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Re: point inside polygon [message #11495 is a reply to message #11494]

Wed, 01 April 1998 00:00

wmc
Messages: 117
Registered: February 1995

Senior Member

In article DF0F2DFA@fys.uio.no, "B�rd Krane" <bard.krane@fys.uio.no> writes:
> 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
(some bits cut)
> sx = size(px)
> sy = size(py)
> N=sx(1)
> 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

Interesting... there had to be a better way and this looks like it.
I'm now trying to work out why it works... I think you're counting up the angles
going round the polygon to the point, and the sum is zero outside and 2*!pi
inside.

Only one criticism: 1e-8 is too tight a test for single precision:
inside(.5,1.5,[0,1,1,0],[0,0,1,1]) returns 1
since total(theta) is -1.19e-7.
But inside(.5,1.5d0,[0,1,1,0],[0,0,1,1]) returns 0 as it should.
So I think the test should be 1e-5 or somesuch (though presumably .1 would work
just as well?).

- William

---
William M Connolley | wmc@bas.ac.uk | http://www.nbs.ac.uk/public/icd/wmc/
Climate Modeller, British Antarctic Survey | Disclaimer: I speak for myself

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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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