Marching squares: Difference between revisions
J: lame partial implementation -- no treatment of saddle points (this version will throw an error for saddle points)
(J: lame partial implementation -- no treatment of saddle points (this version will throw an error for saddle points)) |
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See: [https://en.wikipedia.org/wiki/Marching_squares Marching squares]
=={{header|J}}==
This is a partial implementation, see the talk page for some discussion of the untouched issues.
<lang J>step1=: {{2 2 #.@(0 1 3 2&{)@,;._3 ,&0^:2@|:@|.^:4 y}}
step2=: {{(($y)#:i.$y) {{<x step2a y{::LUT}}"1 0 y}}
step2a=: {{ if. #y do. x+"1 y else. y end. }}
LUT=: <@".;._2 {{)n
EMPTY NB. 0
0 0,:1 1 NB. 1
0 1,:1 0 NB. 2
0 0,:0 1 NB. 3
0 0,:1 1 NB. 4
a: NB. 5 not used
0 0,:1 0 NB. 6
a: NB. 7
1 0,:0 1 NB. 8
1 1,:0 1 NB. 9
a: NB. 10 not used
a: NB. 11 not used
a: NB. 12 not used
a: NB. 13 not used
EMPTY NB. 14
EMPTY NB. 15
}}
unwind=: {{
near=. 7 8 5 1 3 {,(+/~ *&({:$y))i:1
r=., c=. EMPTY
TODO=. I.(<EMPTY)~:Y=.,y
j=. _
while.#TODO=. TODO-.j do.
adj=. (j+near) ([-.-.) TODO
if. #adj do.
j=. {.adj
else.
if. #c do. c=.EMPTY [r=. r,<~.c end.
j=. {.TODO
end.
c=. c, j{::Y
end.
r,<~.c
}}</lang>
Task example:
<lang J> img=: 4~:i.3 2
img
1 1
1 1
0 1
unwind step2 step1 img
┌───┐
│1 2│
│2 1│
│3 1│
│4 2│
│5 3│
│4 4│
│3 4│
│2 4│
│1 3│
└───┘</lang>
Here, <code>img</code> is a bitmap. We pad the bitmap by surrounding it with zeros during processing. The box at the end contains a contour corresponding to the bitmap. Here, the first column represents row number (row 0 at the top) and the second column represents column number (column 0 at the left). Each contour represents a closed loop (so the final coordinate pair would connect with the first coordinate pair).
=={{header|Julia}}==
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