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Re: pollyfillv, area of a blob, etc... [message #59657 is a reply to message #59581]

Fri, 04 April 2008 04:03

jameskuyper
Messages: 79
Registered: October 2007

Member

JP wrote:
> On Apr 4, 1:16 am, David Fanning <n...@dfanning.com> wrote:
>> I suppose the centroid of the polygon is the average
>> value of it vertices, or maybe the mean if you didn't
>> want to overbias outliers.
>
> I guess you meant "the average value of it vertices, or maybe the
> MEDIAN" ??
>
> In any case, thank you. I didn't realize that the centroid is just the
> mean of the vertices in the two dimensions.

It isn't. David's routine calculates the centroid of the vertices
themselves; that's not the same as the centroid of the polygon that they
define.

Consider the polygon [[0,0],[1,0],[1,1],[0,1],[0,0.5]]. It's centroid is
clearly [0.5,0.5], the same as the one for [[0,0],[1,0],[1,1],[0,1]],
but the vertices have a different average. Taking the mean of the
vertices is at best an approximation.

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[Message index]

		pollyfillv, area of a blob, etc... By: JP on Wed, 02 April 2008 23:16
		Re: pollyfillv, area of a blob, etc... By: pgrigis on Fri, 04 April 2008 07:18
		Re: pollyfillv, area of a blob, etc... By: David Fanning on Fri, 04 April 2008 05:31
		Re: pollyfillv, area of a blob, etc... By: jameskuyper on Fri, 04 April 2008 04:03

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