blob.py

import math

class Blob:
    """
    Represents a blob.
    """

    def __init__(self):
        """
        Constructs an empty blob.
        """

        self._P = 0    # number of pixels
        self._x = 0.0  # x-coordinate of center of mass, i.e.
                            # the avg x-coordinate
        self._y = 0.0  # y-coordinate of center of mass, i.e.
                            # the avg y-coordinate

    def add(self, i, j):
        """
        Adds pixel (i, j) to this blob.
        """
        
        # use running average to update CoM coordinates.
        self._x = (self._x * self._P + i) / (self._P + 1)
        self._y = (self._y * self._P + j) / (self._P + 1)
        # increment mass
        self._P += 1


    def mass(self):
        """
        Returns the number of pixels added to this blob, ie, its mass.
        """
        return self._P

    def distanceTo(self, other):
        """
        Returns the Euclidean distance between the center of mass of this blob
        and the center of mass of other blob.
        """
        result = (other._x - self._x) * (other._x - self._x) \
            + (other._y - self._y) * (other._y - self._y)
        return result ** 0.5

    def __str__(self):
        """
        Returns a string representation of this blob.
        """
        return '%d (%.4f, %.4f)' % (self._P, self._x, self._y)