source: inundation/ga/storm_surge/pyvolution/util.py @ 276

def angle(v):
    """Compute angle between e1 (the unit vector in the x-direction)
    and the specified vector
    """

    from math import acos, pi, sqrt
    from Numeric import sum, array

    l = sqrt( sum (array(v)**2))
    v1 = v[0]/l
    v2 = v[1]/l
        
    theta = acos(v1)

    if v2 < 0:
        #Quadrant 3 or 4
        theta = 2*pi-theta

    return theta


def anglediff(v0, v1):
    """Compute difference between angle of vector x0, y0 and x1, y1.
    This is used for determining the ordering of vertices,
    e.g. for checking if they are counter clockwise. 
    
    Always return a positive value
    """
        
    from math import pi
    
    a0 = angle(v0)
    a1 = angle(v1)

    #Ensure that difference will be positive
    if a0 < a1:
        a0 += 2*pi
            
    return a0-a1


def mean(x):
    from Numeric import sum
    return sum(x)/len(x)

####################################################################
#Python versions of function that are also implemented in util_gateway.c
#

def gradient_python(x0, y0, x1, y1, x2, y2, q0, q1, q2):
    """
    """
    
    det = (y2-y0)*(x1-x0) - (y1-y0)*(x2-x0)            
    a = (y2-y0)*(q1-q0) - (y1-y0)*(q2-q0)
    a /= det

    b = (x1-x0)*(q2-q0) - (x2-x0)*(q1-q0)
    b /= det            

    return a, b





##############################################
#Initialise module

import compile
if compile.can_use_C_extension('util_ext.c'):
    from util_ext import gradient
else:
    gradient = gradient_python


if __name__ == "__main__":
    pass