CODEWITHSUNDEEP
python
Geo573
Geo573

Reputation: 142

Getting list of triangles from polygone

I have created two classes named Point and Triangle and I have a function pts_polygone that gives me the list of points in a polygon.

For example PolyPts = pts_polygone(Point(0., 0.), 9, 4) gives me a polygon of 9 points, centered in O(0, 0) and of radius 4.

I'm trying to triangulate this polygon, but first I have to extract the list of triangles, and I'm almost there but one is missing, here is what I have:

def tri_polygone(PolyPts):
    O = Point(0., 0.)
    TriLst = []
    for i in range(len(PolyPts)-1):
        P2 = PolyPts[i]
        P3 = PolyPts[i+1]
        TriLst.append(Triangle(O, P2, P3))
    return TriLst

The last triangle is missing, to make it a 'circle' if you get what I mean. I know there's something wrong with my dimensions in the list I create, it is of len 8 instead of 9, but I can't figure out how to fix it.

Upvotes: 2

Views: 175

Answers (2)

Tinu
Tinu

Reputation: 2513

The last triangle should consist of the first point, the last point in PolyPts and the origin, right? So you can do the following:

def tri_polygone(PolyPts):
    O = Point(0.,0.)
    TriLst = []
    for i in range(len(PolyPts)):
        P2 = PolyPts[i]
        P3 = PolyPts[i-1]
        TriLst.append(Triangle(O,P2,P3))
    return TriLst

That way you append 9 points, origin, current point i and the point behind i. Python has the nice feature that you can pass negative indices to lists, which means you read the array starting from the end, i.e. PolyPts[-1] is the last point, PolyPts[-2] is the second last point and so on.

Upvotes: 1

Tomerikoo
Tomerikoo

Reputation: 19403

You are looping "around" the polygon on couples of points. The first with second, second with third, etc... For this, you are rightfully looping on range(len(PolyPts)-1). But that means that you never get to the last point inside the loop. Because you are going "around" the polygon, the last point should couple with the first, so just add this line after the loop:

TriList.append(Triangle(O, P3, PolyPts[0]))

Upvotes: 0

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