Draw Text as 3D Objects with OpenGL

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Draw Text as 3D objects with OpenGL

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 &amp; 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, &amp;vertices[off], &amp;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, &amp;vertices[off], &amp;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:

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);
 }

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
 }