start(args, format);

    //  Return the number of characters in the string referenced the list of arguments.
    // _vscprintf doesn't count terminating '\0' (that's why +1)
    len = _vscprintf(format, args) + 1;

    //  Allocate memory for a string of the specified size
    text = malloc(len * sizeof(char));

    //  Write formatted output using a pointer to the list of arguments
    vsprintf_s(text, len, format, args);

    //  End using variable argument list
    va_end(args);

    //  Specify the raster position for pixel operations.
    glRasterPos3f (x, y, z);

    //  Draw the characters one by one
    for (i = 0; text[i] != '\0'; i++)
        glutBitmapCharacter(font_style, text[i]);

    //  Free the allocated memory for the string
    free(text);
}

Now that printw is defined, using it is similar to printf:

printf(         "char: %c, decimal: %d, float: %f, 
		string: %s", 'X', 1618, 1.618, "text");
printw(x, y, z, "char: %c, decimal: %d, float: %f, 
		string: %s", 'X', 1618, 1.618, "text");

Libraries to include before copying the function are:

#include <stdio.h>    //  Standard Input\Output C Library
#include <stdarg.h>   //  To use functions with variables arguments
#include <stdlib.h>   //  for malloc
#include <gl/glut.h>  //  Include GLUT, OpenGL, and GLU libraries

Here is a sample source code that contains the printw function. If you have any issues compiling or running the app, check out this section for details about compiling and running an OpenGL app that uses the GLUT library. Below is a screen shot of the demo app:

printw