Lecture 6 - Maps and Alignment Flashcards
What are the 4 different categories of maps?
Metric
• Map accurately represents lengths and angles
Topological
• Map is reduced to a graph representation of the structure of free space
Topometric
• Atlas: a combination of local metric maps (nodes) connected via edges
Sequence of raw time-series observations (e.g. video)
• No metric or topological information directly represented by the map
What is a common type of metric map?
Occupancy Grids
What are two common problems with Octrees and Quadtrees?
Problem 1: quadtrees and octrees are not balanced trees. So, in the worst case an occupancy query could be O(n) in the number of nodes. Problem 2: quadtrees and octrees are sensitive to small changes in the location of obstacles.
Give one advantage and disadvantage of Occupancy Grids
Lookup is O(1).
Doesn’t scale well in higher dimensions.
What’s the purpose of a signed distance function?
It allows smooth surfaces.
What are the advantages and disadvantages of pointclouds
Advantages:
- can make local changes to the map without affecting the pointcloud globally
- can alight pointclouds
- nearest neighbor queries are easy with kd-trees or locality-sensitive hashing
Disadvantages:
- need to segment objects in the map
- raytracing is approximate and nontrivial
What map category does a Generalized Voronoi Graph belong to?
A topological map. It is the set of edges that are at equal distances to obstacles.
What is deformation retraction?
Generalized voronoi map with edges and obstacle points that are also equidistant.
How does small changes in the object’s boundary affect the skeleton of a Voronoi Graph?
The Voronoi graph is highly sensitive to even the smallest changes.
What is one advantage of Voronoi Graphes?
Can specify whether we pass on the left or right of each obstacle on our path to the goal.
What are classes of paths that can be deformed without hitting obstacles called?
Homotopy Classes
What’s the ICP-algorithm?
Iterated Closest Points (ICP).
• Associates each point in the source scan to its nearest neighbor in the target scan.
• Finds the optimal rotation and translation for this correspondence.
• Repeat until no significant drop in error.
What’s the most common sensor to collect ranges?
Laser range-finder (2D or 3D) LIDAR.
Moves the vehicle, scans each location, then aligns all scans.
How can you build a map with LIDAR?
Move the vehicle, scan each location, then align all scans.
What is an advantage of topometric maps?
Main advantage: allows us combine accurate local maps into a global, typically inconsistent map that nevertheless provides sufficient navigation information.