Members of the Forum -
I'm stymied by what seems like a simple problem that goes something like
this: I have a set of points longer in one direction than the other, like
this:
$pts=. <.(]-"1 mean) ~.|:+/&>?&.>(<3 1000)$&.>20 80 NB. mean=: +/%#
913 2
viewmat 1 bordFill pts
NB. where
bordFill=: 4 : '(1)(<"1 y-"1 x-~<./y)}0$~2$(>:+:x)+(>./y)-<./y' NB. x
border around integer point pairs y
I want to identify points near the perimeter. One idea I had for doing this
is to use polar coordinates,
pp=. c2p"1 pts NB. Polar points: length, angle
NB. Where:
p2c=: +. @ (r./) NB.* p2c: polar to cartesian (point pair)
c2p=: ({.,2p1&|@{:)@:(*. @ (j./)) NB.* c2p: cartesian to polar (point pair)
I want to get a rough idea of the perimeter by dividing the angles into,
say, 32 even groups and selecting the 10 most distant points within each
"wedge".
gv=. /:1{"1 pp NB. Order by angle
I'm not sure why the following does not seem to work correctly:
lens=. (<.32*gv%#gv) </. 0{"1 pp NB. Group lengths by angle sector
farpps=. (<.32*gv%#gv) </. pts NB. Same grouping for points
$farpps=. ;((10{.\:)&.>lens){&.>farpps NB. Points corresponding to 10
longest lengths
320 2
[The expression "<.32*gv%#gv" is supposed to map the grade vector to key
values 0-31.]
If this worked correctly, I'd expect the following image to show a ring of
differently-colored points as a perimeter:
viewmat (2) (<"1]1+farpps-"1 <./pts)}1 bordFill pts
Instead, the coloring seems to apply only to the points furthest from the
center, which tend to be those on either side along the longer axis.
Does this make sense? Can anyone see what I'm doing wrong or suggest
another method?
Thanks,
Devon
--
Devon McCormick, CFA
^me^ at acm.
org is my
preferred e-mail
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