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RetrieveVertexData         Retrieving vertex data from a mesh
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This example is taken from monster's OgreODE project. The full source can be found under ogreaddons/ogreode in the Ogre SVN.
It has been adopted, so that it can be used separately. Just copy/paste it into your own project.

Note that this code assumes sizeof(long) == sizeof(uint32_t), which is not true on AMD64 Linux.

A port to Mogre can be found in this article.

Original version

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void getMeshInformation(Mesh* mesh,size_t &vertex_count,Vector3* &vertices,
    size_t &index_count, unsigned* &indices,
    const Vector3 &position = Vector3::ZERO,
    const Quaternion &orient = Quaternion::IDENTITY,const Vector3 &scale = Vector3::UNIT_SCALE)
{
    vertex_count = index_count = 0;

    bool added_shared = false;
    size_t current_offset = vertex_count;
    size_t shared_offset = vertex_count;
    size_t next_offset = vertex_count;
    size_t index_offset = index_count;
    size_t prev_vert = vertex_count;
    size_t prev_ind = index_count;

    // Calculate how many vertices and indices we're going to need
    for(int i = 0;i < mesh->getNumSubMeshes();i++)
    {
        SubMesh* submesh = mesh->getSubMesh(i);

        // We only need to add the shared vertices once
        if(submesh->useSharedVertices)
        {
            if(!added_shared)
            {
                VertexData* vertex_data = mesh->sharedVertexData;
                vertex_count += vertex_data->vertexCount;
                added_shared = true;
            }
        }
        else
        {
            VertexData* vertex_data = submesh->vertexData;
            vertex_count += vertex_data->vertexCount;
        }

        // Add the indices
        Ogre::IndexData* index_data = submesh->indexData;
        index_count += index_data->indexCount;
    }

    // Allocate space for the vertices and indices
    vertices = new Vector3[vertex_count];
    indices = new unsigned[index_count];

    added_shared = false;

    // Run through the submeshes again, adding the data into the arrays
    for(int i = 0;i < mesh->getNumSubMeshes();i++)
    {
        SubMesh* submesh = mesh->getSubMesh(i);

        Ogre::VertexData* vertex_data = submesh->useSharedVertices ? mesh->sharedVertexData : submesh->vertexData;
        if((!submesh->useSharedVertices)||(submesh->useSharedVertices && !added_shared))
        {
            if(submesh->useSharedVertices)
            {
                added_shared = true;
                shared_offset = current_offset;
            }

            const Ogre::VertexElement* posElem = vertex_data->vertexDeclaration->findElementBySemantic(Ogre::VES_POSITION);
            Ogre::HardwareVertexBufferSharedPtr vbuf = vertex_data->vertexBufferBinding->getBuffer(posElem->getSource());
            unsigned char* vertex = static_cast<unsigned char*>(vbuf->lock(Ogre::HardwareBuffer::HBL_READ_ONLY));
            Ogre::Real* pReal;

            for(size_t j = 0; j < vertex_data->vertexCount; ++j, vertex += vbuf->getVertexSize())
            {
                posElem->baseVertexPointerToElement(vertex, &pReal);

                Vector3 pt;

                pt.x = (*pReal++);
                pt.y = (*pReal++);
                pt.z = (*pReal++);

                pt = (orient * (pt * scale)) + position;

                vertices[current_offset + j].x = pt.x;
                vertices[current_offset + j].y = pt.y;
                vertices[current_offset + j].z = pt.z;
            }
            vbuf->unlock();
            next_offset += vertex_data->vertexCount;
        }

        Ogre::IndexData* index_data = submesh->indexData;

        size_t numTris = index_data->indexCount / 3;
        unsigned short* pShort;
        unsigned int* pInt;
        Ogre::HardwareIndexBufferSharedPtr ibuf = index_data->indexBuffer;
        bool use32bitindexes = (ibuf->getType() == Ogre::HardwareIndexBuffer::IT_32BIT);
        if (use32bitindexes) pInt = static_cast<unsigned int*>(ibuf->lock(Ogre::HardwareBuffer::HBL_READ_ONLY));
        else pShort = static_cast<unsigned short*>(ibuf->lock(Ogre::HardwareBuffer::HBL_READ_ONLY));

        for(size_t k = 0; k < numTris; ++k)
        {
            size_t offset = (submesh->useSharedVertices)?shared_offset:current_offset;

            unsigned int vindex = use32bitindexes? *pInt++ : *pShort++;
            indices[index_offset + 0] = vindex + offset;
            vindex = use32bitindexes? *pInt++ : *pShort++;
            indices[index_offset + 1] = vindex + offset;
            vindex = use32bitindexes? *pInt++ : *pShort++;
            indices[index_offset + 2] = vindex + offset;

            index_offset += 3;
        }
        ibuf->unlock();
        current_offset = next_offset;
    }
}

Example usage:

