Graph Search Basis

Configuration Space

Robot degree of freedom (DOF): The minimum number n of real-valued coordinates need to represent the robot configuration.

Robot configuration space: a n-dim space containing all possible robot configurations, denoted as C-space

Each robot pose is a point in the C-space

Configuration Space Obstacle

Workspace - Configuration Space

• Obstacles need to be represented in configuration space (one-time work prior to motion planning), called configuration space obstacle, or C-obstacle.
• C-space = (C-obstacle) $\cup$ (C-free)
• The path planning is finding a path between start point $q_{start}$ and goal point $q_{goal}$ with C-free.

Workspace and Configuration Space Obstacle

• In workspace
  Robot has shape and size
• In configuration space: C-space
  Robot is a point
  Obstacle are represented in C-space prior to motion planning.

Graph and Search Method

Graphs: Graphs have nodes and edges
State space graph: a mathematical representation of a search algorithm.

Graph Search Overview

• Maintain a container to store all the nodes to be visited
• The container is initialized with the start state $X_S$
• Loop
  – Remove a node from the container according to some pre-defined score function.
  – Expansion: Obtain all neighbors of the node
  – Push them (neighbors) into the container.
• End Loop

Question 1: When to end the loop?
Possible option: End the loop when the container is empty.

Question 2: What if the graph is cyclic?
When a node is removed from the container (expand / visited), it should never be added back to the container again.

Question 3: In what way to remove the right node such that the goal state can be reached as soon as possible, which results in less expansion of the graph node.

Graph Traversal

Breadth First Search (BFS)

BFS uses “first in first out” = queue

• Strategy: remove / expand the shallowest node in the container

Depth First Search (DFS)

DFS uses “last in first out” = stack

• Strategy: remove / expand the deepest node in the container
• Implementation: maintain a last in first out (LIFO) container (i.e. stack)

versus

Breadth First Search（BFS，FIFO）能找出最短路径

Heuristic Search

Greedy Best First Search

• BFS and DFS pick the next node off the frontiers based on which was “first in” or “last in”.

• Greedy Best First picks the “best” node according to some rule, called a heuristic.

• Definition: A heuristic is a guess of how close you are to the target.
  

• A heuristic guides you in the right direction.

• A heuristic should be easy to compute.
  

Costs on Actions

• A practical search problem has a cost "C" from a node to its neighbor
  • Length, time, energy, etc.
• When all weight are 1, BFS finds the optimal solution
• For general cases, how to find the least-cost path as soon as possible?

Dijkstra and A*

Algorithm Workflow

Dijkstra’s Algorithm

• Strategy: expand/visit the node with cheapest accumulated cost g(n)
  • g(n): The current best estimates of the accumulated cost from the start state to node “n”
  • Update the accumulated costs g(m) for all unexpected neighbors “m” of node “n”
  • A node that has been expanded/visited is guaranteed to have the smallest cost from the start state.

伪代码

• Maintain a priority queue to store all the nodes to be expanded
• The priority queue is initialized with the start state $X_s$
• Assign $g(X_s)=0$, and g(n)=infinite for all other nodes in the graph
• Loop
  • If the queue is empty, return FALSE; break;
  • Remove the node “n” with the lowest g(n) from the priority queue
  • Mark node “n” as expanded
  • If the node “n” is the goal state, return True; break;
  • For all unexpanded neighbors “m” of node “n”
    • If g(m) = infinite
      • g(m) = g(n) + $C_{nm}$
      • Push node “m” into the queue
    • If g(m) > g(n) + $C_{nm}$
      • g(m) = g(n) + $C_{nm}$
  • End
• End Loop

priority queue = container = open list
已经扩展过的节点放到 close list

Pros and Cons of Dijkstra’s Algorithm

• The good:
  • Complete and optimal
• The bad:
  • Can only see the accumulated so far (i.e. the uniform cost), thus exploring next state in every “direction”
  • No information about goal location

Search Heuristics

• Recall the heuristic introduced in Greedy Best First Search
• Overcome the shortcomings of uniform cost search by inferring the least cost to goal (i.e. goal cost)
• Designed for particular search problem
• Examples: Manhattan distance VS. Euclidean distance

