clipping.py

# from http://rosettacode.org/wiki/Sutherland-Hodgman_polygon_clipping#Python

def clip(subjectPolygon, clipPolygon):
    def inside(p):
        return(cp2[0]-cp1[0])*(p[1]-cp1[1]) > (cp2[1]-cp1[1])*(p[0]-cp1[0])

    def computeIntersection():
        dc = [ cp1[0] - cp2[0], cp1[1] - cp2[1] ]
        dp = [ s[0] - e[0], s[1] - e[1] ]
        n1 = cp1[0] * cp2[1] - cp1[1] * cp2[0]
        n2 = s[0] * e[1] - s[1] * e[0] 
        n3 = 1.0 / (dc[0] * dp[1] - dc[1] * dp[0])
        return tuple([(n1*dp[0] - n2*dc[0]) * n3, (n1*dp[1] - n2*dc[1]) * n3])
 
    outputList = subjectPolygon
    cp1 = clipPolygon[-1]
 
    for clipVertex in clipPolygon:
        cp2 = clipVertex
        inputList = outputList
        outputList = []
        try:
            s = inputList[-1]
    
            for subjectVertex in inputList:
                e = subjectVertex
                if inside(e):
                    if not inside(s):
                        outputList.append(computeIntersection())
                    outputList.append(e)
                elif inside(s):
                    outputList.append(computeIntersection())
                s = e
            cp1 = cp2
        except:
            continue

    return(outputList)