Draw Text as 3D Objects with OpenGL: Difference between revisions

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[[Category:3D]]
= Draw Text as 3D objects with OpenGL =
There are a couple of functions in WGL ([http://msdn.microsoft.com/en-us/library/windows/desktop/ee417756(v=vs.85).aspx Windows Graphics Library]) which can be used to draw text as nice 3D objects in OpenGL. There is a well known example at [http://nehe.gamedev.net/tutorial/outline_fonts/15004/ 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 [http://stackoverflow.com/questions/3514935/3d-text-on-qglwidget-in-qt-4-6-3/3516254#3516254 this example on Stackexchange] that got me started.
There are a couple of functions in WGL ([http://msdn.microsoft.com/en-us/library/windows/desktop/ee417756(v=vs.85).aspx Windows Graphics Library]) which can be used to draw text as nice 3D objects in OpenGL. There is a well known example at [http://nehe.gamedev.net/tutorial/outline_fonts/15004/ 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 [http://stackoverflow.com/questions/3514935/3d-text-on-qglwidget-in-qt-4-6-3/3516254#3516254 this example on Stackexchange] that got me started.


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{
{
public:
public:
Text3D();
    Text3D();
void initfont(QFont & f, int thickness); // set up a font and specify the "thickness"
    void initfont(QFont & f, int thickness); // set up a font and specify the "thickness"
void print(QString text); // print it in 3D!
    void print(QString text); // print it in 3D!
 
private:
private:
void buildglyph(GLuint b, int c); // create one displaylist for character "c"
    void buildglyph(GLuint b, int c); // create one displaylist for character "c"
QFont * font;
    QFont* font;
QFontMetricsF *fm;
    QFontMetricsF* fm;
float glyphthickness;
    float glyphthickness;
GLuint base; // the "base" of our displaylists
    GLuint base; // the "base" of our displaylists
};
};
</code>
</code>
Line 50: Line 48:


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


The initialization just loops through the first 256 char's and calls buildglyph() for each of them.
The initialization just loops through the first 256 char's and calls buildglyph() for each of them.
<code>
<code>
void
void Text3D::initfont(QFont& f, float thickness)
Text3D::initfont(QFont & f, float thickness)
{
{
font = &f;
    font = &f;
fm = new QFontMetricsF(f);
    fm = new QFontMetricsF(f);
glyphthickness = thickness;
    glyphthickness = thickness;
if(base) // if we have display lists already, delete them first
    if (base) // if we have display lists already, delete them first
glDeleteLists(base, 256);
        glDeleteLists(base, 256);
    base = glGenLists(256); // generate 256 display lists
    if (base==0) {
        qDebug() << "cannot create display lists.";
        throw;
    }


base = glGenLists(256); // generate 256 display lists
    for (int i=0; i<256;++i) // loop to build the first 256 glyphs
if(base == 0)
        buildglyph(base+i, (char)i);
{
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);
}
}
</code>
</code>
The print() function uses glCallLists() to "interpret" a complete string. See below how the char-by-char advance works.
The print() function uses glCallLists() to "interpret" a complete string. See below how the char-by-char advance works.
<code>
<code>
void
void Text3D::print(QString text)
Text3D::print(QString text)
{
{
glPushAttrib(GL_LIST_BIT); // Pushes The Display List Bits
    glPushAttrib(GL_LIST_BIT); // Pushes The Display List Bits
glListBase(base); // Sets The Base Character to 0
    glListBase(base); // Sets The Base Character to 0
glCallLists(text.length(), GL_UNSIGNED_BYTE, text.toLocal8Bit()); // Draws The Display List Text
    glCallLists(text.length(), GL_UNSIGNED_BYTE, text.toLocal8Bit()); // Draws The Display List Text
glPopAttrib(); // Pops The Display List Bits
    glPopAttrib(); // Pops The Display List Bits
}
}
</code>
</code>
At the beginning we need to set up both, the tesselation and the display list.
At the beginning we need to set up both, the tesselation and the display list.
<code>
<code>
void
void Text3D::buildglyph(GLuint listbase, int c) // this is the main "workhorse" function. Create a displaylist with
Text3D::buildglyph(GLuint listbase, int c) // this is the main "workhorse" function. Create a displaylist with
  // ID "listbase" from character "c"
  // ID "listbase" from character "c"
    GLUtriangulatorObj *tobj;
    QPainterPath path;
    path.addText(QPointF(0,0),*font, QString((char)c));


