glutBitmapCharacter positions text wrong

Question

I'm trying to draw a simple string (overlay) on the screen.

From what I've found over the internet, I'm using it this way:

void write(string text, int x, int y){
    glRasterPos2i(x,y);

    for(int i = 0; i < text.length(); i++){ 
        glutBitmapCharacter(GLUT_BITMAP_HELVETICA_12, text.data()[i]);
    }
}

But it draws the string according to the world coordinates. Say, if x and y are set to 10, they are drawn at (10,10,0) coordinates in the world. But I simple need this string at window's (10,10) coordinates in 2D.

This is part of a small project, and the draw method is given as below. I don't want to change it much as it may break something else in the project.

void disp(){
 // set viewing translation and object rotations
 glMatrixMode( GL_MODELVIEW );
 glLoadIdentity ();
 glTranslatef( INIT_VIEW_X, INIT_VIEW_Y, INIT_VIEW_Z );
 glRotatef( xRot, 1.0, 0.0, 0.0 );
 glRotatef( zRot, 0.0, 0.0, 1.0 );
 glScalef( scaleFactor, scaleFactor, scaleFactor );
 glClear(GL_COLOR_BUFFER_BIT);
 draw();
 glFlush();
}

I also don't exactly know what they do, and I think drawing the text to world coordinates have to do with something in this code. I've also tried Using GLUT bitmap fonts but it doesn't work either.

How can I simple draw onto the screen. OpenGL is over complicating things; I try to simply write to the window, but it takes the whole thing and translates into 3D world. I just don't want this.

Answer 1

From twall, yes you need to clear BOTH the modelview and projection matrices

//TEXT
glMatrixMode( GL_PROJECTION ) ;
glPushMatrix() ; // save
glLoadIdentity();// and clear
glMatrixMode( GL_MODELVIEW ) ;
glPushMatrix() ;
glLoadIdentity() ;

glDisable( GL_DEPTH_TEST ) ; // also disable the depth test so renders on top

glRasterPos2f( 0,0 ) ; // center of screen. (-1,0) is center left.
glColor4f(1.0f, 1.0f, 1.0f, 1.0f);
char buf[300];
sprintf( buf, "Oh hello" ) ;
const char * p = buf ;
do glutBitmapCharacter( GLUT_BITMAP_HELVETICA_18, *p ); while( *(++p) ) ;

glEnable( GL_DEPTH_TEST ) ; // Turn depth testing back on

glMatrixMode( GL_PROJECTION ) ;
glPopMatrix() ; // revert back to the matrix I had before.
glMatrixMode( GL_MODELVIEW ) ;
glPopMatrix() ;

Answer 2

From what I can gather, it seems what you're after is orthographic projection.

I'm not sure about OpenGL specifically (I'm used to higher-level graphics libraries), but here's a couple of links that could be a starting point:

Setting a orthographic projection matrix?

http://www.cprogramming.com/tutorial/opengl_projections.html

Answer 3

you can step back to "2D" coordinates like so:

//save your current matrix on the stack, so you don't lose it
glPushMatrix();
//load identity matrix, so you don't have any 3d transformations
glLoadIdentity ();
//now you also can transform the text
//to the position just as you like
//glTransformf( ... )
//make sure the text is draw, even though it might be outside the viewing area
glDisable( GL_DEPTH_TEST )
drawMyFunkyText();
glEnable( GL_DEPTH_TEST )
//restore the matrix as it was before,
//so you can draw all your shapes as before
glPopMatrix();