Picking with glRenderMode

Question

I try to do OpenGL picking so I followed this post (OpenGL Picking on Selection Mode ) which seems to have a working solution but I can't make mine work. I have a 3D textured polygon in my scene and I would like to know when I click on it.

Here is my code :

The mouse method which launches the picking when right click :

void mousePressEvent(QGraphicsSceneMouseEvent *event)
{
    if(event->button()==Qt::RightButton){
        pickObjects(event->pos().x(),event->pos().y());
    }
}

The pickObjects method:

#define BUFSIZE 512
void pickObjects(int x, int y)
{
    GLint viewport[4];
    GLint hits;

    GLuint selectBuf[BUFSIZE];
    glSelectBuffer (BUFSIZE, selectBuf);

    glRenderMode (GL_SELECT);

    glMatrixMode (GL_PROJECTION);
    glPushMatrix ();
    glLoadIdentity ();

    glGetIntegerv (GL_VIEWPORT, viewport);

    gluPickMatrix ((GLdouble) x, (GLdouble) (viewport[3] - y), 5.0, 5.0, viewport);
    gluPerspective(fov, this->width() / this->height(), 0.1, 1000);

    glMatrixMode(GL_MODELVIEW);

    glInitNames();

    makeCustomAnnot(GL_SELECT);

    int hits=0;

    glMatrixMode (GL_PROJECTION);
    glPopMatrix ();

    glMatrixMode(GL_MODELVIEW);

    hits = glRenderMode (GL_RENDER);
    if (hits != 0)
    {
        cout<<"FOUND " << hits << " hit(s)"<<endl; //ALWAYS GIVES 0 HITS
        processHits(hits,selectBuf);
    }

}

The processHits method :

void processHits(GLint inHits, GLuint buffer[])
{
    unsigned int i, j;
    GLuint names, *ptr, minZ,*ptrNames, numberOfNames;

    ptr = (GLuint *) buffer;
    minZ = 0xffffffff;
    for (i = 0; i < inHits; i++) {
        names = *ptr;
        ptr++;
        if (*ptr < minZ) {
            numberOfNames = names;
            minZ = *ptr;
            ptrNames = ptr+2;
        }
        ptr += names+2;
    }

    cout << "Nearest: ";
    ptr = ptrNames;
    for (j = 0; j < numberOfNames; j++,ptr++) {
        cout<< *ptr ;
    }
}

The drawing obejct method :

void makeCustomAnnot(GLenum mode){

    glEnable(GL_TEXTURE_2D);
    glColor3f(1,1,1);
    GLuint j=0;

    QImage img("img.jpg");
    img=QGLWidget::convertToGLFormat(img);

    GLuint texturesAnnot[1];
    glBindTexture( GL_TEXTURE_2D, texturesAnnot[0] );
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, img.width(), img.height(), 0, GL_RGBA, GL_UNSIGNED_BYTE, img.bits());
    glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR);

    if(mode == GL_SELECT){
        glPushName (j);
    }

    glBegin(GL_QUADS);
        glTexCoord2d(0,0);glVertex3d(x4,y4,z4);
        glTexCoord2d(1,0);glVertex3d(x3,y3,z3);
        glTexCoord2d(1,1);glVertex3d(x2,y2,z2);
    glTexCoord2d(0,1);glVertex3d(x1,y1,z1);
    glEnd();

    if(mode == GL_SELECT)
        glPopName ();
    }

    glDisable(GL_TEXTURE_2D);
}

The rendering method :

void render()
{
    glEnable(GL_DEPTH_TEST);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();

    glViewport(0,0,this->width() , this->height());

    gluPerspective(fov, this->width() / this->height(), 0.1, 1000);

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

    glRotatef(xRot / 5.0f, 1.0f, 0.0f, 0.0f);
    glRotatef(yRot / 5.0f, 0.0f, 1.0f, 0.0f);
    glRotatef(zRot / 5.0f, 0.0f, 0.0f, 1.0f);

    glPushMatrix();
        makeCustomAnnot(GL_RENDER);
    glPopMatrix();

    glDisable(GL_DEPTH_TEST);
}

So hits = glRenderMode (GL_RENDER); always returns 0 hits. What am I doing wrong?