A*: Dijkstra with a Heuristic

• Accumulated cost
  • g(n): The current best estimates of the accumulated cost from the start state to node “n”
• Heuristic
  • h(n): The estimated least cost from node n to goal state (i.e. goal cost)
• The least estimated cost from start node to goal state passing through node “n” is f(n)=g(n) + h(n)
• Strategy: expand the node with cheapest f(n) = g(n )+ h(n)
  • Update the accumulated costs g(m) for all unexpanded neighbors “m” of node “n”
  • A node that has been expanded is guaranteed to have the smallest cost from the start node

伪代码

• Maintain a priority queue to store all the nodes to be expanded
• The heuristic function h(n) for all nodes are pre-defined
• The priority queue is initialized with the start state $X_s$
• Assign $g(X_s)=0$, and g(n)=infinite for all other nodes in the graph
• Loop
  • If the queue is empty, return FALSE; break;
  • Remove the node “n” with the lowest f(n) = g(n)+h(n) from the priority queue
  • Mark node “n” as expanded
  • If the node “n” is the goal state, return True; break;
  • For all unexpanded neighbors “m” of node “n”
    • If g(m) = infinite
      • g(m) = g(n) + $C_{nm}$
      • Push node “m” into the queue
    • If g(m) > g(n) + $C_{nm}$
      • g(m) = g(n) + $C_{nm}$
  • End
• End Loop

A* Optimality

• What went wrong?
• For node A: actual least cost to goal (i.e. goal cost) < estimated least cost to goal (i.e. heuristic)
• We need the estimate to be less than actual least cost to goal (i.e. goal cost) for all nodes!

Admissible Heuristics

• A Heuristic h is admissible(optimistic) if:
  • h(n) <= h*(n) for all node “n”, where h*(n) is the true least cost to goal from node “n”
• If the heuristic is admissible, the A* search is optimal
• Coming up with admissible heuristics is most of what’s involved in using A* in practice.

Heuristic Design

An admissible heuristic function has to be designed case by case.

• Euclidean Distance
  Is Euclidean distance (L2 norm) admissible? Always

• Manhattan Distance
  Is Manhattan distance (L1 norm) admissible? Depends

Is L $\infty$ norm distance admissible? Always
Is 0 distance admissible? Always

A* expands mainly towards the goal, but does not hedge its bets to ensure optimality.

Sub-optimal Solution

What if we intend to use an over-estimate heuristic?
Suboptimal path and Faster
Weighted A*: Expands states based on $f=g+\epsilon h$,$\epsilon >1$=bias towards states that are closer to goal.

• Weighted A* Search
  • Optimality vs. speed
  • $\epsilon$-suboptimal: cost(solution) <= $\epsilon cost(optimalsolution)$
  • It can be orders of magnitude faster than A*

Weighted A* -> Anytime A* -> ARA* -> D*

Greedy Best First Search vs. Weighted A* vs. A*

Engineering Considerations

Grid-based Path Search

• How to represent grids as graphs?
  Each cell is a node. Edges connect adjacent cells.

Implementation

• Create a dense graph.
• Link the occupancy status stored in the grid map.
• Neighbors discovered by grid index.
• Perform A* search.

\;

• Priority queue in C++
• std::priority_queue
• std::make_heap
• std::multimap

The Best Heuristic

They are useful, but none of them is the best choice, why?
Because none of them is tight
Tight means who close they measure the true shortest distance.

Why so many nodes expanded?
Because Euclidean distance is far from the truly theoretical optimal solution.

\;

How to get the truly theoretical optimal solution
Fortunately, the grid map is highly structural.

• You don’t need to search the path.
• It has the closed-form solution!
  $$\begin{gathered} dx=abs(node.x-goal.x) \\ dy=abs(node.y-goal.y) \\ h=(dx+dy)+(\sqrt{2}-2)\ast min(dx,dy) \end{gathered}$$

diagonal Heuristic h(n) = h*(n)

Tie Breaker

• Many paths have the same f value.

• No differences among them making them explored by A* equally.
  

• Manipulate the $f$ value breaks tie

• Make same $f$ values differ.

