How to create a line from more than two points?

Question

I have a loop which calculates two points for each iteration (x3,y3) and (x4,y4). As seen in the code these two points are added for each iteration, which gives i-1 lines. Each line has a individual id.

What I want is to add all these points at the same time, so it makes one polyline with only one id. This is done by adding all the points in the line:

fet.setGeometry( QgsGeometry.fromMultiPolyline([[QgsPoint(x3,y3), QgsPoint(x4,y4)]]))

instead of only these two points at the time. But I can not figure out how to change the loop to make it work. I guess for each iteration, I have to save a string with QgsPoint(x3,y3), QgsPoint(x4,y4) for the given iteration and add the next two points as a string in the next iteration. But I can not see how?

for i in range(0,len(points)-1):
    coor1 = points[i]
    x1 = coor1[0]
    y1 = coor1[1]
    dist = 0
    coor2 = points[i+1]
    x2 = coor2[0]
    y2 = coor2[1]
    dn = ((x1-x2)**2 + (y1-y1)**2)**0.5
    x3 = x1 + dist*(y1-y2) / dn
    y3 = y1 - dist*(x1-x2) / dn     
    x4 = x2 + dist*(y1-y2) / dn
    y4 = y2 - dist*(x1-x2) / dn  

    fet.setGeometry( QgsGeometry.fromMultiPolyline([[QgsPoint(x3,y3), QgsPoint(x4,y4)]]))
    pr.addFeatures( [ fet ] )

Can you clarify what you need to do a bit more please? Do you want to build out a polygon as you go though each iteration? — Nathan W, Apr 01 '13 at 07:59
Yes, I want to build a polyline with only one id. The polyline should contain all x3,y3 and x4,y4 for all i. — Toke, Apr 03 '13 at 01:02

score 2 · Answer 1 · edited May 23 '17 at 12:39

use a Python iterator. See How to calculate distances in a point sequence? or How do I find vector line bearing in QGIS or GRASS?

example (iterator from Most pythonic way to get the previous element):

# iterator 
def pair_points(iterable):
    prev = None
    for elem in iterable:
        yield prev, elem
        prev = elem

Application with a simple example:

points = ((1,2),(3,4),(5,6),(7,8))
for pt in pair_points(points):
    print pt
 (None, (1, 2))
 ((1, 2), (3, 4))
 ((3, 4), (5, 6))
 ((5, 6), (7, 8))

So, to avoid the None

for i, pair in enumerate(pair_points(points)):
    if i > 0: # None
        print pair
 ((1, 2), (3, 4))
 ((3, 4), (5, 6))
 ((5, 6), (7, 8))

and in your case:

def new_point(point1,point2):
   dist = 0
   x1,y1 = point1
   x2,y2 = point2
   dn = ((x1-x2)**2 + (y1-y1)**2)**0.5
   x3 = x1 + dist*(y1-y2) / dn
   y3 = y1 - dist*(x1-x2) / dn     
   x4 = x2 + dist*(y1-y2) / dn
   y4 = y2 - dist*(x1-x2) / dn
   return x3,y3,x4,y4    

for i, pair in enumerate(pair_points(points)):
    if i > 0:
       x3,y3, x4,y4 = new_point(pair[0],pair[1])
       fet.setGeometry( QgsGeometry.fromMultiPolyline([[QgsPoint(x3,y3), QgsPoint(x4,y4)]]))
       pr.addFeatures( [ fet ] )

But I don't understand the dist = 0 because:

x3 = x1 + dist*(y1-y2) / dn = x1 /dn

You can even do: https://gist.github.com/NathanW2/5284046 with the x3, y3, etc lines. Just for a bit of clean up. — Nathan W, Apr 01 '13 at 09:47
yes but with dist = 0 it is the same as x3, y3, x4, y4 = (x1 / dn, y1 / dn, x2/ dn, y2/ dn) — gene, Apr 01 '13 at 09:52

score 0 · Answer 2 · answered Apr 02 '13 at 08:33

The dist is not zero, I just forgot to give it a number. The example you give works, but it does not do what I want. What I want is to get the following line:

fet.setGeometry(QgsGeometry.fromMultiPolyline([[QgsPoint(x3,y3), QgsPoint(x4,y4), Qgspoint(x3,y3), QgsPoint(x4,y4), Qgs...)]]))

I want only one setGeometry line with all the x3,y3 and x4,y4 generated for each i. So all these lines have the same id. I hope it makes sense. It is my first python code, and are soooo close to make it work :D

Another possibility maybe is to this for each single point:

 fet.setGeometry(QgsGeometry.fromPoint(QgsPoint(x3,y3))
 pr.addFeatures( [ fet ] )

and afterwards convert this to a polyline. But I do not know if that can be done.

How to create a line from more than two points?

2 Answers2