@j-p I tried but it still doesn't work, and I think hits = glRenderMode (GL_RENDER); is the good one, although it's tempting to put GL_SELECT. — SteveTJS
No ideas? I looked at several other posts on the same topic but I can't figure out what I am doing wrong. — SteveTJS

SteveTJS SteveTJS · Accepted Answer · 2015-02-27T09:48:47

So here is what I did to have it work perfectly.

The processHits method :

void processHits(GLint inHits, GLuint buffer[])
{
    unsigned int i, j;
    GLuint names, *ptr, minZ,*ptrNames, numberOfNames;

    ptr = (GLuint *) buffer;
    minZ = 0xffffffff;
    for (i = 0; i < inHits; i++) {
        names = *ptr;
        ptr++;
        if (*ptr < minZ) {
            numberOfNames = names;
            minZ = *ptr;
            ptrNames = ptr+2;
        }
        ptr += names+2;
    }

    cout << "Nearest: ";
    ptr = ptrNames;
    for (j = 0; j < numberOfNames; j++,ptr++) {
        cout<< *ptr ;
    }
}

The drawing object method with creates the objects I need to pick :

void makeCustomAnnot(GLenum mode){

    glEnable(GL_TEXTURE_2D);
    glColor3f(1,1,1);
    GLuint j=0;

    QImage img("img.jpg");
    img=QGLWidget::convertToGLFormat(img);

    GLuint texturesAnnot[1];
    glBindTexture( GL_TEXTURE_2D, texturesAnnot[0] );
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, img.width(), img.height(), 0, GL_RGBA, GL_UNSIGNED_BYTE, img.bits());
    glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR);

    if(mode == GL_SELECT)
        glLoadName (j);

    glBegin(GL_QUADS);
        glTexCoord2d(0,0);glVertex3d(x4,y4,z4);
        glTexCoord2d(1,0);glVertex3d(x3,y3,z3);
        glTexCoord2d(1,1);glVertex3d(x2,y2,z2);
    glTexCoord2d(0,1);glVertex3d(x1,y1,z1);
    glEnd();

    glDisable(GL_TEXTURE_2D);
}

The pickObjects method, identical to the rendering function but with instructions for picking :

#define BUFSIZE 512
unsigned int selectBuf[BUFSIZE];

void pickObjects(int x, int y)
{
    GLint viewport[4];
    GLint hits;

    glSelectBuffer (BUFSIZE, selectBuf);
    glRenderMode (GL_SELECT);

    glMatrixMode (GL_PROJECTION);
    glPushMatrix ();
    glLoadIdentity ();

    glGetIntegerv (GL_VIEWPORT, viewport);

    gluPickMatrix ((GLdouble) x, (GLdouble) (viewport[3] - y), 5.0, 5.0, viewport);
    gluPerspective(fov, this->width() / this->height(), 0.1, 1000);

    glMatrixMode(GL_MODELVIEW);

    glLoadIdentity();

    glRotatef(xRot / 5.0f, 1.0f, 0.0f, 0.0f);
    glRotatef(yRot / 5.0f, 0.0f, 1.0f, 0.0f);
    glRotatef(zRot / 5.0f, 0.0f, 0.0f, 1.0f);

    glRotatef((float)angleV, 1.0f, 0.0f, 0.0f);

    glInitNames();
    glPushName( 10000 );

    glPushMatrix();
        makeCustomAnnot(GL_SELECT);//draw scene in GL_SELECT mode to create the names
    glPopMatrix();

    hits = glRenderMode (GL_RENDER);//get the hits in GL_RENDER mode
    if (hits != 0)
    {
        processHits(hits,selectBuf);
    }

    glPopMatrix();
    glDisable(GL_DEPTH_TEST);
}

The rendering method :

void render()
{
    glEnable(GL_DEPTH_TEST);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();

    glViewport(0,0,this->width() , this->height());

    gluPerspective(fov, this->width() / this->height(), 0.1, 1000);

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

    glRotatef(xRot / 5.0f, 1.0f, 0.0f, 0.0f);
    glRotatef(yRot / 5.0f, 0.0f, 1.0f, 0.0f);
    glRotatef(zRot / 5.0f, 0.0f, 0.0f, 1.0f);

    glRenderMode (GL_RENDER);

    glPushMatrix();
        makeCustomAnnot(GL_RENDER);
    glPopMatrix();

    glDisable(GL_DEPTH_TEST);
}

And finally hits = glRenderMode (GL_RENDER); in pickObjects method and then the processHits method return exactly the picked object.

Picking with glRenderMode

1 Answers