• Interfere $h$ slightly.

  • $h=h\times (1.0+p)$
  • $p<\frac{minimumcostofonestep}{expectedmaximumpathcost}$

Core idea of tie breaker:
Find a preference among same cost paths

• When nodes having same $f$, compare their $h$

• Add deterministic random numbers to the heuristic or edge costs (A hash of the coordinates).

• Prefer paths that are along the straight line from the starting point to the goal.
  $$\begin{gathered} dx1=abs(node.x-goal.x) \\ dy1=abs(node.y-goal.y) \\ dx2=abs(start.x-goal.x) \\ dy2=abs(start.y-goal.y) \\ cross=abs(dx1\times dy2-dx2\times dy1) \\ h=h+cross\times 0.001 \end{gathered}$$

• … Many customized ways

\;

• Prefer paths that are along the straight line from the starting point to the goal.
  

Jump Point Search

Core idea of JPS
Find symmetry and break them.

JPS explores intelligently, because it always looks ahead based on a rule.

Look Ahead Rule

Consider

• current node $x$
• $x$’s expanded direction
  

Neighbor Pruning

• Gray nodes: inferior neighbors, when going to them, the path without $x$ is cheaper. Discard.
• White nodes: natural neighbors
• We only need to consider natural neighbors when expand the search.

Forced Neighbors

• There is obstacle adjacent to $x$
• Red nodes are forced neighbors.
• A cheaper path from $x$’s parent to them is blocked by obstacle.

Jumping Rules

• Recursively apply straight pruning rule and identify y as a jump point successor of $x$. This node is interesting because it has a neighbor z that cannot be reached optimally except by a path that visit $x$ then $y$.
• Recursively apply the diagnol pruning rule and identify y as a jump point successor of $x$
• Before each diagonal step we first recurse straight. Only if both straight recursions fail to identify a jump point do we step diagnally again.
• Node w, a forced neighbor of $x$, is expanded as normal. (also push into the open list, the priority queue)

pruning rule = look ahead rule

pesudo-code

Recall A* 's pseudo-code, JPS’s is all the same!

• Maintain a priority queue to store all the nodes to be expanded
• The heuristic function h(n) for all nodes are pre-defined
• The priority queue is initialized with the start state $X_s$
• Assign $g(X_s)=0$, and g(n)=infinite for all other nodes in the graph
• Loop
  • If the queue is empty, return FALSE; break;
  • Remove the node “n” with the lowest f(n) = g(n) + h(n) from the priority queue
  • Mark node “n” as expanded
  • If the node “n” is the goal state, return TRUE, break;
  • For all unexpanded neighbors “m” of node “n”
    • If g(m) = infinite
      • g(m) = g(n) + $C_{nm}$
      • Push node “m” into the queue
    • If g(m) > g(n) + $C_{nm}$
      • g(m) = g(n) + $C_{nm}$
  • end
• End Loop

Example

Extension

3D JPS

Planning Dynamically Feasible Trajectories for Quadrotors using Safe Flight Corridors in 3-D Complex Environments, Sikang Liu, RAL 2017
KumarRobotics/jps3d

Is JPS always better?

• This is a simple example saying “No.”
• This case may commonly occur in robot navigation.
• Robot with limited FOV, but a global map/large local map.
  
  Conclusion:
• Most time, especially in complex environment, JPS is better, but far away from “always”. Why?
• JPs reduces the number of nodes in Open List, but increase the number of status query.
• You can try JPS in Homework2.
• JPS's limitation: only applicable to uniform grid map.

Homework

Assignment: Basic

• This project work will focus on path finding and obstacle avoidance in a 2D grid map.
• A 2D grid map is generated randomly every time the Project is run, which contains the obstacles, start point and target point locations will be provided. You can also change the probability of obstacles in the map in obstacle_map.m
• You need to implement a 2D A* path search method to plan an optimal path with safety guarantee.

Assignment: Advance

• I highly suggest you implement Dijkstra/A* with C++/ROS
• Complex 3d map can be generated randomly. The sparsity of obstacles in this map is tunable.
• An implementation of JPS is also provided. Comparisons can be made between A* and JPS in different map set-up.