Marching squares: Difference between revisions
→{{header|J}}: yes, an oval, but...
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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).
While the above implementation is incomplete, it seems to adequately handle an oval of cassini with focal points at X=1, -1 and Y=0:
<lang J>a=: 1
X=: |:Y=:201#"0]0.02*i:100
Z0=: (*:(*:X)+(*:Y)) + (_2*(*:a)*X -&*: Y) + *:a
Z=: (Z0>:0.8)*Z0<:1.2
C=: unwind step2 step1 Z</lang>
Here, Z is a bitmap, and C is a list of three contours (one with 336 distinct coordinate pairs, and two with 134 distinct coordinate pairs) which surround that bitmap. These can be inspected (after <code>require'viewmat'</code>) with <code>viewmat Z</code> and <code>viewmat 1 (<"1;C)} 200 200$0</code>, and look plausible.
(Presumably the above implementation would fail if a threshold had been picked such that the bitmap exhibited a saddlepoint at the origin.)
=={{header|Julia}}==
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