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)]

def scaleadd(origin, offset, vectorx):

"""

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[0], pt2[1], 0.0)

v1 = eu.Vector3(pt1[0] - pt2[0],

pt1[1] - pt2[1], 0.0)

v1.normalize()

v2 = eu.Vector3(pt3[0] - pt2[0],

pt3[1] - pt2[1], 0.0)

v2.normalize()

v3 = copy.copy(v1)

v1 = v1.cross(v2)

v3 += v2

if v1.z < 0.0:

retval = scaleadd(origin, -OFFSET, v3)

else:

retval = scaleadd(origin, OFFSET, v3)

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[0], poly[1]))

polyinset.append(getinsetpoint(poly[0],

poly[1], poly[2]))

The result: