[postgis-users] shp2pgsql inner/outer ring detection bug FIX

[strk]

I'm sorry but you got me wrong: the bug is NOT in postgis_fn.c, but
in loader/shp2pgsql.c. There is a PIP function also there with the
loop check running while i<n ... It is for the rest identical to
the one you posted here (apart from vt declared inside the loop and
itscomputatio casted to (float)).

I'm not familiar with shapelib code myself, I'll check the 
last-vertex issue, however even if the last vertex != first vertex
referencing V[n] will go past the allocated memory for the
array... in this case shp2pgsql should check the value of i
and using V[i], V[0] when i=n-1.

--strk;

dblasby wrote:
> > Anyway for my case shapefile was not "dirty". The PIP function was ...
> >
> > Checking the same function in postgis_fn.c I've found the bug:
> > the edge loop check has to be i<(n-1) instead of i<n
> > Fixing this everything works fine.
> 
> I dont think this is correct - the V array has n points in it - V[0] ... V[n-1]
> 
> If you change "i<(n-1)" to "i<n" then (in the loop), "i" will eventually get to be
> the value (n-1).
> 
> The V[i+1] reference will then be V[(n-1)+1] = V[n] which is past the end of the
> array (bad news).
> 
> "i" is the edge number - there are (n-1) edges in a [PostGIS] polygon that has the
> 1st point = last point.
> 
> Now, I've looked at the shapefile spec, and it doesnt seem to say that the first
> point of a polygon must equal the last point.
> 
> So, I think the postgis code is correct (could you verify that for me?), but the
> shapefile code maybe not checking the last edge (connecting the last point to the
> first point).  I'm not familiar with the shapefile code....
> 
> dave
> 
> 
> 
> 
> int PIP( POINT3D *P, POINT3D *V, int n )
> {
>     int    cn = 0;    // the crossing number counter
>    int i;
> 
>  double vt;
> 
>     // loop through all edges of the polygon
> 
>      for (i=0; i< (n-1) ; i++)
>  {    // edge from V[i] to V[i+1]
> 
>         if (((V[i].y <= P->y) && (V[i+1].y > P->y))    // an upward crossing
>           || ((V[i].y > P->y) && (V[i+1].y <= P->y))) // a downward crossing
>    {
> 
>            vt = (double)(P->y - V[i].y) / (V[i+1].y - V[i].y);
>              if (P->x < V[i].x + vt * (V[i+1].x - V[i].x))    // P.x <intersect
>                    ++cn;   // a valid crossing of y=P.y right of P.x
>        }
>     }
>     return (cn&1);    // 0 if even (out), and 1 if odd (in)
> 
> }
> 
> 
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