OpenGL glDrawElements

Question

Given a set of Faces with each face containing the number of vertices and a pointer to a vertex in a vector std::Vector, i want to iterate over all faces and use glDrawElements to draw each face:

Edit: I just noticed i forgot to activate the vertex_array

for(std::vector<Face>::iterator it = faces.begin();it != faces.end();++it) {
    const Face &f = *it;

    std::Vector<GLint> indices;
    std::Vector<GLfloat> positions;

    for(int i=0;i<f.vcount;++i){
        const Vertex &v = vertices[f.vertices[i]];

        positions.push_back(v.x);
        positions.push_back(v.y);
        positions.push_back(v.z);

        indices.push_back(f.vertices[i]);
    }
    glEnableClientState(GL_VERTEX_ARRAY);
    glVertexPointer(3,GL_FLOAT,3*sizeof(GL_FLOAT),&positions[0]);                     

    glDrawElements(GL_POLYGON,indices.size(),GL_UNSIGNED_INT,&indices[0]);
    glDisableClientState(GL_VERTEX_ARRAY);
    positions.clear();
    indices.clear();
}

But apparently this does not work correctly and there is nothing displayed.

Edit: Enabling the GL_VERTEX_ARRAY draws something on the screen but not the model i tried to create. So there seems to be something wrong with the addressing.

Answer 1

Your index array doesn't make sense. The indices glDrawElements will use just refer to the vertex arrays you have set up - and you are setting up a new array for each separate polygon.

This means that

indices.push_back(f.vertices[i]);

should be conceptually just

indices.push_back(i);

which in the end means that you could skip the indices completely and just use

glDrawArrays(GL_POLYGON,0,f.vcount);

Note that what you are doing here is a very inefficent way to render the ojects. You would be much better if you would use a single draw call for the whole object. You could do that by manually triangulating the polygons into triangles as a pre-processing step.