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README.md

htm

Hierarchical Triangular Mesh

This is a copy of the SDSS HTM implementation from [1] developed by O'Mullane, Kunszt, Szalay

Original Readme

   	       	       	       	       	       	       	       	       	       |

Configurtion notes: There is a Makefile.generic in this directory. Makefile is made by the configure shell script. DO NOT EDIT Makefile, edit Makefile.generic instead. The plan is to have all system related things defined as (see Makefile.generic for an example) stuff = @@stuff## and have a stream editor change them everywhere. Only u*x (unix, linux, irix, darwin, bsd, solaris...) benefits, Windows flavors have their own project files, and one day, nmakefiles

There are several changes in this release. The bitlist is gone. Partial and full lists are gone. They have been merged into a data datype called HtmRange.

This single data type can represent many ways of looking at large (or small) numbers of HTMIDs.

The (inaptly named) application "intersect" reads domain descripion files and spits out hids (HTM-ID's) to the standard output. Depending on what the user wants the number of hids can be potentially very large. Consider a case where the DOMAIN consists of a large area, and the application asks to produce all hids at level 20! For a concrete example, consider a small disk around the "north pole" described by the DOMAIN description file: (testTiny in this directory) ---------------------- #DOMAIN 1 #CONVEX 1 0.0 0.0 1.0 0.9999 ---------------------- The number of level 20 triangles or, as we now call them, trixels (triangle shaped pixels) is on the order of about a billion (10^9). It may be unfeasible to put all those numbers (hids) into the output stream, so HtmRanges allow us to exploit the numeric coherence implicit in large sets of hids. Although in general, it is not necessarily true that two consecutive hids represent adjacent trixels, it is true, that the set of all hids derived from a single parent triangle form a single connected component. In plain words, if you sort them there are no gaps. Therefore, it is convenient to represent these large chunks of hids as a range, or more precisely, an ordered pair: (low_hid, high_hid). Any arbitrary DOMAIN is therefore represented as a collection (list, vector, array, etc...) of (lo, hi) pairs.

The best way to play with the HTM interface is by playing with the intersect application.

Consider the DOMAIN description ---------------testTiny ------------------------ #DOMAIN 1 #CONVEX 1 0.0 0.0 1.0 0.9999 ---------------END: testTiny--------------------

intersect produces the ranges

% intersect 20 testTiny
13469017440256 13469285875711
14568529068032 14568797503487
15668040695808 15668309131263
16767552323584 16767820759039

Consider the DOMAIN description If you desire to see the largest triangles represented by an hid that is still contained in the DOMAIN, use an option that tells the application to produce variable length hids instead of ranges.

% intersect -varlength 20 testTiny
50176
54272
58368
62464

--------- further examples -

The file testInputIntersect contains the following DOMAIN description:

------------------- testInputIntersect -----------------
#DOMAIN
1
#CONVEX
3
0.9 0.007107 0.05 0.780775301220802
0.5 0.5 0.707107 0.63480775301220802
0.707107 -0.5 0.3 0.8480775301220802
------------------- END: testInputIntersect -------------

Run the intersect program produces 141 ranges. The first three and the last three are shown here:

% intersect  20 testInputIntersect
13211319402496 13213466886143
13213802430464 13213803479039
13214003757056 13214305746943
...
14214513033216 14214529810431
14215536443392 14215670661119
14215771324416 14215788101631

The variable length option would produce 207 hids. The first and last are respectively:

3089
14170322436984

Representing trixels 'N0100000' and 'N032031021002100031320'. Run intersect without arguments to get usage info:

-----------------------------------------------------------------------------------
usage: 
intersect [-save savelevel] [-verbose] [-olevel olevel] level domainfile
[-save savelevel]   : store up to this depth (default 2)
[-verbose]          : verbose
[-olevel out_level] : output HTMID level (must be >= level)
[-symbolic]         : output HTMID as a symbolic name, must be used with -varlength
[-varlength]        : output natural HTMID (conflicts with -olevel)
[-nranges nrages]   : keep number of ranges below this number
level               : Maximal spatialindex depth 
domainfile          : filename of domain 
-----------------------------------------------------------------------------------



	    Comments about this READ_ME? Please send me mail
	    mailto:gfekete@pha.jhu.edu

Some stats:

At level 20 the largets HID is N333333333333333333333 = 17592186044415 The smallest HID is S000000000000000000000 = 8796093022208 The difference is 17592186044415

  • 8796093022208 = .................... = 8796093022207

17,592,186,044,415 8,796,093,022,207

about 17 and a half trillion. difference is about 8 and three quarters trillion.

At level 12 The largest HID is N3333333333333 = 268435455 The smallest HID is S0000000000000 = 134217728 268435455-134217728 The difference is = 134217727 16777219 268,435,455 134,217,728

about 268 million. difference is about 134 million.

(16777219 16777230 369098836) 16777219

[1] http://www.skyserver.org/htm/implementation.aspx

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