Re: [ompl-users] Saving and Reusing RRT* tree

[Beth B. <bbo...@en...>]

Mark,

Thanks for the reply.

My plan is to write a new planner that does replanning. My paper that has the basic concept (somethings have now changed) on what I’m trying to implement is
http://permalink.lanl.gov/object/tr?what=info:lanl-repo/lareport/LA-UR-14-24990 <http://permalink.lanl.gov/object/tr?what=info:lanl-repo/lareport/LA-UR-14-24990>

I’ve focus on using the RRT* as the planner the robot would be using. Ideally the RRT* would be rooted at the goal state not the initial state, which is something else I know I will have to change in the code, but for now I can work around it.
Once a new obstacle is introduced, the tree is pruned of all invalid states, which I think i have figured out how to do in the framework of the OMPL and RRT*.
I would then run this new, pruned, tree back through the RRT* to continue building it in the space defined by the states that were removed.

What I need to save from the first run of the RRT* is the full tree. All vertices, edges, edge costs, and possibly cost-to-go though I can work around that. Does PlannerData (PlannerDataStorage) save all vertices, edges, edge costs? I’m look through PlannerDataStorage and this looks like the case using PlannerDataVertex and PlannerDataEdge. How are the edge end points ordered, do you know? For a tree would they be ordered parent then child? 
Using these I should be able to reconstruct the tree in the structure used by the OMPL RRT*. 

Bottom line is I need to know how to save the graph vertices, edges, and edge costs/weights and then load them inside another planner.

Beth



> On Sep 21, 2016, at 9:02 PM, Mark Moll <mm...@ri...> wrote:
> 
> Hi Beth,
> 
> Support for replanning is not built into OMPL. PlannerData (or PlannerDataStorage if you want to save to / load from disk) is probably a good starting point for replanning. If the environment is changing (or at least the model used for planning is changing), then it’s not clear how much information you want to retain between re-planning cycles. Extracting the subtree of states rooted at the last valid state along the path that was executed is something that has to be done separately for each planner.
> 
> Perhaps you could derive a new planner from LazyPRM*, where you add a method to reset the validity of all states and edges to UNKNOWN when you wan to replan? This way you start with a roadmap of samples that probably still collision-free (i.e., you don’t have to sample a whole bunch of states that are in collision), you can easily restart at any state in the roadmap, more alternative routes than RRT* in case unknown obstacles appear, and still preserve asymptotic optimality.
> 
> Mark
> 
> 
> 
>> On Sep 21, 2016, at 1:02 PM, Beth Boardman <bbo...@en... <mailto:bbo...@en...>> wrote:
>> 
>> Hi,
>> 
>> I am looking for advice on how to save a tree, created using RRT*, and then be able to load the saved tree into another planner to be reused.
>> 
>> This pertains to my work on a replanning when an unexpected obstacle shows up in the environment. Ideally, what I want to do is build a tree that is rooted at the goal (the goal part isnt important for this post). Then, after the robot moves to some new location it realizes its path is blocked by a previously unknown obstacle. The tree would then be pruned to remove any conflicting states and edges (this part I think I've figured out).
>> Then I want to run this pruned tree back through the RRT* to fill in the hole that was made from pruning.
>> 
>> I have most of this figured out, except how to either
>> 1) save the tree and then open it again for reuse; OR
>> 2) update the initial (or goal) state and obstacle set midway through a planning session
>> 
>> It seems like PlannerData might have some of the information I need, but not entirely sure.
>> 
>> Any ideas or places I should look are much appreciated. Most of my coding has been done in matlab, where I have a basic working version of the above described algorithm.
>> 
>> 
>> Thanks,
>> - Beth 
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