OpenGL Programming Guide

Chapter 8Drawing Pixels, Bitmaps, Fonts, and Images

Bitmaps and Fonts

A bitmap is a rectangular array of 0s and 1s that serves as a drawing mask for a corresponding rectangular portion of the window. Suppose you're drawing a bitmap and that the current color is red. Everywhere there's a 1 in the bitmap, the corresponding pixel is replaced by a red pixel (or combined with the red pixel, depending on which per-fragment operations are in effect; see "Testing and Operating on Fragments" ). If there's a 0 in the bitmap, the contents of the pixel are unaffected. The most common use for bitmaps is for drawing characters on the screen.

OpenGL provides only the lowest level of support for drawing strings of characters and manipulating fonts. The commands glRasterPos*() and glBitmap() position and draw a single bitmap on the screen. In addition, through the display-list mechanism, you can use a sequence of character codes to index into a corresponding series of bitmaps representing those characters. See Chapter 4 for more information about display lists. You'll have to write your own routines to provide any other support you need for manipulating bitmaps, fonts, and strings of characters.

As an example, consider Example 8-1 , which draws the character F three times on the screen. Figure 8-1 shows the F as a bitmap and its corresponding bitmap data.

Figure 8-1 : A Bitmapped F and Its Data

Example 8-1 : Drawing a Bitmapped Character: drawf.c

#include <GL/gl.h>
#include <GL/glu.h>
#include "aux.h"

GLubyte rasters[24] = {
    0xc0, 0x00, 0xc0, 0x00, 0xc0, 0x00, 0xc0, 0x00, 
    0xc0, 0x00, 0xff, 0x00, 0xff, 0x00, 0xc0, 0x00, 
    0xc0, 0x00, 0xc0, 0x00, 0xff, 0xc0, 0xff, 0xc0};

void myinit(void)
{
    glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
    glClearColor(0.0, 0.0, 0.0, 0.0);
}

void display(void)
{
    glClear(GL_COLOR_BUFFER_BIT);
    glColor3f(1.0, 1.0, 1.0);
    glRasterPos2i (20.5, 20.5);
    glBitmap(10, 12, 0.0, 0.0, 12.0, 0.0, rasters);
    glBitmap(10, 12, 0.0, 0.0, 12.0, 0.0, rasters);
    glBitmap(10, 12, 0.0, 0.0, 12.0, 0.0, rasters);
    glFlush();
}

void myReshape(GLsizei w, GLsizei h)
{
    glViewport(0, 0, w, h);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrtho(0, w, 0, h, -1.0, 1.0);
    glMatrixMode(GL_MODELVIEW);
}


int main(int argc, char** argv)
{
    auxInitDisplayMode(AUX_SINGLE | AUX_RGBA);
    auxInitPosition(0, 0, 500, 500);
    auxInitWindow(argv[0]);
    myinit();
    auxReshapeFunc(myReshape);
    auxMainLoop(display);
}

In Figure 8-1 , note that the visible part of the F character is at most 10 bits wide. Bitmap data is always stored in chunks that are multiples of 8 bits, but the width of the actual bitmap doesn't have to be a multiple of 8. The bits making up a bitmap are drawn starting from the lower left corner: First, the bottom row is drawn, then the next row above it, and so on. As you can tell from the code, the bitmap is stored in memory in this order - the array of rasters begins with 0xc0, 0x00, 0xc0, 0x00 for the bottom two rows of the F and continues to 0xff, 0xc0, 0xff, 0xc0 for the top two rows.

The commands of interest in this example are glRasterPos2i() and glBitmap(); they're discussed in detail in the next section. For now, ignore the call to glPixelStorei(); it describes how the bitmap data is stored in computer memory. This topic is discussed in "Controlling Pixel-Storage Modes."

The Current Raster Position
Drawing the Bitmap
Fonts and Display Lists
Defining and Using a Complete Font

OpenGL Programming Guide