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Re: point inside polygon [message #11504 is a reply to message #11494]

Wed, 01 April 1998 00:00

B}rd Krane
Messages: 5
Registered: April 1997

Junior Member

Alex Schuster wrote:
>
> William Connolley wrote:
>
>> In article C0684CDB@oma.be, Philippe Peeters <philp@oma.be> writes:
>>> Does anybody knows of an IDL function to test whether a given point is
>>> inside a polygon?
>>
>> I needed to solve this recently (in a mapping context). The solution I came up
>> with works but its not elegant: use poly_fill to actually draw your polygon
>> (in a pixmap not the screen window if you prefer), then read off the pixel value
>> of your point to see if its in or out.
>>
>> This is grotesquely inelegant, but its very simple and it works. I can
>> post the code if you're interested. A better solution
>> would be to look at polyfill and see how it does the fill... but sadly
>> polyfill seems to be one of the few routines not written in IDL.
>

This function determines if a point is inside a polygon or not. If you
have
several points I believe you are better off with the polyfillv approach.
Baard

FUNCTION inside, x, y, px, py

sx = size(px)
sy = size(py)
IF (sx[0] EQ 1) THEN NX=sx[1] ELSE return,-1 ; error if px not a
vector
IF (sy[0] EQ 1) THEN NY=sy[1] ELSE return,-1 ; error if py not a
vector
IF (NX EQ NY) THEN N = NX ELSE return,-1 ; Incompatible
dimensions

tmp_px = [px, px[0]] ; Close Polygon in x
tmp_py = [py, py[0]] ; Close Polygon in y

i = indgen(N) ; indices 0...N-1
ip = indgen(N) + 1 ; indices 1...N

X1 = tmp_px(i) - x & Y1 = tmp_py(i) - y
X2 = tmp_px(ip) - x & Y2 = tmp_py(ip) - y

dp = X1*X2 + Y1*Y2 ; Dot-product
cp = X1*Y2 - Y1*X2 ; Cross-product
theta = atan(cp,dp)

IF (abs(total(theta)) GT 1.0E-8) THEN return,1 ELSE return,0
END

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[Message index]

		Re: point inside polygon By: LC's No-Spam Newsread on Thu, 09 April 1998 00:00
		Re: point inside polygon By: manizade on Wed, 01 April 1998 00:00
		Re: point inside polygon By: wmc on Wed, 01 April 1998 00:00
		Re: point inside polygon By: Philippe Peeters on Wed, 01 April 1998 00:00
		Re: point inside polygon By: B}rd Krane on Wed, 01 April 1998 00:00
		Re: point inside polygon By: Alex Schuster on Wed, 01 April 1998 00:00
		Re: point inside polygon By: wmc on Wed, 01 April 1998 00:00

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