Strange mesh import problem with Assimp and OpenGL

Question

Using the assimp library for importing 3D data into an OpenGL application. I get some strange problems regarding indexing of the vertices:

If I use the following code for importing vertex indices:

for (unsigned int t = 0; t < mesh->mNumFaces; ++t) 
    {
        const struct aiFace * face = &mesh->mFaces[t];
        if (face->mNumIndices == 3)
        {
        indices->push_back(face->mIndices[0]);
        indices->push_back(face->mIndices[1]);
        indices->push_back(face->mIndices[2]);
        }
    }

I get the following result:

Instead, if I use the following code:

for(int k = 0; k < 2 ; k++)
    {
        for (unsigned int t = 0; t < mesh->mNumFaces; ++t) 
        {
            const struct aiFace * face = &mesh->mFaces[t];
            if (face->mNumIndices == 3)
            {
            indices->push_back(face->mIndices[0]);
            indices->push_back(face->mIndices[1]);
            indices->push_back(face->mIndices[2]);
            }
        }
    }

I get the correct result:

Hence adding the indices twice, renders the correct result?

The OpenGL buffer is populated, like so:

glBufferData(GL_ELEMENT_ARRAY_BUFFER, 
             indices->size() * sizeof(unsigned int), 
             indices->data(), 
             GL_STATIC_DRAW);

And rendered as follows:

glDrawElements(GL_TRIANGLES, 
               vertexCount*3, 
               GL_UNSIGNED_INT, 
               indices->data());

Answer 1

The second Argument of the glDrawElements function should be the number of indices and not the number of vertices.

It should look something like this: glDrawElements(GL_TRIANGLES, indices->size(), GL_UNSIGNED_INT, indices->data());

If you have more vertices than indices, this would explain your problem. Because you try to draw more indices than which are actually stored in the buffer unless you put the indices twice.