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Re: Understanding IDLanROI [message #73195 is a reply to message #73156]

Wed, 27 October 2010 08:44

guillermo.castilla.ca is currently offline

guillermo.castilla.ca
Messages: 27
Registered: September 2008

Junior Member

On Oct 25, 1:17 pm, Matt <sav...@nsidc.org> wrote:
> Is this the preferred method to determine the vertex's
> direction? internal vs external?

The below function will tell you whether a ring (i.e., a closed chain
of vertices defined by their x y coordinates) is a hole (i.e., is
ordered counterclockwise). It is based on the same principle than
Kelly's trick, but it uses only the first 3 vertices of the ring
(which form a triangle from which area can be computed), and therefore
it should be a little more efficient than the other method.

;+
; :Description: Determines whether a list of vertices forming a ring
is a hole
; (i.e., is ordered counterclockwise) or not (and then is clockwise)
;
; :Params:
; xy : in, required, type="dblarr(2\, nVertices)"
;
; :Returns: 1, if the ring is a hole; 0, otherwise
; :Categories: IDLffShape
;-
FUNCTION ishole, xy

dims = SIZE(xy, /DIMENSIONS)
IF dims[0] NE 2 THEN MESSAGE, 'The xy array must have two columns'
IF dims[1] LE 2 THEN MESSAGE, 'xy must contain at least three
vertices'

answer = (xy[0,2] - xy[0,0]) * (xy[1,1] - xy[1,0]) - $
(xy[0,1] - xy[0,0]) * (xy[1,2] - xy[1,0]) LT 0 ? 1 : 0

RETURN, answer

END

Cheers

Guillermo

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[Message index]

		Understanding IDLanROI By: KRDean on Fri, 08 October 2010 14:13
		Re: Understanding IDLanROI By: KRDean on Tue, 19 October 2010 08:02
		Re: Understanding IDLanROI By: David Fanning on Tue, 19 October 2010 10:38
		Re: Understanding IDLanROI By: guillermo.castilla.ca on Wed, 20 October 2010 10:01
		Re: Understanding IDLanROI By: KRDean on Fri, 22 October 2010 07:16
		Re: Understanding IDLanROI By: Matt[2] on Mon, 25 October 2010 09:17
		Re: Understanding IDLanROI By: guillermo.castilla.ca on Wed, 27 October 2010 08:44
		Re: Understanding IDLanROI By: guillermo.castilla.ca on Mon, 01 November 2010 06:23
		Re: Understanding IDLanROI By: guillermo.castilla.ca on Mon, 01 November 2010 06:24
		Re: Understanding IDLanROI By: David Fanning on Tue, 19 October 2010 14:02
		Re: Understanding IDLanROI By: KRDean on Tue, 19 October 2010 13:19

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