Visualization of the NARF descriptor and descriptor distances

.. _narf_descriptor_visualization:

This tutorial is about the visualization of how the NARF descriptor is calculated and to test how the descriptor distances between certain points in a range image behave. Compared to the other tuturials, this one is not really about the code, but about trying the program and looking at the visualization. Of course, nothing keeps you from having a look at it anyway.

The code

First, create a file called, let's say, narf_descriptor_visualization.cpp in your favorite editor, and place the following code inside it:

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

Compiling and running the program

Add the following lines to your CMakeLists.txt file:

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

You can now try it with a point cloud file from your hard drive::

$ ./narf_descriptor_visualization <point_cloud.pcd>

It will take a few second, during which you will see the status in the terminal. During this time, a NARF feature is extracted in every point of the range image created from the given point cloud. When it is done, a widget showing the range image pops up. Now click on a point in the range image. If it is a valid image point, three additional widgets will pop up. One visualizing the actual descriptor as a row of gray values, one showing a local range image patch of the area on which you clicked, overlaid with a star shaped pattern. Each beam corresponds to one of the cells in the descriptor. The one facing upwards to the first cell and then going clockwise. The basic intuition is, that the more the surface changes under the beam, the higher (brighter) the value of the corresponding descriptor cell. There is also one or more red beams, which mark the extracted dominant orientations of the image patch, which, together with the normal, is used to create a unique orientation for the feature coordinate frame. The last image visualizes the descriptor distances to every other point in the scene. The darker the value, the more similar the point is to the clicked image point.

The result should look similar to this:

.. image:: images/narf_descriptor_visualization.png

Also have a look at::

$ ./narf_descriptor_visualization -h

for a list of parameters.