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<title>OMPL: State Validity Checking</title>
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<div class="title">State Validity Checking </div> </div>
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<div class="textblock"><p>OMPL itself does not include code for state validity checking. This is intentional, since defining this notion depends on the type of problems to be solved. For instance, OMPL.app defines state validity checking in terms of collision checking between loaded CAD models. The ROS interface to OMPL defines state validity checking to be collision checking between the robot model and sensed objects in the environment. If planning for protein folding, the state validity check has to do with the evaluation of energy functions.</p>
<p>In order to allow the user to specify the notion of state validity, OMPL defines two abstract classes:</p>
<ul>
<li><a class="el" href="classompl_1_1base_1_1StateValidityChecker.html" title="Abstract definition for a class checking the validity of states. The implementation of this class mus...">ompl::base::StateValidityChecker</a></li>
<li><a class="el" href="classompl_1_1base_1_1MotionValidator.html" title="Abstract definition for a class checking the validity of motions -- path segments between states...">ompl::base::MotionValidator</a></li>
</ul>
<p>These classes abstract away properties specific to the systems we are planning for. For instance, OMPL does not need to represent robot or obstacle geometry. The user can specify which implementations to use when configuring <a class="el" href="classompl_1_1base_1_1SpaceInformation.html" title="The base class for space information. This contains all the information about the space planning is d...">ompl::base::SpaceInformation</a>. The instances of <a class="el" href="classompl_1_1base_1_1StateValidityChecker.html" title="Abstract definition for a class checking the validity of states. The implementation of this class mus...">ompl::base::StateValidityChecker</a> and <a class="el" href="classompl_1_1base_1_1MotionValidator.html" title="Abstract definition for a class checking the validity of motions -- path segments between states...">ompl::base::MotionValidator</a> need to be thread safe.</p>
<p>The <a class="el" href="classompl_1_1base_1_1StateValidityChecker.html" title="Abstract definition for a class checking the validity of states. The implementation of this class mus...">ompl::base::StateValidityChecker</a> class defines the <a class="el" href="classompl_1_1base_1_1StateValidityChecker.html#a1e70a8c8c51e42c81408c5eae6108a4b" title="Return true if the state state is valid. Usually, this means at least collision checking. If it is possible that ompl::base::StateSpace::interpolate() or ompl::control::ControlSpace::propagate() return states that are outside of bounds, this function should also make a call to ompl::base::SpaceInformation::satisfiesBounds().">ompl::base::StateValidityChecker::isValid()</a> function, which allows planners to evaluate the validity of a state. If the user does not specify this class, an instance of <a class="el" href="classompl_1_1base_1_1AllValidStateValidityChecker.html" title="The simplest state validity checker: all states are valid.">ompl::base::AllValidStateValidityChecker</a> is assumed. This will consider all states to be valid. </p>
<div class="fragment"><pre class="fragment"> <span class="comment">// define this class:</span>
<span class="keyword">class </span>myStateValidityCheckerClass : <span class="keyword">public</span> base::StateValidityChecker
{
<span class="keyword">public</span>:
myStateValidityCheckerClass(<span class="keyword">const</span> base::SpaceInformationPtr &si) :
base::StateValidityChecker(si)
{
}
<span class="keyword">virtual</span> <span class="keywordtype">bool</span> isValid(<span class="keyword">const</span> base::State *state)<span class="keyword"> const</span>
<span class="keyword"> </span>{
<span class="keywordflow">return</span> ...;
}
};
<span class="comment">// or this function:</span>
<span class="keywordtype">bool</span> myStateValidityCheckerFunction(<span class="keyword">const</span> base::State *state)
{
<span class="keywordflow">return</span> ...;
}
base::SpaceInformationPtr si(space);
<span class="comment">// either this call:</span>
si-><a class="code" href="classompl_1_1base_1_1SpaceInformation.html#a0449ec0893e02aca2bdfc63323b624e5" title="Set the instance of the state validity checker to use. Parallel implementations of planners assume th...">setStateValidityChecker</a>(base::StateValidityCheckerPtr(<span class="keyword">new</span> myStateValidityCheckerClass(si)));
<span class="comment">// or this call:</span>
si-><a class="code" href="classompl_1_1base_1_1SpaceInformation.html#a0449ec0893e02aca2bdfc63323b624e5" title="Set the instance of the state validity checker to use. Parallel implementations of planners assume th...">setStateValidityChecker</a>(boost::bind(&myStateValidityCheckerFunction, _1));
si-><a class="code" href="classompl_1_1base_1_1SpaceInformation.html#a2bcd47fd9b7cf54b086d2122646736bf" title="Set the resolution at which state validity needs to be verified in order for a motion between two sta...">setStateValidityCheckingResolution</a>(0.03); <span class="comment">// 3%</span>
si-><a class="code" href="classompl_1_1base_1_1SpaceInformation.html#ac39aa0c4b92e3ca5acfa75eeb56b080f" title="Perform additional setup tasks (run once, before use). If state validity checking resolution has not ...">setup</a>();
</pre></div><p>The <a class="el" href="classompl_1_1base_1_1MotionValidator.html" title="Abstract definition for a class checking the validity of motions -- path segments between states...">ompl::base::MotionValidator</a> class defines <a class="el" href="classompl_1_1base_1_1MotionValidator.html#a9b6d254308ea12799a72049a8dff3fc6" title="Check if the path between two states (from s1 to s2) is valid. This function assumes s1 is valid...">ompl::base::MotionValidator::checkMotion()</a> routines that evaluate the validity of motions between two specified states. By default, the implementation of this class is assumed to be <a class="el" href="classompl_1_1base_1_1DiscreteMotionValidator.html" title="A motion validator that only uses the state validity checker. Motions are checked for validity at a s...">ompl::base::DiscreteMotionValidator</a>. The advantage of <a class="el" href="classompl_1_1base_1_1DiscreteMotionValidator.html" title="A motion validator that only uses the state validity checker. Motions are checked for validity at a s...">ompl::base::DiscreteMotionValidator</a> is that it can be implemented solely using functionality from <a class="el" href="classompl_1_1base_1_1StateValidityChecker.html" title="Abstract definition for a class checking the validity of states. The implementation of this class mus...">ompl::base::StateValidityChecker</a>. The disadvantage is that the motion is discretized to some resolution and states are checked for validity at that resolution. If the resolution at which the motions are checked for validity is too large, there may be invalid states along the motion that escape undetected. If the resolution is too small, there can be too many states to be checked along each motion, slowing down the planner significantly. The resolution at which motions are discretized is computed using <a class="el" href="classompl_1_1base_1_1StateSpace.html#a17443b992677f5043d3fc38e9288b9a7" title="Count how many segments of the "longest valid length" fit on the motion from state1 to state2...">ompl::base::StateSpace::validSegmentCount()</a>: each state space provides the ability to compute how many segments the motion should be split into so that the number of states checked for validity is satisfactory. The user can define this resolution in terms of percentages of the space's maximum extent (<a class="el" href="classompl_1_1base_1_1StateSpace.html#a46ed7d102b6a65a2e2a03e70c7464a37" title="Get the maximum value a call to distance() can return (or an upper bound). For unbounded state spaces...">ompl::base::StateSpace::getMaximumExtent()</a>) by calling <a class="el" href="classompl_1_1base_1_1SpaceInformation.html#a2bcd47fd9b7cf54b086d2122646736bf" title="Set the resolution at which state validity needs to be verified in order for a motion between two sta...">ompl::base::SpaceInformation::setStateValidityCheckingResolution()</a> or by calling <a class="el" href="classompl_1_1base_1_1StateSpace.html#ad8e6450ad306d272df39751aa7e690b8" title="When performing discrete validation of motions, the length of the longest segment that does not requi...">ompl::base::StateSpace::setLongestValidSegmentFraction()</a> for individual state spaces. Different resolutions can be used in subspaces, if using <a class="el" href="classompl_1_1base_1_1CompoundStateSpace.html" title="A space to allow the composition of state spaces.">ompl::base::CompoundStateSpace</a>. If continuous collision checking is available, it is recommended that a different implementation of <a class="el" href="classompl_1_1base_1_1MotionValidator.html" title="Abstract definition for a class checking the validity of motions -- path segments between states...">ompl::base::MotionValidator</a> is provided, one that does not rely on discretizing the motion at a specific resolution. </p>
<div class="fragment"><pre class="fragment"> <span class="comment">// define this class:</span>
<span class="keyword">class </span>myMotionValidator : <span class="keyword">public</span> base::MotionValidator
{
<span class="keyword">public</span>:
<span class="comment">// implement checkMotion()</span>
};
base::SpaceInformationPtr si(space);
si-><a class="code" href="classompl_1_1base_1_1SpaceInformation.html#a69e58548f91db3e30ac7f41c87459279" title="Set the instance of the motion validity checker to use. Parallel implementations of planners assume t...">setMotionValidator</a>(base::MotionValidatorPtr(<span class="keyword">new</span> myMotionValidator(si)));
si-><a class="code" href="classompl_1_1base_1_1SpaceInformation.html#ac39aa0c4b92e3ca5acfa75eeb56b080f" title="Perform additional setup tasks (run once, before use). If state validity checking resolution has not ...">setup</a>();
</pre></div> </div></div>
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<a href="http://www.kavrakilab.org">Physical and Biological Computing Group</a> •
<a href="http://www.cs.rice.edu">Department of Computer Science</a> •
<a href="http://www.rice.edu">Rice University</a><br>
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