# [Qgis-developer] python plugin - layer intersection ?

Michaël Douchin michael.douchin at laposte.net
Tue Jan 15 03:32:39 EST 2008

```Hi list
I am playing with the python bindings.... I would like to know if we can
use python to develop a layer intersection plugin.
For example, I have 2 shp layers :
* mypoly = polygon layer (id_poly, name, area, valuea, valueb, etc.)
* mypoint = point layer (id_point, name, value1, value2, etc.)

Is it possible with python to test for each point if it intersects one
of the polygons, and then give a point layer like this:
myresult(id_result, id_point, id_poly, valuea, value2)
In this case, I keep the id of each source layer, and choose to keep
also one value of each layer. Each new point has the same geometry of
the source point

For those who know postgis, it would be a query like this
SELECT p1.id_poly, p1.valuea, p2.id_point, p2.value2, p2.the_geom
FROM mypoly p1, mypoint p2
WHERE INTERSECTS (p1.the_geom, p2.the_geom)

PS: I can use the geometric algorytm like this instead, but it would be
useless if Qgis core has intersection capabilities:
*****
# is target point inside a 2D polygon?
#   poly = 2 dimenson list with all the coords of each polygon,
#   x (horizontal)=  X of the point to test
#   y (vertical) = Y of the point to test
def inpoly( poly, xt,  yt):
inside = False #valeur que retournera la fonction
npoints = len(poly)  #calcul du nombre de lignes du tableau = nombre
de sommets du polygone
if (npoints < 3): #si il y a moins que 3 sommets, ce n'est pas un
polygone --> on arrete
return(0)
xold=poly[npoints-1]
yold=poly[npoints-1]

for i in range(0,npoints):
xnew=poly[i]
ynew=poly[i]
if (xnew > xold) :
x1=xold
x2=xnew
y1=yold
y2=ynew
else :
x1=xnew
x2=xold
y1=ynew
y2=yold
# edge "open" at one end
if ( (xnew < xt) == (xt <= xold) and (yt-y1)*(x2-x1)  <
(y2-y1)*(xt-x1) ):
inside= not inside

xold=xnew
yold=ynew
return inside
*****

```