//alloca un vettore con tutti gli indici
        pcl::IndicesPtr keypointsIndices_1(new std::vector<int>);
        keypointsIndices_1->resize(cloud_1->width * cloud_1->height);
        for(int id=0; id<keypointsIndices_1->size(); id++)
        {
            (*keypointsIndices_1)[id] = id;
        }

        //li disordina
        unsigned int nSamples = 10000;
        std::random_shuffle(keypointsIndices_1->begin(), keypointsIndices_1->end());

        //ne prende i primi nSamples
        keypointsIndices_1->resize(nSamples);
        pcl::PointCloud<pcl::PointXYZ>::Ptr cloud_keypoints_1 (new pcl::PointCloud<pcl::PointXYZ> ());
        pcl::copyPointCloud(*cloud_1, *keypointsIndices_1, *cloud_keypoints_1);