>>> """
===============
Rain simulation
===============

Simulates rain drops on a surface by animating the scale and opacity
of 50 scatter points.

Author: Nicolas P. Rougier
"""
... 
... import numpy as np
... import matplotlib.pyplot as plt
... from matplotlib.animation import FuncAnimation
... 
... # Fixing random state for reproducibility
... np.random.seed(19680801)
... 
... 
... # Create new Figure and an Axes which fills it.
... fig = plt.figure(figsize=(7, 7))
... ax = fig.add_axes([0, 0, 1, 1], frameon=False)
... ax.set_xlim(0, 1), ax.set_xticks([])
... ax.set_ylim(0, 1), ax.set_yticks([])
... 
... # Create rain data
... n_drops = 50
... rain_drops = np.zeros(n_drops, dtype=[('position', float, (2,)),
...                                       ('size',     float),
...                                       ('growth',   float),
...                                       ('color',    float, (4,))])
... 
... # Initialize the raindrops in random positions and with
... # random growth rates.
... rain_drops['position'] = np.random.uniform(0, 1, (n_drops, 2))
... rain_drops['growth'] = np.random.uniform(50, 200, n_drops)
... 
... # Construct the scatter which we will update during animation
... # as the raindrops develop.
... scat = ax.scatter(rain_drops['position'][:, 0], rain_drops['position'][:, 1],
...                   s=rain_drops['size'], lw=0.5, edgecolors=rain_drops['color'],
...                   facecolors='none')
... 
... 
... def update(frame_number):
...     # Get an index which we can use to re-spawn the oldest raindrop.
...     current_index = frame_number % n_drops
... 
...     # Make all colors more transparent as time progresses.
...     rain_drops['color'][:, 3] -= 1.0/len(rain_drops)
...     rain_drops['color'][:, 3] = np.clip(rain_drops['color'][:, 3], 0, 1)
... 
...     # Make all circles bigger.
...     rain_drops['size'] += rain_drops['growth']
... 
...     # Pick a new position for oldest rain drop, resetting its size,
...     # color and growth factor.
...     rain_drops['position'][current_index] = np.random.uniform(0, 1, 2)
...     rain_drops['size'][current_index] = 5
...     rain_drops['color'][current_index] = (0, 0, 0, 1)
...     rain_drops['growth'][current_index] = np.random.uniform(50, 200)
... 
...     # Update the scatter collection, with the new colors, sizes and positions.
...     scat.set_edgecolors(rain_drops['color'])
...     scat.set_sizes(rain_drops['size'])
...     scat.set_offsets(rain_drops['position'])
... 
... 
... # Construct the animation, using the update function as the animation director.
... animation = FuncAnimation(fig, update, interval=10)
... plt.show()
...