opengl in qt: render string to QImage to texture

gistfile1.txt

void displayText(const QString &text, const QColor &backgroundColor, const QColor &textColor, const Vec3D &pos)
    {
        // some magic number (margin, spacing, font size, etc..)
        int fontSize = 15;
        int text_width=text.size()*(fontSize/1.5)+5, text_height=fontSize+20;
        
        // create the QImage and draw txt into it
        QImage textimg(text_width, text_height, QImage::Format_RGB888);
        {
            QPainter painter(&textimg);
            painter.fillRect(0, 0, text_width, text_height, backgroundColor);
            painter.setBrush(textColor);
            painter.setPen(textColor);
            painter.setFont(QFont("Sans", fontSize));
            painter.drawText(5, 20, text);
        }

//        // very stupid debug. warning: do not use in animation loop
//        QWidget *w = new QWidget();
//        QLabel *l = new QLabel();
//        l->setPixmap(QPixmap::fromImage(textimg));
//        l->setParent(w);
//        w->show();
//        std::cout << "test image!" << std::endl;

        // now the texture
        glEnable(GL_TEXTURE_2D);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);

        glPixelStorei(GL_UNPACK_SWAP_BYTES,   GL_FALSE);
        glPixelStorei(GL_UNPACK_LSB_FIRST,    GL_FALSE);
        glPixelStorei(GL_UNPACK_ROW_LENGTH,   0);
        glPixelStorei(GL_UNPACK_IMAGE_HEIGHT, 0);
        glPixelStorei(GL_UNPACK_SKIP_ROWS,    0);
        glPixelStorei(GL_UNPACK_SKIP_PIXELS,  0);
        glPixelStorei(GL_UNPACK_SKIP_IMAGES,  0);
        glPixelStorei(GL_UNPACK_ALIGNMENT,    4);

        glTexImage2D(GL_TEXTURE_2D, 0,
                    GL_RGB8, text_width, text_height, 0,
                    GL_RGB, GL_UNSIGNED_BYTE, textimg.constBits());


        // old style GL drawing, very stupid. use gl vertex array instead
        // just for reduce the gist!
        glPushMatrix();
        glTranslatef(pos.x, pos.y, pos.z);
        glBegin(GL_QUADS);
        glTexCoord2f(1, 0); glVertex3f(0, 0, 0);
        glTexCoord2f(0, 0); glVertex3f(text_width, 0, 0);
        glTexCoord2f(0, 1); glVertex3f(text_width, text_height, 0);
        glTexCoord2f(1, 1); glVertex3f(0, text_height, 0);
        glEnd();
        glPopMatrix();
        

        glDisable(GL_TEXTURE_2D);

        glFlush();
    }