Re: Point inside/outside area constructed by an expanded path

  On 10/14/10 12:00 PM, Josi Perez (3T Systems) wrote:
> Hi,
>
> I use postgreSQL.8.4.2 and don't have postGIS installed.
> I have a geometric type polygon and I know that it is possible to 
> determine if a point it is inside or not of this polygon, using 
> anything like that:
> select 
> polygon('(-23.5672,-46.6510),(-23.5670,-46.65095),(-23.5672,-46.6508),(-23.5672,-46.6510)') 
> @> point('-23.567534,-46.64942')
>
> The question:
> Anyone knows an algorithm or have ideas how to determine if a point it 
> is inside an area constructed by geometric type path expanded for a 
> known distance?
> Graphically:
>
> the path
>             x=========x=======x
>
> the path expanded by a distance
>
>             |         | A     |
>             |         |       |
>      -------x=========x=======x-------  C
>             |         |       |
>         B   |         |       |
>
> the points A and B are inside and the point C are outside .

that's pretty tricky.    are the 'end caps' on that expansion 
rectangular or circular?  if circular is acceptable,  you could perhaps 
calculate the shortest distance of your point from the path (non trivial 
if the path is complex), then if that distance is < your 'expansion', 
its 'inside'.  probably be a good idea to use an overall bounding box to 
reject points that are far away to save a lot of overhead.