GLUtriangulatorObj *tobj;
    QList<QPolygonF> poly = path.toSubpathPolygons(); // get the glyph outline as a list of paths
QPainterPath path;
    // set up the tesselation
path.addText(QPointF(0,0),*font, QString((char)c));
    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);


QList<QPolygonF> poly = path.toSubpathPolygons(); // get the glyph outline as a list of paths
    glNewList(listbase, GL_COMPILE); // start a new list
    glShadeModel(GL_FLAT);
    gluTessBeginPolygon(tobj, 0 ); // start tesselate


// set up the tesselation
    // first, calculate number of vertices.
tobj = gluNewTess();
    int elements = 0; // number of total vertices in one glyph, counting all paths.
gluTessCallback(tobj, GLU_TESS_BEGIN, (TessFuncPtr)glBegin);
    for (QList<QPolygonF>::iterator it = poly.begin(); it != poly.end(); it) {
gluTessCallback(tobj, GLU_TESS_VERTEX, (TessFuncPtr)glVertex3dv);
        elements= (*it).size();
gluTessCallback(tobj, GLU_TESS_END, (TessFuncPtr)glEnd);
    }
gluTessProperty(tobj, GLU_TESS_WINDING_RULE, GLU_TESS_WINDING_ODD);
</code>
 
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();
}
</code>
Now it's ready to tesselate the "front plate" polygon.
Now it's ready to tesselate the "front plate" polygon.
<code>
<code>
GLdouble''' vertices = (GLdouble ''') malloc(elements''' 3 * sizeof(GLdouble));
    GLdouble* vertices = (GLdouble*) malloc(elements* 3 * sizeof(GLdouble));
int j = 0;
    int j=0;
for (QList<QPolygonF>::iterator it = poly.begin(); it != poly.end(); it+'') // enumerate paths
    for (QList<QPolygonF>::iterator it = poly.begin(); it != poly.end(); it+'') { // enumerate paths
{
        gluTessBeginContour(tobj);
gluTessBeginContour(tobj);
        int i=0;
int i = 0;
        for (QPolygonF::iterator p = (*it).begin(); p != it->end(); p) { // enumerate vertices
for (QPolygonF::iterator p = (*it).begin(); p != it->end(); p) // enumerate vertices
            int off = j+i;
{
            vertices[off+0] = p->rx();
int off = j+i;
            vertices[off+1] = -p->ry();
vertices[off+0] = p->rx();
            vertices[off+2] = 0; // setting Z offset to zero.
vertices[off+1] = -p->ry();
            gluTessVertex(tobj, &vertices[off], &vertices[off] );
vertices[off+2] = 0; // setting Z offset to zero.
            i=3; // array math
gluTessVertex(tobj, &vertices[off], &vertices[off] );
        }
i''=3; // array math
        gluTessEndContour(tobj);
}
        j= (*it).size()*3; // some more array math
gluTessEndContour(tobj);
    }
j ''= (*it).size()*3; // some more array math
    gluTessEndPolygon(tobj);
}
gluTessEndPolygon(tobj);
</code>
</code>
Do the whole tesselation a second time with an offset applied for the "back plate". The "offset" (thickness) is set in
Do the whole tesselation a second time with an offset applied for the "back plate". The "offset" (thickness) is set in
<code>
<code>
gluTessBeginPolygon(tobj, 0 );
    gluTessBeginPolygon(tobj, 0 );
j = 0;
    j = 0;
for (QList<QPolygonF>::iterator it = poly.begin(); it != poly.end(); it)
    for (QList<QPolygonF>::iterator it = poly.begin(); it != poly.end(); it) {
{
        gluTessBeginContour(tobj);
gluTessBeginContour(tobj);
        int i = 0;
int i = 0;
        for (QPolygonF::iterator p = (*it).begin(); p != it->end(); p) {
for (QPolygonF::iterator p = (*it).begin(); p != it->end(); p)
            int off = j+i;
{
            vertices[off+0] = p->rx();
int off = j+i;
            vertices[off+1] = -p->ry();
vertices[off+0] = p->rx();
            vertices[off+2] = -glyphthickness; // Z offset set to "minus glyphtickness"
vertices[off+1] = -p->ry();
            gluTessVertex(tobj, &vertices[off], &vertices[off] );
vertices[off+2] = -glyphthickness; // Z offset set to "minus glyphtickness"
            i=3;
gluTessVertex(tobj, &vertices[off], &vertices[off] );
        }
i''=3;
        gluTessEndContour(tobj);
}
        j = (*it).size()*3;
gluTessEndContour(tobj);
    }
j ''= (*it).size()*3;
    gluTessEndPolygon(tobj);
}
    free(vertices); // no need for the vertices anymore
gluTessEndPolygon(tobj);
</code>


