octave-dev

[OctDev] improve polyarea

[Dr.-Ing. T. F. <fi...@ig...>]

Hello Octave-Forge developers, 

may I suggest a little improvement to the function polyarea which calculates the
area a of a given polygon with vertices (x, y). 

Now it is calculated as (line 47 of polyarea.m): 

  a = abs ( sum (x .* shift (y,-1))  - sum (y .* shift (x, -1)) ) / 2;

Since 

  sum (y .* shift (x, -1)) == sum (shift(y, 1) .* x)

one can reduce the amount of summation and multiplication to just the half: 

  a = abs (sum (x .* (shift (y, 1) - shift (y, -1))) / 2);


Thank You and best regards


Torsten Finke

  

-- 

------------------------------------------------------------------------

Dr.-Ing. Torsten Finke
Ingenieurgemeinschaft IgH
Heinz-Baecker-Str. 34
D-45356 Essen
Tel.: +49 201 / 360-14-17
Fax.: +49 201 / 360-14-14
E-mail: tor...@ig...

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Re: [OctDev] improve polyarea

[Paul K. <pki...@us...>]

On Nov 13, 2005, at 8:00 AM, Dr.-Ing. Torsten Finke wrote:

> Hello Octave-Forge developers,
>
> may I suggest a little improvement to the function polyarea which 
> calculates the
> area a of a given polygon with vertices (x, y).
>
> Now it is calculated as (line 47 of polyarea.m):
>
>   a = abs ( sum (x .* shift (y,-1))  - sum (y .* shift (x, -1)) ) / 2;
>
> Since
>
>   sum (y .* shift (x, -1)) == sum (shift(y, 1) .* x)
>
> one can reduce the amount of summation and multiplication to just the 
> half:
>
>   a = abs (sum (x .* (shift (y, 1) - shift (y, -1))) / 2);

Applied.

Thanks,

	- Paul
>