.. _animation-rain:

animation example code: rain.py
===============================

[`source code <rain.py>`_]

::

    """
    ===============
    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
    
    
    # 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, 1),
                                          ('growth',   float, 1),
                                          ('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()
    

Keywords: python, matplotlib, pylab, example, codex (see :ref:`how-to-search-examples`)