free(vertices); // no need for the vertices anymore
</code>
The "wrapping" between the two "plates" is simple compared to the tesselation.
The "wrapping" between the two "plates" is simple compared to the tesselation.
<code>
<code>
    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();
    }
</code>


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();
}
</code>
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:]]!)
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:]]!)
<code>
<code>
GLfloat gwidth = (float)fm->width©;
    GLfloat gwidth = (float)fm->width();
glTranslatef(gwidth ,0.0f,0.0f);
    glTranslatef(gwidth ,0.0f,0.0f);
 
    glEndList();
glEndList();
    gluDeleteTess(tobj);
gluDeleteTess(tobj);
}
}
</code>
</code>
The whole thing can actually be used in a init() and render() functions within a OpenGL object like this:
The whole thing can actually be used in a init() and render() functions within a OpenGL object like this:
<code>
<code>
init()
void init() {
{
    text = "Qt is great!";
text = "Qt is great!";
    QFont dfont("Comic Sans MS", 20);
QFont dfont("Comic Sans MS", 20);
    QFontMetrics fm(dfont);
QFontMetrics fm(dfont);
    textwidth = fm.width(text);
textwidth = fm.width(text);
    qDebug() << "width of text: " << textwidth;
qDebug() << "width of text: " << textwidth;
    initfont(dfont,5);
}


initfont(dfont,5);
void render() {
}
    glEnable(GL_DEPTH_TEST);


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


glMatrixMode(GL_MODELVIEW); // To operate on model-view matrix
    glRotatef(rot, 1.0f, 0.0f, 0.0f); // Rotate On The X Axis
glLoadIdentity(); // Reset the model-view matrix
    glRotatef(rot*1.5f, 0.0f, 1.0f, 0.0f); // Rotate On The Y Axis
glTranslatef(0, 0.0f, –500.0f); // Move right and into the screen
    glRotatef(rot*1.4f, 0.0f, 0.0f, 1.0f); // Rotate On The Z Axis


glRotatef(rot, 1.0f, 0.0f, 0.0f); // Rotate On The X Axis
    glColor3f( 1.0f*float(cos(rot/20.0f)), // Animate the color
glRotatef(rot*1.5f, 0.0f, 1.0f, 0.0f); // Rotate On The Y Axis
        1.0f*float(sin(rot/25.0f)),
glRotatef(rot*1.4f, 0.0f, 0.0f, 1.0f); // Rotate On The Z Axis
        1.0f-0.5f*float(cos(rot/17.0f))
    );


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);


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


glDisable(GL_DEPTH_TEST);
    rot=0.3f; // increase rot value
 
    if (rot > 2000.f)  
rot''=0.3f; // increase rot value
        rot = 0.0f; // wrap around at 2000
if(rot > 2000.f) rot = 0.0f; // wrap around at 2000
}
}
</code>
</code>
== Update: ==

Latest revision as of 15:44, 23 November 2016

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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
}