Extracting indices from a PointCloud

.. _extract_indices:

In this tutorial we will learn how to use an :pcl:ExtractIndices <pcl::ExtractIndices> filter to extract a subset of points from a point cloud based on the indices output by a segmentation algorithm. In order to not complicate the tutorial, the segmentation algorithm is not explained here. Please check the :ref:planar_segmentation tutorial for more information.

.. raw:: html

The code

First, download the dataset table_scene_lms400.pcd <https://raw.github.com/PointCloudLibrary/data/master/tutorials/table_scene_lms400.pcd>_ and save it somewhere to disk.

Then, create a file, let's say, extract_indices.cpp in your favorite editor, and place the following inside it:

.. literalinclude:: sources/extract_indices/extract_indices.cpp :language: cpp :linenos:

The explanation

Now, let's break down the code piece by piece, skipping the obvious.

After the data has been loaded from the input .PCD file, we create a :pcl:VoxelGrid<pcl::VoxelGrid> filter, to downsample the data. The rationale behind data downsampling here is just to speed things up -- less points means less time needed to spend within the segmentation loop.

.. literalinclude:: sources/extract_indices/extract_indices.cpp :language: cpp :lines: 24-27

The next block of code deals with the parametric segmentation. To keep the tutorial simple, its explanation will be skipped for now. Please see the segmentation tutorials (in particular :ref:planar_segmentation) for more information.

.. literalinclude:: sources/extract_indices/extract_indices.cpp :language: cpp :lines: 38-48

The line

.. literalinclude:: sources/extract_indices/extract_indices.cpp :language: cpp :lines: 51

and

.. literalinclude:: sources/extract_indices/extract_indices.cpp :language: cpp :lines: 67-70

represent the actual indices :pcl:extraction filter <pcl::ExtractIndices>. To process multiple models, we run the process in a loop, and after each model is extracted, we go back to obtain the remaining points, and iterate. The inliers are obtained from the segmentation process, as follows:

.. literalinclude:: sources/extract_indices/extract_indices.cpp :language: cpp :lines: 58-59

Compiling and running the program

Add the following lines to your CMakeLists.txt file:

.. literalinclude:: sources/extract_indices/CMakeLists.txt :language: cmake :linenos:

After you have made the executable, you can run it. Simply do::

$ ./extract_indices

You will see something similar to::

PointCloud before filtering: 460400 data points. PointCloud after filtering: 41049 data points. PointCloud representing the planar component: 20164 data points. PointCloud representing the planar component: 12129 data points.