Drawing bitmap fonts with OpenGL, what does glRasterPos2i() do? - c++

Question

This is another one of those "I have a blank screen, please help me fix it" moments.
This example is from The OpenGL Programming Guide, Version 2.1, Page 311-312.
The example is supposed to draw 2 lines of text on the screen.
Past of the problem I think is that I don't understand how glRasterPos2i() works. Does it:
A:) Set the position of bitmaps to be drawn in the 3D world in homogeneous / "OpenGL coordinates"
B:) Set the position of bitmaps to be drawn on the screen in pixel coordinates
Here is the code I have so far: You can pretty much ignore the first big lump which defines what the bitmaps are.
#include <GL/glut.h>
#include <cstdlib>
#include <iostream>
#include <cstring>
// This first bit is kind of irreverent, it sets up some fonts in memory as bitmaps
GLubyte space[] = {0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 };
GLubyte letters[][13] = {
{ 0x00, 0x00, 0xc3, 0xc3, 0xc3, 0xc3, 0xff, 0xc3, 0xc3, 0xc3, 0x66, 0xc3, 0x18 },
{ 0x00, 0x00, 0xfe, 0xc7, 0xc3, 0xc3, 0xc7, 0xfe, 0xc7, 0xc3, 0xc3, 0xc7, 0xfe },
{ 0x00, 0x00, 0x7e, 0xe7, 0xc0, 0xc0, 0xc0, 0xc0, 0xc0, 0xc0, 0xc0, 0xe7, 0x7e },
{ 0x00, 0x00, 0xfc, 0xce, 0xc7, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3, 0xc7, 0xce, 0xfc },
{ 0x00, 0x00, 0xff, 0xc0, 0xc0, 0xc0, 0xc0, 0xfc, 0xc0, 0xc0, 0xc0, 0xc0, 0xff },
{ 0x00, 0x00, 0xc0, 0xc0, 0xc0, 0xc0, 0xc0, 0xc0, 0xfc, 0xc0, 0xc0, 0xc0, 0xff },
{ 0x00, 0x00, 0x7e, 0xe7, 0xc3, 0xc3, 0xcf, 0xc0, 0xc0, 0xc0, 0xc0, 0xe7, 0x7e },
{ 0x00, 0x00, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3, 0xff, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3 },
{ 0x00, 0x00, 0x7e, 0x18, 0x18, 0x18, 0x18, 0x18, 0x18, 0x18, 0x18, 0x18, 0x7e },
{ 0x00, 0x00, 0x7c, 0xee, 0xc6, 0x06, 0x06, 0x06, 0x06, 0x06, 0x06, 0x06, 0x06 },
{ 0x00, 0x00, 0xc3, 0xc6, 0xcc, 0xd8, 0xf0, 0xe0, 0xf0, 0xd8, 0xcc, 0xc6, 0xc3 },
{ 0x00, 0x00, 0xff, 0xc0, 0xc0, 0xc0, 0xc0, 0xc0, 0xc0, 0xc0, 0xc0, 0xc0, 0xc0 },
{ 0x00, 0x00, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3, 0xdb, 0xff, 0xff, 0xe7, 0xc3 },
{ 0x00, 0x00, 0xc7, 0xc7, 0xcf, 0xcf, 0xdf, 0xdb, 0xfb, 0xf3, 0xf3, 0xe3, 0xe3 },
{ 0x7e, 0xe7, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3, 0xe7, 0x7e },
{ 0xc0, 0xc0, 0xc0, 0xc0, 0xc0, 0xfe, 0xc0, 0xf3, 0xc7, 0xc3, 0xc3, 0xc7, 0xfe },
{ 0x00, 0x00, 0x3f, 0x6e, 0xdf, 0xdb, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3, 0x66, 0x3c },
{ 0x00, 0x00, 0xc3, 0xc6, 0xcc, 0xd8, 0xf0, 0xfe, 0xc7, 0xc3, 0xc3, 0xc7, 0xfe },
{ 0x00, 0x00, 0x7e, 0xe7, 0x03, 0x03, 0x07, 0x7e, 0xe0, 0xc0, 0xc0, 0xe7, 0x7e },
{ 0x00, 0x00, 0x18, 0x18, 0x18, 0x18, 0x18, 0x18, 0x18, 0x18, 0x18, 0x18, 0xff },
{ 0x00, 0x00, 0x7e, 0xe7, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3 },
{ 0x00, 0x00, 0x18, 0x3c, 0x3c, 0x66, 0x66, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3 },
{ 0x00, 0x00, 0xc3, 0xe7, 0xff, 0xff, 0xdb, 0xdb, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3 },
{ 0x00, 0x00, 0xc3, 0x66, 0x66, 0xc3, 0xc3, 0x18, 0xc3, 0xc3, 0x66, 0x66, 0xc3 },
{ 0x00, 0x00, 0x18, 0x18, 0x18, 0x18, 0x18, 0x18, 0xc3, 0xc3, 0x66, 0x66, 0xc3 },
{ 0x00, 0x00, 0xff, 0xc0, 0xc0, 0x60, 0x30, 0x7e, 0x0c, 0x06, 0x03, 0x03, 0xff }
};
// This is just copying from the book
GLuint fontOffset;
void makeRasterFont()
{
GLuint i, j;
glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
