[OctDev] improve polyarea

[Dr.-Ing. Torsten Finke]

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: [hidden email]

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[Paul Kienzle-3]

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
>



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