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Entity *ent = mSceneMgr->createEntity("head", "ogrehead.mesh");

size_t vertex_count,index_count;
Vector3* vertices;
unsigned* indices;

getMeshInformation(ent->getMesh(),vertex_count,vertices,index_count,indices);

LogManager::getSingleton().logMessage(LML_NORMAL,"Vertices in mesh: %u",vertex_count);
LogManager::getSingleton().logMessage(LML_NORMAL,"Triangles in mesh: %u",index_count / 3);

delete[] vertices;
delete[] indices;

Optimized version

Below is a modified version of getMeshInformation(). It was based on "Monsters" function. This modified version does exactly the same like the one above, but is slightly modified to be more understandable, has a few fixes and a few speed optimizations.
I just thought I share it back to the community, so here it is:

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void getMeshInformation(const Ogre::Mesh* const mesh,
                        size_t &vertex_count,
                        Ogre::Vector3* &vertices,
                        size_t &index_count,
                        unsigned long* &indices,
                        const Ogre::Vector3 &position,
                        const Ogre::Quaternion &orient,
                        const Ogre::Vector3 &scale)
{
    bool added_shared = false;
    size_t current_offset = 0;
    size_t shared_offset = 0;
    size_t next_offset = 0;
    size_t index_offset = 0;

    vertex_count = index_count = 0;

    // Calculate how many vertices and indices we're going to need
    for ( unsigned short i = 0; i < mesh->getNumSubMeshes(); ++i)
    {
        Ogre::SubMesh* submesh = mesh->getSubMesh(i);
        // We only need to add the shared vertices once
        if(submesh->useSharedVertices)
        {
            if( !added_shared )
            {
                vertex_count += mesh->sharedVertexData->vertexCount;
                added_shared = true;
            }
        }
        else
        {
            vertex_count += submesh->vertexData->vertexCount;
        }
        // Add the indices
        index_count += submesh->indexData->indexCount;
    }

    // Allocate space for the vertices and indices
    vertices = new Ogre::Vector3[vertex_count];
    indices = new unsigned long[index_count];

    added_shared = false;

    // Run through the submeshes again, adding the data into the arrays
    for (unsigned short i = 0; i < mesh->getNumSubMeshes(); ++i)
    {
        Ogre::SubMesh* submesh = mesh->getSubMesh(i);

        Ogre::VertexData* vertex_data = submesh->useSharedVertices ? mesh->sharedVertexData : submesh->vertexData;

        if ((!submesh->useSharedVertices) || (submesh->useSharedVertices && !added_shared))
        {
            if(submesh->useSharedVertices)
            {
                added_shared = true;
                shared_offset = current_offset;
            }

            const Ogre::VertexElement* posElem =
                vertex_data->vertexDeclaration->findElementBySemantic(Ogre::VES_POSITION);

            Ogre::HardwareVertexBufferSharedPtr vbuf =
                vertex_data->vertexBufferBinding->getBuffer(posElem->getSource());

            unsigned char* vertex =
                static_cast<unsigned char*>(vbuf->lock(Ogre::HardwareBuffer::HBL_READ_ONLY));

            // There is _no_ baseVertexPointerToElement() which takes an Ogre::Real or a double
            //  as second argument. So make it float, to avoid trouble when Ogre::Real will
            //  be comiled/typedefed as double:
            //Ogre::Real* pReal;
            float* pReal;

            for( size_t j = 0; j < vertex_data->vertexCount; ++j, vertex += vbuf->getVertexSize())
            {
                posElem->baseVertexPointerToElement(vertex, &pReal);
                Ogre::Vector3 pt(pReal[0], pReal[1], pReal[2]);
                vertices[current_offset + j] = (orient * (pt * scale)) + position;
            }
            
            vbuf->unlock();
            next_offset += vertex_data->vertexCount;
        }

        Ogre::IndexData* index_data = submesh->indexData;
        size_t numTris = index_data->indexCount / 3;
        Ogre::HardwareIndexBufferSharedPtr ibuf = index_data->indexBuffer;
        
        bool use32bitindexes = (ibuf->getType() == Ogre::HardwareIndexBuffer::IT_32BIT);

        unsigned long* pLong = static_cast<unsigned long*>(ibuf->lock(Ogre::HardwareBuffer::HBL_READ_ONLY));
        unsigned short* pShort = reinterpret_cast<unsigned short*>(pLong);

        size_t offset = (submesh->useSharedVertices)? shared_offset : current_offset;

        if ( use32bitindexes )
        {
            for ( size_t k = 0; k < numTris*3; ++k)
            {
                indices[index_offset++] = pLong[k] + static_cast<unsigned long>(offset);
            }
        }
        else
        {
            for ( size_t k = 0; k < numTris*3; ++k)
            {
                indices[index_offset++] = static_cast<unsigned long>(pShort[k]) +
                                          static_cast<unsigned long>(offset);
            }
        }

        ibuf->unlock();
        current_offset = next_offset;
    }
}

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