fontOffset = glGenLists(128);
for(i = 0, j = 'A'; i < 26; i ++, j ++)
{
glNewList(fontOffset + ' ', GL_COMPILE);
glBitmap(8, 13, 0.0, 2.0, 10.0, 0.0, letters[i]);
glEndList();
}
glNewList(fontOffset + ' ', GL_COMPILE);
glBitmap(8, 13, 0.0, 2.0, 10.0, 0.0, space);
glEndList();
}
void init()
{
glShadeModel(GL_FLAT);
makeRasterFont();
}
void printString(char* s)
{
glPushAttrib(GL_LIST_BIT);
glListBase(fontOffset);
glCallLists(std::strlen(s), GL_UNSIGNED_BYTE, (GLubyte*)s);
glPopAttrib();
}
void display()
{
GLfloat white[3] = {1.0, 1.0, 1.0 };
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glColor3fv(white);
// Print some text on the screen at (20,60) and (20,40)
glRasterPos2i(20, 60);
printString("THE QUICK BROWN FOX JUMPS");
glRasterPos2i(20, 40);
printString("OVER A LAZY DOG");
glFlush();
}
void reshape(int w, int h)
{
// Set the viewport
glViewport(0, 0, (GLsizei)w, (GLsizei)h);
// Set viewing mode
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(45.0, (GLfloat)w / (GLfloat)h, 0.01, 100.0);
glMatrixMode(GL_MODELVIEW);
}
int main(int argc, char** argv)
{
/* Init glut with a single buffer display mode,
* window size, position and title */
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB | GLUT_DEPTH);
glutInitWindowSize(500, 500);
glutInitWindowPosition(100, 100);
glutCreateWindow(argv[0]);
// Call init routine to set OpenGL specific initialization values
init();
// Set callback function
glutDisplayFunc(display);
glutReshapeFunc(reshape);
// Enter main loop
glutMainLoop();
return EXIT_SUCCESS;
}
Sorry for the type of question - I hate just asking "please fix my code", because really I should be able to fix it myself. On this occasion I find myself, stuck, basically. Thanks for you time and help.
Solution:
For those interested, to "get it to work", the changes made were:
1: Change gluPerspective to gluOrtho2D(0, width, 0, height).
2: Change glnewList(fontOffset + ' ', GL_COMPILE) to glnewList(fontOffset + j, GL_COMPILE) - not BOTH, just the FIRST ONE IN THE LOOP.
3: Set the glRasterPos2i to be anywhere within the region specified by glOrtho2D. My width and height are both 500, so I used coordinates (20, 60) and then (20, 40).
You could have just left it with gluPerspective, and used coordinates about (0,0) without specifying any transformations. However, since a bitmap is 2D I think this is less intuitive.

Answer 1

As to your rendering problem, hint, you don't use j...
In the for loop:
glNewList(fontOffset + ' ', GL_COMPILE);
replace your space with the letter you want.

Answer 2

The glRasterPos function specifies the raster position in object coordinates. Those are passed through the current modelview and projection matrices (at the time of the glRasterPos-call) to get the actual raster position in window (viewport) coordinates to be used for things like glDrawPixels and glBitmap (thus option A). So given your current perspective projection and identity modelview, those (20,40) (which are probably meant as pixels) are quite off the screen. If you want to specify it in pixels (which is usually the case), you need to setup your transformation pipeline accordingly.
But I wouldn't recommend using those old and deprecated (and likely slooow) pixel drawing functions at all (and neither to learn from the unfortunately awfully outdated Redbook). Just draw a textured quad with a custom shader that just takes window coordinates.