taogashi/backgroud_subtraction.md

## backgroud_subtraction.md

      
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              backgroud_subtraction.md
            
          
    OctreePointCloud类型的setInputCloud方法只是赋值，不会做任何reset。
inline void setInputCloud (const PointCloudConstPtr &cloud_arg,
             const IndicesConstPtr &indices_arg = IndicesConstPtr ())
{
	input_ = cloud_arg;
	indices_ = indices_arg;
}
所以，方法addPointsFromInputCloud可以重复调用。

OctreePointCloudDensity用于计算空间网格的密度。
实际上只是对叶节点进行计数。它的container类型是OctreePointCloudDensityContainer。
/** \brief Read input data. Only an internal counter is increased.
  */
void addPointIndex (int)
{
	point_counter_++;
}
这个函数的调用路径是：OctreePointCloud::addPointsFromInputCloud->OctreePointCloud::addPointIdx->LeafContainerT::addPointIdx。

因此，可以逐帧累加，静止的背景所在的网格点的密度会越来越高。对于新的点云，只要检查每个点对应的密度，就可以判断这个点是前景还是背景。
PointCloudColor::const_iterator cloud_it = cloud->begin();
PointCloudColor::const_iterator cloud_it_end = cloud->end();
for (; cloud_it != cloud_it_end; ++cloud_it) {
	pcl::PointXYZRGBA pt = *cloud_it;
	uint64_t density = oct.getVoxelDensityAtPoint(*cloud_it);
	if (density > 2) {
		pt.rgba = 0xFFFF0000;
	} else if (density > 1) {
		pt.rgba = 0xFF00FF00;
	} else {
		pt.rgba = 0xFF0000FF;
	}
	display_cloud->points.push_back(pt);
}
伪代码：
read cloud;
octree_density.add(new cloud);
update density threshold;
for point in new cloud:
  if octree_density.density_at(point) > threshold:
    remove point from cloud;