Making animations by matplotlib and saving the video files
<p>I have been studying the 1D wave equations and making the animation of the equation. But there are some problems when using the <code>anim.save</code> of <code>animation</code> to save the video file. I have already installed <code>ffmpeg</code> on my computer (a Windows machine) and set the environment variables. But it keeps telling me this:</p>

<pre><code>UserWarning: MovieWriter ffmpeg unavailable
...
RuntimeError: Error creating movie, return code: 4 Try running with --verbose-debug
</code></pre>

<p><a href="https://i.stack.imgur.com/GYKBu.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/GYKBu.png" alt="enter image description here"></a></p>

<p>Here is my code:</p>

<pre><code>from numpy import zeros,linspace,sin,pi
import matplotlib.pyplot as mpl
from matplotlib import animation
mpl.rcParams['animation.ffmpeg_path'] = 'C:\\ffmpeg\\bin\\ffmpeg.exe'

def I(x):
    return sin(2*x*pi/L)

def f(x,t):
    return sin(0.5*x*t)

def solver0(I,f,c,L,n,dt,t):
    # f is a function of x and t, I is a function of x
    x = linspace(0,L,n+1)
    dx = L/float(n)
    if dt &lt;= 0:
        dt = dx/float(c)
    C2 = (c*dt/dx)**2
    dt2 = dt*dt
    up = zeros(n+1)
    u = up.copy()
    um = up.copy()
    for i in range(0,n):
        u[i] = I(x[i])
    for i in range(1,n-1):
        um[i] = u[i]+0.5*C2*(u[i-1] - 2*u[i] + u[i+1]) + dt2*f(x[i],t)
    um[0] = 0
    um[n] = 0    
    while t &lt;= tstop:
        t_old = t
        t += dt
        #update all inner points:
        for i in range(1,n-1):
            up[i] = -um[i] + 2*u[i] + C2*(u[i-1] - 2*u[i] + u[i+1]) + dt2*f(x[i],t_old)    
        #insert boundary conditions:
        up[0] = 0
        up[n] = 0
        #update data structures for next step
        um = u.copy()
        u = up.copy()   
    return u

c = 1.0 
L = 10
n = 100
dt = 0
tstop = 40

fig = mpl.figure()
ax = mpl.axes(xlim=(0,10),ylim=(-1.0,1.0))
line, = ax.plot([],[],lw=2)

def init():
    line.set_data([],[])
    return line,

def animate(t):
    x = linspace(0,L,n+1)
    y = solver0(I, f, c, L, n, dt, t)
    line.set_data(x,y)
    return line,

anim = animation.FuncAnimation(fig, animate, init_func=init,
                               frames=200, interval=20, blit=True)

anim.save('seawave_1d_ani.mp4',writer='ffmpeg',fps=30)
mpl.show()
</code></pre>

<p>I believe the problem is in the part of the animation instead of the three functions above. Please help me find what mistake I have made.</p>