E* Interpolated Graph Replanner
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E-star Interpolated Graph Replanner

Copyright (C) 2004-2005 Swiss Federal Institute of Technology, Lausanne

Copyright (C) 2005-2007 Roland Philippsen

Released under the GNU General Public License, version 2. See LICENSE.GPL for more details.


We still use the Sourceforge E* project for mailing lists and file distribution. The code has moved to a github repository though, and the github wiki may be a good place to look, too.


The E* algorithm is a path planner for (mobile) robotics. Unlike A*, which constrains movements to graph edges, it produces smooth trajectories by interpolating between edges. Like D*, it supports dynamic replanning after local path cost changes. The file ABOUT has some more details.




  • To build the development version, you need GNU Automake, Autoconf, and Libtool. If you're building from a release tarball, you don't need this.
  • An OpenGL implementation. This is sort of optional, the configure script tries to guess if you have OpenGL, but it has never been really tested on a system without it.
  • Doxygen for creating API documentation

Building E*

Release tarballs are available form the download page on Sourceforge.

The instructions here assume you're using the build-stage.sh script. It has several options for fine-grained control, read the output of ./build-stage.sh -h for more information. You can also do a manual "configure / make / make install" of course.

Download and Build a Release Tarball

$ tar xfvz estar-X.Y.tar.gz
$ cd estar-X.Y
$ ./build-stage.sh -s

Note the -s option to the build-stage.sh script, which is especially important if you do not have GNU Automake, Autoconf, and Libtool. This will build everything using a separate build directory, and "install" it in the stage/ directory.

Cloning the git Repository

$ git clone git://github.com/poftwaresatent/estar.git
$ cd estar
$ ./build-stage.sh

Note that build-stage.sh will create a build/ directory that can then be used for recompiling the project after modifications to the code. The GNU tools provide dependency tracking, and in most cases only a small portion of the code gets recompiled after a change. Also, you can pass -s to subsequent calls to ./build-stage.sh to avoid wasting time on recreating the build system files that haven't changed.

The build-stage script has some more tricks up its sleeve. For example, to install E* on your system, for example underneath /usr/local/estar, just do this:

$ ./build-stage.sh -p /usr/local/estar

API Documentation

The online documentation was generated using Doxygen. Each release can also be downloaded with documentation, simply choose one of the tarballs tagged with-doc on the download page. If you downloaded one of those, then simply open doc/html/index.html in a browser.

Otherwise, you can create the API documentation using Doxygen after a successful configuration from within the build directory:

$ cd build
$ make doc


After building and installing, test programs will be in stage/bin/ (unless you overrode the installation path used by build-stage.sh). Some of these test programs are console-based. If you have OpenGL (and the build system detected it), then there will also be graphical programs which are controlled using the keyboard:

  • SPACE: Compute one propagation step.
  • c: Continuous propagation, plot each step.
  • f: Flush - Propagate all cells, then plot.
  • q: Quit (and optionally write to file)

Main E* Test: test_estar_gfx

This program is a graphical test of E*. It expects the name of a grid config file and runs the E* algorithm using the goal and robot positions (indices) defined therein. You can use the -t option to disable graphical output, in which case it makes sense to also define an output file using -o filename in order to capture the result (it simply writes resulting cell values in ASCII).

In the misc/ directory there are some grid config files that can be used for test_estar_gfx:

  • grid-nhp-*.txt are grid definitions created by Akin Sisbot at LAAS-CNRS for testing E* on his PhD results for a human-aware motion planner (that was around ca. 2006).
  • grid-small.txt is a small hand-crafted grid
  • grid-hex-small.txt is a trial triangular mesh grid... this has eternally been work in progress, triangular meshes are not working reliably.

Main PNF Test: test_pnf_gfx

With this program you can test the PNF algorithm: a planner that takes into account dynamic obstacles and relies on E* to create smooth navigation functions. More information about PNF can be found in the references.

test_pnf_gfx expects the name of a config file. If you specify the string "paper" as second argument, then the plots will be in greyscale and with a different layout (supposedly better for inclusion in publications). The misc/ directory contains some grid config files that can be used for test_pnf_gfx:

  • pnf-static-impulse.pnf: A setup with a single wall between the robot and the goal.
  • pnf-dynamic-impulse.pnf: A setup with a single dynamic object between the robot and the goal in an otherwise empty environment.
  • pnf-setup-iros06-*.pnf: Setups for the IROS'06 paper, not all were used though.
  • pnf-setup-star06-*.pnf: Setups for the STAR paper.

There are also two utility scripts for creating vector-format figures of PNF plots.

  • plot_pnf.sh uses Gnuplot to create a contour plot of PNF values in XFig format.
  • pnf_3d_riskplot.sh uses Gnuplot and fig2dev to create a 3D plot of PNF risk data in PDF format. (Note that nowadays, you can directly produce PDF output from Gnuplot).

Other Programs

  • test_estar_replan_gfx: Program for comparing two instances of E*, with the goal of checking that dynamic replanning produces the same results as re-initializing and replanning from scratch. You can use the mouse to add and remove obstacles.
  • test_dbg_opt: Small test for checking whether debug messages get pruned properly (they do).
  • test_estar: An old text-only test program for E*. See the source-code for options.
  • test_estar_queue: Text-only debugging of the E* queue-ing mechanism. Will hopefully still be useful one day when work on triangular meshes is taken up again...
  • test_fake_os: Totally simple compile-test to see whether the rudimentary estar::fake_os class behaves correctly.
  • test_pnf_cooc and test_pnf_cooc3d: Old test programs for verifying the co-occurrence probability computations of PNF (which was ported from Matlab code that was written by Björn Jensen at ASL-EPFL back in 2002 or 2003).
  • test_pnf_riskmap1: Checks that pnf::PNFRiskMap` does what it should.
  • test_shape: Tests the estar::Sprite class.


E* poster presented at IROS 2007 (and the corresponding flyer) during the Workshop on Algorithmic Motion Planning for Autonomous Robots in Challenging Environments.

 author    = {Roland Philippsen},
 title     = {E* Interpolated Graph Replanner},
 booktitle = {Workshop Proceedings on Algorithmic Motion Planning for Autonomous Robots
              in Challenging Environments, held in conjunction with the IEEE International
              Conference on Intelligent Robots and Systems (IROS)},
 year      = 2007

Technical Report on "light" E* written in 2006. This describes the formulation underlying this implementation.

 author      = {Roland Philippsen},
 title       = {A Light Formulation of the E\* Interpolated Path Replanner},
 institution = {Autonomous Systems Lab, Ecole Polytechnique Federale de Lausanne},
 year        = 2006

E* paper presented at ICRA 2005 (describes an outdated formulation, but gives basic insights)

 author    = {Roland Philippsen and Roland Siegwart},
 title     = {An Interpolated Dynamic Navigation Function},
 booktitle = {Proceedings of the IEEE International Conference on Robotics and Automation (ICRA)},
 year      = 2005

PNF paper published in STAR 2006.

 author       = {Philippsen, R. and Jensen, B. and Siegwart, R.},
 editor       = {Laugier, C. and Chatila, R.},
 chapter      = {Towards Real-Time Sensor-Based Path Planning in Highly Dynamic Environments},
 title        = {Autonomous Navigation in Dynamic Environments},
 publisher    = {Springer Tracts on Advanced Robotics},
 year         = 2006