Python. How to get the x,y coordinates of a offset spline from a x,y list of points and offset distance

Question

I need to make an offset parallel enclosure of an airfoil profile curve, but I cant figure out how to make all the points be equidistant to the points on the primary profile cur

Answer 1

If you are willing (and able) to install a third-party tool, I'd highly recommend the Shapely module. Here's a small sample that offsets both inward and outward:

from StringIO import StringIO
import matplotlib.pyplot as plt
import numpy as np
import requests
import shapely.geometry as shp

# Read the points    
AFURL = 'http://m-selig.ae.illinois.edu/ads/coord_seligFmt/ah79100c.dat'
afpts = np.loadtxt(StringIO(requests.get(AFURL).content), skiprows=1)

# Create a Polygon from the nx2 array in `afpts`
afpoly = shp.Polygon(afpts)

# Create offset airfoils, both inward and outward
poffafpoly = afpoly.buffer(0.03)  # Outward offset
noffafpoly = afpoly.buffer(-0.03)  # Inward offset

# Turn polygon points into numpy arrays for plotting
afpolypts = np.array(afpoly.exterior)
poffafpolypts = np.array(poffafpoly.exterior)
noffafpolypts = np.array(noffafpoly.exterior)

# Plot points
plt.plot(*afpolypts.T, color='black')
plt.plot(*poffafpolypts.T, color='red')
plt.plot(*noffafpolypts.T, color='green')
plt.axis('equal')
plt.show()

And here's the output; notice how the 'bowties' (self-intersections) on the inward offset are automatically removed: