Draw Text as 3D Objects with OpenGL

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There are a couple of functions in WGL (Windows Graphics Library) which can be used to draw text as nice 3D objects in OpenGL. There is a well known example at NeHe. However, this is not portable at all, and since I'm using Qt anyway, I was looking for a way to have this done with Qt. I was surprised that there was no such function already available within Qt, but then I stumbled across this example on Stackexchange that got me started.

Before I get to the code, some drawbacks of this example:

  • It uses the fixed-function pipeline. (GL_QUAD_STRIP's and DisplayLists). Im sure this can be done in a "more modern" way with VBO's, but my OpenGL knowlegde is not yet at that level.
  • it relies on GLU for polygon tesselation. There might be better alternatives around or even some within Qt.
  • No real character set (or even UTF) handling. It only uses the first 256 characters.
  • side effects on the matrix.

The example uses QFont to get the font outline for each character (glyph). The basic idea is to create two flat outline-polygons for the front- and back-"plane" of a glyph and then create the "wrapping" in between the front- and backplane. Although it seems more difficult at first, it was pretty easy to create the wrapping in between the two outline-polygons with GL_QUAD_STRIP. The tricky bit was the polygon tesselation of the glyph outline, because the glyph-polygons are not concave and may have one or more holes. I'm using the polygon tesselation facility available in GLU.

The text3d class can be subclassed by a GLWidget or GLWindow object. There are only 2 functions required to draw text: initfont() and print(). The initialization of the font cannot easily be done in the constructor, because the contest is probably not initialized during construction. Therefore the initfont().

text3d.h

#include <QOpenGLFunctions>
#include <QString>
#include <QFont>
#include <QFontMetricsF>

class Text3D
{
public:
    Text3D();
    void initfont(QFont & f, int thickness); // set up a font and specify the "thickness"
    void print(QString text); // print it in 3D!
private:
    void buildglyph(GLuint b, int c); // create one displaylist for character "c"
    QFont* font;
    QFontMetricsF* fm;
    float glyphthickness;
    GLuint base; // the "base" of our displaylists
};

The implementation file: text3d.cpp

#include <QFont>
#include <QList>
#include <QPainter>
#include <QOpenGLFunctions>
#include <QChar>
#include <gl/GLU.h>
#include "text3d.h"

typedef void (__stdcall *TessFuncPtr)(); // defintion of the callback function type

Text3D::Text3D() // nothing special in the constructor
    : glyphthickness(1.0f)
    , base(0)
{
}

The initialization just loops through the first 256 char's and calls buildglyph() for each of them.

void Text3D::initfont(QFont& f, float thickness)
{
    font = &f;
    fm = new QFontMetricsF(f);
    glyphthickness = thickness;
    if (base) // if we have display lists already, delete them first
        glDeleteLists(base, 256);
    base = glGenLists(256); // generate 256 display lists
    if (base==0) {
        qDebug() << "cannot create display lists.";
        throw;
    }

    for (int i=0; i<256;++i) // loop to build the first 256 glyphs
        buildglyph(base+i, (char)i);
}

The print() function uses glCallLists() to "interpret" a complete string. See below how the char-by-char advance works.

void Text3D::print(QString text)
{
    glPushAttrib(GL_LIST_BIT); // Pushes The Display List Bits
    glListBase(base); // Sets The Base Character to 0
    glCallLists(text.length(), GL_UNSIGNED_BYTE, text.toLocal8Bit()); // Draws The Display List Text
    glPopAttrib(); // Pops The Display List Bits
}

At the beginning we need to set up both, the tesselation and the display list.

void Text3D::buildglyph(GLuint listbase, int c) // this is the main "workhorse" function. Create a displaylist with
 // ID "listbase" from character "c"
    GLUtriangulatorObj *tobj;
    QPainterPath path;
    path.addText(QPointF(0,0),*font, QString((char)c));

    QList<QPolygonF> poly = path.toSubpathPolygons(); // get the glyph outline as a list of paths
    // set up the tesselation
    tobj = gluNewTess();
    gluTessCallback(tobj, GLU_TESS_BEGIN, (TessFuncPtr)glBegin);
    gluTessCallback(tobj, GLU_TESS_VERTEX, (TessFuncPtr)glVertex3dv);
    gluTessCallback(tobj, GLU_TESS_END, (TessFuncPtr)glEnd);
    gluTessProperty(tobj, GLU_TESS_WINDING_RULE, GLU_TESS_WINDING_ODD);

    glNewList(listbase, GL_COMPILE); // start a new list
    glShadeModel(GL_FLAT);
    gluTessBeginPolygon(tobj, 0 ); // start tesselate

    // first, calculate number of vertices.
    int elements = 0; // number of total vertices in one glyph, counting all paths.
    for (QList<QPolygonF>::iterator it = poly.begin(); it != poly.end(); it) {
        elements= (*it).size();
    }

Now it's ready to tesselate the "front plate" polygon.

    GLdouble* vertices = (GLdouble*) malloc(elements* 3 * sizeof(GLdouble));
    int j=0;
    for (QList<QPolygonF>::iterator it = poly.begin(); it != poly.end(); it+'') { // enumerate paths
        gluTessBeginContour(tobj);
        int i=0;
        for (QPolygonF::iterator p = (*it).begin(); p != it->end(); p) { // enumerate vertices
            int off = j+i;
            vertices[off+0] = p->rx();
            vertices[off+1] = -p->ry();
            vertices[off+2] = 0; // setting Z offset to zero.
            gluTessVertex(tobj, &vertices[off], &vertices[off] );
            i=3; // array math
        }
        gluTessEndContour(tobj);
        j= (*it).size()*3; // some more array math
    }
    gluTessEndPolygon(tobj);

Do the whole tesselation a second time with an offset applied for the "back plate". The "offset" (thickness) is set in

    gluTessBeginPolygon(tobj, 0 );
    j = 0;
    for (QList<QPolygonF>::iterator it = poly.begin(); it != poly.end(); it) {
        gluTessBeginContour(tobj);
        int i = 0;
        for (QPolygonF::iterator p = (*it).begin(); p != it->end(); p) {
            int off = j+i;
            vertices[off+0] = p->rx();
            vertices[off+1] = -p->ry();
            vertices[off+2] = -glyphthickness; // Z offset set to "minus glyphtickness"
            gluTessVertex(tobj, &vertices[off], &vertices[off] );
            i=3;
        }
        gluTessEndContour(tobj);
        j = (*it).size()*3;
    }
    gluTessEndPolygon(tobj);
    free(vertices); // no need for the vertices anymore

The "wrapping" between the two "plates" is simple compared to the tesselation.

    for (QList<QPolygonF>::iterator it = poly.begin(); it != poly.end(); it) {
        glBegin(GL_QUAD_STRIP);
        QPolygonF::iterator p;
        for (p = (*it).begin(); p != it->end(); p) {
            glVertex3f(p->rx(), -p->ry(), 0.0f);
            glVertex3f(p->rx(), -p->ry(), -glyphthickness);
        }
        p = (*it).begin();
        glVertex3f(p->rx(), -p->ry(), 0.0f); // draw the closing quad
        glVertex3f(p->rx(), -p->ry(), -glyphthickness); // of the "wrapping"
        glEnd();
    }

This is where the char-by-char advance is done. Get the width from the font metrics and apply a glTranslate() with that value. This goes into the displaylist as well. (This may have side-effects as the matrix is not in the same "state" as before the call[[Image:|Image:]]!)

    GLfloat gwidth = (float)fm->width();
    glTranslatef(gwidth ,0.0f,0.0f);
    glEndList();
    gluDeleteTess(tobj);
}

The whole thing can actually be used in a init() and render() functions within a OpenGL object like this:

void init() {
    text = "Qt is great!";
    QFont dfont("Comic Sans MS", 20);
    QFontMetrics fm(dfont);
    textwidth = fm.width(text);
    qDebug() << "width of text: " << textwidth;
    initfont(dfont,5);
}

void render() {
    glEnable(GL_DEPTH_TEST);

    glMatrixMode(GL_MODELVIEW); // To operate on model-view matrix
    glLoadIdentity(); // Reset the model-view matrix
    glTranslatef(0, 0.0f, 500.0f); // Move right and into the screen

    glRotatef(rot, 1.0f, 0.0f, 0.0f); // Rotate On The X Axis
    glRotatef(rot*1.5f, 0.0f, 1.0f, 0.0f); // Rotate On The Y Axis
    glRotatef(rot*1.4f, 0.0f, 0.0f, 1.0f); // Rotate On The Z Axis

    glColor3f( 1.0f*float(cos(rot/20.0f)), // Animate the color
        1.0f*float(sin(rot/25.0f)),
        1.0f-0.5f*float(cos(rot/17.0f))
    );


    glTranslatef(-textwidth/2.0f, 0.0f, 0.0f); // textwidth holds the pixel width of the text
    // Print GL Text To The Screen
    print(text);

    glDisable(GL_DEPTH_TEST);

    rot=0.3f; // increase rot value
    if (rot > 2000.f) 
        rot = 0.0f; // wrap around at 2000
}