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
}