How do I render text on to a square (4 vertices) in OpenGL?

Question

I'm using Linux and GLUT. I have a square as follows:

glVertex3f(-1.0, +1.0, 0.0); // top left
glVertex3f(-1.0, -1.0, 0.0); // bottom left
glVertex3f(+1.0, -1.0, 0.0); // bottom right
glVertex3f(+1.0, +1.0, 0.0); // top right

I guess I can't use glutBitmapCharacter, since this is only ideal for 2D ortho.

Simple enough, I'd like to render "Hello world!" anywhere on said square. Should I create a texture and then apply it to the vertices using glTexCoord2f?

Answer 1

The simplest way is to load a font map from a image, such as those generated by the bitmap font builder (I know it's windows but I can't find one for linux), eg:

The example is a 256x256 gif, though you may what to convert it to a png/tga/bmp. It's full ASCII mapped gird, 16x16 characters. Load the texture and use glTexCoord2f to line it up on your quad, and you should be good to go.

Here's an example using a bitmap of the above:

unsigned texture = 0;

void LoadTexture()
{
    // load 24-bit bitmap texture
    unsigned offset, width, height, size;
    char *buffer;
    FILE *file = fopen("text.bmp", "rb");
    if (file == NULL)
        return;
    fseek(file, 10, SEEK_SET);
    fread(&offset, 4, 1, file);
    fseek(file, 18, SEEK_SET);
    fread(&width, 1, 4, file);
    fread(&height, 1, 4, file);

    size = width * height * 3; 
    buffer = new char[size];

    fseek(file, offset, SEEK_SET);
    fread(buffer, 1, size, file);

    glEnable(GL_TEXTURE_2D);
    glGenTextures(1, &texture);
    glBindTexture(GL_TEXTURE_2D, texture);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, width, height, 0, GL_RGB, GL_UNSIGNED_BYTE, buffer);

    fclose(file);
    printf("Loaded\n");
}

void DrawCharacter(char c)
{
    int column = c % 16, row = c / 16;
    float x, y, inc = 1.f / 16.f;
    x = column * inc;
    y = 1 - (row * inc) - inc;

    glBegin(GL_QUADS);
      glTexCoord2f( x,       y);       glVertex3f( 0.f, 0.f, 0.f);
      glTexCoord2f( x,       y + inc); glVertex3f( 0.f, 1.f, 0.f);
      glTexCoord2f( x + inc, y + inc); glVertex3f( 1.f, 1.f, 0.f);
      glTexCoord2f( x + inc, y);       glVertex3f( 1.f, 0.f, 0.f);
    glEnd();
}

Answer 2

Indeed rendering to a bitmap is a solution. There's a decent tutorial on how to do it here on GameDev.