Lecture 6 - Maps and Alignment Flashcards

1
Q

What are the 4 different categories of maps?

A

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

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2
Q

What is a common type of metric map?

A

Occupancy Grids

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3
Q

What are two common problems with Octrees and Quadtrees?

A
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.
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4
Q

Give one advantage and disadvantage of Occupancy Grids

A

Lookup is O(1).

Doesn’t scale well in higher dimensions.

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5
Q

What’s the purpose of a signed distance function?

A

It allows smooth surfaces.

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6
Q

What are the advantages and disadvantages of pointclouds

A

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
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7
Q

What map category does a Generalized Voronoi Graph belong to?

A

A topological map. It is the set of edges that are at equal distances to obstacles.

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8
Q

What is deformation retraction?

A

Generalized voronoi map with edges and obstacle points that are also equidistant.

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9
Q

How does small changes in the object’s boundary affect the skeleton of a Voronoi Graph?

A

The Voronoi graph is highly sensitive to even the smallest changes.

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10
Q

What is one advantage of Voronoi Graphes?

A

Can specify whether we pass on the left or right of each obstacle on our path to the goal.

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11
Q

What are classes of paths that can be deformed without hitting obstacles called?

A

Homotopy Classes

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12
Q

What’s the ICP-algorithm?

A

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.

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13
Q

What’s the most common sensor to collect ranges?

A

Laser range-finder (2D or 3D) LIDAR.

Moves the vehicle, scans each location, then aligns all scans.

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14
Q

How can you build a map with LIDAR?

A

Move the vehicle, scan each location, then align all scans.

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15
Q

What is an advantage of topometric maps?

A

Main advantage: allows us combine accurate local maps into a global, typically inconsistent map that nevertheless provides sufficient navigation information.

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16
Q

What’s the main idea behind maps of raw observations?

A
  • Map = entire (unprocessed) sequence of observations, e.g. video.
  • Do not try to support distance, collision, and raytracing queries.
  • Instead, provide only a similarity/nearest neighbors query
  • “Find the image in the video that is most similar to the one I’m seeing now.”
  • History of observations determines a (set of) location(s) in the map
17
Q

What do we say about scan alignments where the correct correspondences are known?

A

We say that the data association is known/unambiguous.

18
Q

Where could scan correspondences come from?

A
  • We could have an extra piece of knowledge
    like room numbers beside office doors
  • We could run an algorithm to “match”
    similar looking places in the world (ideas in
    Computer Vision COMP 558)