Module moog.physics.distance_fn_force

Two-body distance-based forces.

Expand source code 
"""Two-body distance-based forces."""

from . import abstract_force
import abc
import numpy as np


class DistanceForce(abstract_force.AbstractNewtonianForce,
                    metaclass=abc.ABCMeta):
    """Distance force base class.
    
    This class can be used for any force that attracts or repels two sprites as
    a function of their distance alone.
    """

    def __init__(self, force_fn, symmetric=False):
        """Constructor

        Args:
            force_fn: Force function that takes a scalar distance and produces a
                scalar force magnitude, the magnitude of the force to be applied
                between sprites at the given distance apart.
            symmetric: Bool. Whether to apply the force to both sprites
                involved. If False, only applies force to the second sprite, the
                second argument to .step().
        """
        self._force_fn = force_fn
        self._symmetric = symmetric

    def _compute_forces(self, sprite_0, sprite_1):
        """Compute forces on sprite_0 and sprite_1."""
        diff = sprite_1.position - sprite_0.position
        dist = np.linalg.norm(diff)
        if dist == 0.:
            return np.zeros(2), np.zeros(2)
        force_direction = diff / dist
        force_magnitude = self._force_fn(dist)
        sprite_1_force = force_magnitude * force_direction

        if self._symmetric:
            sprite_0_force = -1 * sprite_1_force
        else:
            sprite_0_force = np.array([0., 0.])

        return sprite_0_force, sprite_1_force


def linear_force_fn(zero_intercept,
                    slope,
                    apply_distant_force=False,
                    apply_nearby_force=True):
    """Force is a linear function of distance.
    
    Args:
        zero_intercept: Scalar. Magnitude of force between sprites a distance
            zero apart.
        slope: Scalar. Slope of linear change in force with respect to distance.
        apply_distant_force: Bool. Whether to apply force at distances greater
            than the event horizon (distance at which the force is zero).
        apply_distant_force: Bool. Whether to apply force at distances less than
            the event horizon.

    Returns:
        force_fn: Function distance -> force magnitude.
    """
    event_horizon = -1. * zero_intercept / slope
    def force_fn(distance):
        force_magnitude = zero_intercept + slope * distance
        if not apply_distant_force and distance > event_horizon:
            force_magnitude = 0
        if not apply_nearby_force and distance < event_horizon:
            force_magnitude = 0
        return force_magnitude
    return force_fn


def spring_force_fn(spring_constant, equilibrium=0):
    """Spring force by Hooke's Law.
    
    Args:
        spring_constant: Scalar. Spring constant.
        equilibrium: Spring equilibrium. Distance at which the force is zero.

    Returns:
        force_fn: Function distance -> force magnitude.
    """
    def force_fn(distance):
        return -1. * spring_constant * (distance - equilibrium)
    return force_fn

Functions

def linear_force_fn(zero_intercept, slope, apply_distant_force=False, apply_nearby_force=True)

Force is a linear function of distance.

Args

zero_intercept
Scalar. Magnitude of force between sprites a distance zero apart.
slope
Scalar. Slope of linear change in force with respect to distance.
apply_distant_force
Bool. Whether to apply force at distances greater than the event horizon (distance at which the force is zero).
apply_distant_force
Bool. Whether to apply force at distances less than the event horizon.

Returns

force_fn
Function distance -> force magnitude.
Expand source code 
def linear_force_fn(zero_intercept,
                    slope,
                    apply_distant_force=False,
                    apply_nearby_force=True):
    """Force is a linear function of distance.
    
    Args:
        zero_intercept: Scalar. Magnitude of force between sprites a distance
            zero apart.
        slope: Scalar. Slope of linear change in force with respect to distance.
        apply_distant_force: Bool. Whether to apply force at distances greater
            than the event horizon (distance at which the force is zero).
        apply_distant_force: Bool. Whether to apply force at distances less than
            the event horizon.

    Returns:
        force_fn: Function distance -> force magnitude.
    """
    event_horizon = -1. * zero_intercept / slope
    def force_fn(distance):
        force_magnitude = zero_intercept + slope * distance
        if not apply_distant_force and distance > event_horizon:
            force_magnitude = 0
        if not apply_nearby_force and distance < event_horizon:
            force_magnitude = 0
        return force_magnitude
    return force_fn
def spring_force_fn(spring_constant, equilibrium=0)

Spring force by Hooke's Law.

Args

spring_constant
Scalar. Spring constant.
equilibrium
Spring equilibrium. Distance at which the force is zero.

Returns

force_fn
Function distance -> force magnitude.
Expand source code 
def spring_force_fn(spring_constant, equilibrium=0):
    """Spring force by Hooke's Law.
    
    Args:
        spring_constant: Scalar. Spring constant.
        equilibrium: Spring equilibrium. Distance at which the force is zero.

    Returns:
        force_fn: Function distance -> force magnitude.
    """
    def force_fn(distance):
        return -1. * spring_constant * (distance - equilibrium)
    return force_fn

Classes

class DistanceForce (force_fn, symmetric=False)

Distance force base class.

This class can be used for any force that attracts or repels two sprites as a function of their distance alone.

Constructor

Args

force_fn
Force function that takes a scalar distance and produces a scalar force magnitude, the magnitude of the force to be applied between sprites at the given distance apart.
symmetric
Bool. Whether to apply the force to both sprites involved. If False, only applies force to the second sprite, the second argument to .step().
Expand source code 
class DistanceForce(abstract_force.AbstractNewtonianForce,
                    metaclass=abc.ABCMeta):
    """Distance force base class.
    
    This class can be used for any force that attracts or repels two sprites as
    a function of their distance alone.
    """

    def __init__(self, force_fn, symmetric=False):
        """Constructor

        Args:
            force_fn: Force function that takes a scalar distance and produces a
                scalar force magnitude, the magnitude of the force to be applied
                between sprites at the given distance apart.
            symmetric: Bool. Whether to apply the force to both sprites
                involved. If False, only applies force to the second sprite, the
                second argument to .step().
        """
        self._force_fn = force_fn
        self._symmetric = symmetric

    def _compute_forces(self, sprite_0, sprite_1):
        """Compute forces on sprite_0 and sprite_1."""
        diff = sprite_1.position - sprite_0.position
        dist = np.linalg.norm(diff)
        if dist == 0.:
            return np.zeros(2), np.zeros(2)
        force_direction = diff / dist
        force_magnitude = self._force_fn(dist)
        sprite_1_force = force_magnitude * force_direction

        if self._symmetric:
            sprite_0_force = -1 * sprite_1_force
        else:
            sprite_0_force = np.array([0., 0.])

        return sprite_0_force, sprite_1_force

Ancestors

Inherited members