## Sunday, July 3, 2011

### pyeuclid, vector math, and polygon offset

UPDATE:  My code here has a bug.  Mr. Rafsanjani corrected it in the comments.  Please refer to them below.  Apologies - this blog is for beginners and for learning, so I've left the original code and the comment as is.  CBT 21NOV2014

I had previously done a series of posts on polygon offset intended as a practical guide for accomplishing the task quickly.  Some kind folks pointed out that I was making things considerably harder than necessary by using trigonometric functions when vector math would be easier and less error prone.

A coworker lent me his Java code that does polygon offset.  I translated it into python (2.5) using the pyeuclid module:

import euclid as eu
import copy

OFFSET = 0.15

# coordinates
# PT 1
MONASTERY = [(1.1, 0.75),
# PT 2
(1.2, 1.95),
. . .
# PT 21
(1.1, 0.75)]

"""
From a vector representing the origin,
a scalar offset, and a vector, returns
a Vector3 object representing a point

offset from the origin.

(Multiply vectorx by offset and add to origin.)
"""
multx = vectorx * offset
return multx + origin

def getinsetpoint(pt1, pt2, pt3):
"""
Given three points that form a corner (pt1, pt2, pt3),
returns a point offset distance OFFSET to the right
of the path formed by pt1-pt2-pt3.

pt1, pt2, and pt3 are two tuples.

Returns a Vector3 object.
"""
origin = eu.Vector3(pt2, pt2, 0.0)
v1 = eu.Vector3(pt1 - pt2,

pt1 - pt2, 0.0)
v1.normalize()
v2 = eu.Vector3(pt3 - pt2,

pt3 - pt2, 0.0)
v2.normalize()
v3 = copy.copy(v1)
v1 = v1.cross(v2)
v3 += v2
if v1.z < 0.0:
else:
return retval

polyinset = []
lenpolygon = len(MONASTERY)
i = 0
poly = MONASTERY
while i < lenpolygon - 2:
polyinset.append(getinsetpoint(poly[i],

poly[i + 1], poly[i + 2]))
i += 1
polyinset.append(getinsetpoint(poly[-2],

poly, poly))
polyinset.append(getinsetpoint(poly,

poly, poly))

The result: