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• attempting to save FuncAnimation changes the animation

  25 mars 2024, par pibion

  I am attempting to create and save an animation of a time-varying vector field using matplotlib's FuncAnimation. When I run the animation without attempting to save it, it updates smoothly as expected. However, when I try to save the animation, the vectors increase in length and the animation only appears to update a few times. I've been able to record the screen to get a video file, but I'd like to understand how to save the animation to a video in python.

  


  Here is the code to create the vector field animation. I ran this in a local jupyter notebook using python 3.9.12 on Windows.

  


  %matplotlib notebook
import numpy as np
import matplotlib.pyplot as plt
import matplotlib.animation as animation

x_min, x_max = -20, 20
y_min, y_max = 0, 25
wire_x = -4
wire_y = 17

# Define the vector field function (example: rotating vector field)
def vector_field(x, y, t):
    x = x - wire_x
    y = y - wire_y
    r = np.sin(t) / (x**2 + y**2)
    u = -r * y
    v = r * x
    return (u, v)

# Define grid
x = np.linspace(x_min, x_max, 20)
y = np.linspace(y_min, y_max, 20)
X, Y = np.meshgrid(x, y)

# Create animation
fig, ax = plt.subplots(1,1)
ax.set_aspect('equal')
ax.set_xlim(x_min, x_max)
ax.set_ylim(y_min, y_max)
ax.set_xlabel('X (m)')
ax.set_ylabel('Y (m)')

u, v = vector_field(X, Y, 1)

quiver = ax.quiver(X, Y, u, v)

time_arr = np.linspace(0,20,200)

plt.show()

def update_quiver(num):
    t = num * 0.1  # Time step
    u, v = vector_field(X, Y, t)
    quiver.set_UVC(u, v)

ani = animation.FuncAnimation(fig, update_quiver, frames=200, interval=50, repeat=False)


  


  This produces an animation whose vectors smoothly vary in length on my computer.

  


  However, when I try to save the animation by adding the lines

  


  writervideo = animation.FFMpegWriter(fps=60) 
ani.save('field_leftWire.mp4', writer=writervideo) 


  


  then the animation changes - the vectors are much longer and it appears to only update a few times throughout the animation.

  


  I'll also note that I had to install ffmpeg and restart my computer as outlined in Installation of FFMPEG for Python in WIndows to get the last two lines to run without error.

  


• FFMPEG Save stream frames to single image file (not sequence) [duplicate]

  5 mars 2021, par Nikola Knežević

  Is there a way to open a stream (usb webcam for example) and save each frame to the same image file ?

  


  That image would be later accessed as a simple resource from whichever protocol/service.

  


  I tried this :

  


  ffmpeg.exe -f dshow -i video="ASUS USB2.0 Webcam" image.png


  


  but it fails to write second frame. Error :

  


  Could not get frame filename number 2 from pattern 'image.png'


  


  Sure, I can make a script to periodically starts ffmpeg and save only one frame, but that's a huge performance overhead.

  


• Revision f3d8e315ac : Fix a bug in neon that has not save and restore q4-q7 registers. Change-Id : Ie2

  27 février 2014, par hkuang

  Changed Paths :
   Modify /vp9/common/arm/neon/vp9_iht8x8_add_neon.asm

  
  
  Fix a bug in neon that has not save and restore q4-q7 registers.

  Change-Id : Ie21b5ae89100389b80f919710839084f935a8545