Arrow animation in Python

Question

First of all, I am just starting to learn Python. I have been struggling during the last hours trying to update the arrow properties in order to change them during a plot animation.

After thoroughly looking for an answer, I have checked that it is possible to change a circle patch center by modifying the attribute 'center' such as circle.center = new_coordinates. However, I don't find the way to extrapolate this mechanism to an arrow patch...

The code so far is:

import numpy as np, math, matplotlib.patches as patches
from matplotlib import pyplot as plt
from matplotlib import animation

# Create figure
fig = plt.figure()    
ax = fig.gca()

# Axes labels and title are established
ax = fig.gca()
ax.set_xlabel('x')
ax.set_ylabel('y')

ax.set_ylim(-2,2)
ax.set_xlim(-2,2)
plt.gca().set_aspect('equal', adjustable='box')

x = np.linspace(-1,1,20) 
y  = np.linspace(-1,1,20) 
dx = np.zeros(len(x))
dy = np.zeros(len(y))

for i in range(len(x)):
    dx[i] = math.sin(x[i])
    dy[i] = math.cos(y[i])
patch = patches.Arrow(x[0], y[0], dx[0], dy[0] )


def init():
    ax.add_patch(patch)
    return patch,

def animate(t):
    patch.update(x[t], y[t], dx[t], dy[t])   # ERROR
    return patch,

anim = animation.FuncAnimation(fig, animate, 
                               init_func=init, 
                               interval=20,
                               blit=False)

plt.show()

After trying several options, I thought that the function update could somehow take me closer to the solution. However, I get the error:

TypeError: update() takes 2 positional arguments but 5 were given

If I just add one more patch per step by defining the animate function as shown below, I get the result shown in the image attached.

def animate(t):
    patch = plt.Arrow(x[t], y[t], dx[t], dy[t] )
    ax.add_patch(patch)
    return patch,

Wrong animation

I have tried to add a patch.delete statement and create a new patch as update mechanism but that results in an empty animation...

Answer 1

Add ax.clear() before ax.add_patch(patch) but will remove all elements from plot.

def animate(t):

    ax.clear() 

    patch = plt.Arrow(x[t], y[t], dx[t], dy[t] )
    ax.add_patch(patch)

    return patch,

---

EDIT: removing one patch

• using ax.patches.pop(index).

  In your example is only one patch so you can use index=0

  def animate(t):
  
      ax.patches.pop(0) 
  
      patch = plt.Arrow(x[t], y[t], dx[t], dy[t] )
      ax.add_patch(patch)
  
      return patch,
  

• using ax.patches.remove(object)

  It needs global to get/set external patch with Arrow

  def animate(t):
  
      global patch
  
      ax.patches.remove(patch) 
  
      patch = plt.Arrow(x[t], y[t], dx[t], dy[t] )
      ax.add_patch(patch)
  
      return patch,
  

---

BTW: to get list of properties which you can use with update()

print( patch.properties().keys() )

dict_keys(['aa', 'clip_path', 'patch_transform', 'edgecolor', 'path', 'verts', 'rasterized', 'linestyle', 'transform', 'picker', 'capstyle', 'children', 'antialiased', 'sketch_params', 'contains', 'snap', 'extents', 'figure', 'gid', 'zorder', 'transformed_clip_path_and_affine', 'clip_on', 'data_transform', 'alpha', 'hatch', 'axes', 'lw', 'path_effects', 'visible', 'label', 'ls', 'linewidth', 'agg_filter', 'ec', 'facecolor', 'fc', 'window_extent', 'animated', 'url', 'clip_box', 'joinstyle', 'fill'])

so you can use update to change color - `facecolor

def animate(t):
    global patch

    t %= 20 # get only 0-19 to loop animation and get color t/20 as 0.0-1.0

    ax.patches.remove(patch)

    patch = patches.Arrow(x[t], y[t], dx[t], dy[t])

    patch.update({'facecolor': (t/20,t/20,t/20,1.0)})

    ax.add_patch(patch)

    return patch,

Answer 2

I found this by mimicking the code in patches.Arrow.__init__:

import numpy as np
import matplotlib.patches as patches
from matplotlib import pyplot as plt
from matplotlib import animation
import matplotlib.transforms as mtransforms

# Create figure
fig, ax = plt.subplots()

# Axes labels and title are established
ax.set_xlabel('x')
ax.set_ylabel('y')

ax.set_ylim(-2,2)
ax.set_xlim(-2,2)
ax.set_aspect('equal', adjustable='box')

N = 20
x = np.linspace(-1,1,N) 
y  = np.linspace(-1,1,N) 
dx = np.sin(x)
dy = np.cos(y)

patch = patches.Arrow(x[0], y[0], dx[0], dy[0])

def init():
    ax.add_patch(patch)
    return patch,

def animate(t):
    L = np.hypot(dx[t], dy[t])

    if L != 0:
        cx = float(dx[t]) / L
        sx = float(dy[t]) / L
    else:
        # Account for division by zero
        cx, sx = 0, 1

    trans1 = mtransforms.Affine2D().scale(L, 1)
    trans2 = mtransforms.Affine2D.from_values(cx, sx, -sx, cx, 0.0, 0.0)
    trans3 = mtransforms.Affine2D().translate(x[t], y[t])
    trans = trans1 + trans2 + trans3
    patch._patch_transform = trans.frozen()
    return patch,

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

plt.show()