Star Selection Part 3

20 Jul 2011 22:29
Last Modified
13 Jan 2013 17:33

I discussed in parts 1 and 2 an approach to allow efficient selection of nearby stars to a reticle. I originally used an index which divided the sphere volume into 50 equal divisions along each axis, giving 25,000 cubes. This gave a distribution of items per key as shown in Figure 1.

Spatial Index

Figure 1. Hipparcos Cartesian Spatial Index.

Switching to a Hierarchical Triangular Mesh (HTM), also known as Quaternary Triangular Mesh (QTM), gives a more even distribtion with less cells containing very few stars, as shown for subdisivion level 5 in Figure 2. This results in a more consistent behaviour in selecting nearby stars.

Spatial Index

Figure 2. Hipparcos HTM L5 Spatial Index.

Using HTM also makes it easy to index at multiple levels of detail, such that an appropriate index can be used at a given field of view.

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