はじめに

matplotlibでgifアニメーションを作成できるが、長すぎるgif（下記のコードのend_frameが大きすぎる場合）を作ろうとするとプロセスがkillされるので困った

app.py

import numpy as np
import matplotlib.pyplot as plt
from matplotlib.animation import FuncAnimation

def create_gif(start_frame, end_frame, part_number, base_name, fps, gif_format, plot_interval):
    fig, ax = plt.subplots(figsize=(8, 8))
    ax.set_xlim(-200, 200)
    ax.set_ylim(-200, 200)
    ax.set_aspect('equal')
    ax.axis('off')

    line, = ax.plot([], [], 'r--', linewidth=2)

    a = 1  # Control the distance between turns

    def animate(frame):
        t = np.arange(0, start_frame + frame + 1) * plot_interval
        x = a * t * np.cos(t)
        y = a * t * np.sin(t)
        
        line.set_data(x, y)
        
        ax.set_xlim(min(x) - 10, max(x) + 10)
        ax.set_ylim(min(y) - 10, max(y) + 10)

        return line,

    frames = end_frame - start_frame
    anim = FuncAnimation(fig, animate, frames=frames, interval=20, blit=True)
    gif_name = gif_format.format(base_name=base_name, part_number=part_number)
    anim.save(gif_name, writer='pillow', fps=fps)
    plt.close(fig)

start_frame = 0
end_frame = 2000
base_name = "archimedes_spiral"
fps = 30
gif_format = "{base_name}_part_{part_number:03d}.gif"
plot_interval = 0.1
create_gif(start_frame, end_frame, 0, base_name, fps, gif_format, plot_interval)

$ python app.py 
Killed

manimの利用も検討したが、仕様がまだ変わりそうなので今回は見送り

解決策

下記の流れでgifを作成するようにした
1. アニメーションを分割してmp4として保存
2. ffmpegでmp4を結合
3. ffmpegでmp4をgifに変換

変更後のコードは下記の通り

app.py

import numpy as np
import matplotlib.pyplot as plt
from matplotlib.animation import FuncAnimation, FFMpegWriter
import subprocess
import os

def create_mp4(start_frame, end_frame, part_number, base_name, fps, plot_interval):
    fig, ax = plt.subplots(figsize=(8, 8))
    ax.set_xlim(-200, 200)
    ax.set_ylim(-200, 200)
    ax.set_aspect('equal')
    ax.axis('off')

    line, = ax.plot([], [], 'r--', linewidth=2)

    a = 1  # Control the distance between turns

    def animate(frame):
        t = np.arange(0, start_frame + frame + 1) * plot_interval
        x = a * t * np.cos(t)
        y = a * t * np.sin(t)

        line.set_data(x, y)

        ax.set_xlim(min(x) - 10, max(x) + 10)
        ax.set_ylim(min(y) - 10, max(y) + 10)

        return line,

    frames = end_frame - start_frame
    anim = FuncAnimation(fig, animate, frames=frames, interval=20, blit=True)

    # Save as MP4 using FFMpegWriter
    mp4_name = f"{base_name}_part_{part_number}.mp4"
    writer = FFMpegWriter(fps=fps)
    anim.save(mp4_name, writer=writer)
    
    plt.close(fig)

def create_all_partial(total_frames, frames_per_part, base_name, fps, gif_format, plot_interval):
    for i in range(0, total_frames, frames_per_part):
        start_frame = i
        end_frame = min(i + frames_per_part, total_frames)
        create_mp4(start_frame=start_frame, 
                   end_frame=end_frame, 
                   part_number=i // frames_per_part + 1, 
                   base_name=base_name, 
                   fps=fps, 
                   plot_interval=plot_interval)

def combine_mp4s_and_convert_to_gif(input_prefix, output_filename, max_size_kb=20000, target_width=None, target_height=None, frame_rate=10):
    # Get list of files matching the prefix
    matching_files = [f for f in os.listdir() if f.startswith(input_prefix) and f.endswith('.mp4')]
    
    if not matching_files:
        print(f"No MP4 files found with prefix '{input_prefix}'")
        return

    # Sort the files to ensure correct order
    matching_files.sort()

    # Create a temporary file with the list of input MP4 files
    with open('input_files.txt', 'w') as f:
        for mp4 in matching_files:
            f.write(f"file '{mp4}'\n")

    # Use FFmpeg to concatenate the MP4 files
    combined_mp4 = 'combined.mp4'
    ffmpeg_command = [
        'ffmpeg',
        '-f', 'concat',
        '-safe', '0',
        '-i', 'input_files.txt',
        '-c', 'copy',
        combined_mp4
    ]

    try:
        subprocess.run(ffmpeg_command, check=True)
        print(f"Combined MP4 saved as {combined_mp4}")

        # Generate a palette for better quality and smaller size
        palette_file = 'palette.png'
        palette_command = [
            'ffmpeg', '-i', combined_mp4,
            '-vf', f'fps={frame_rate},scale={target_width}:{target_height}:flags=lanczos,palettegen',
            palette_file
        ]
        
        subprocess.run(palette_command, check=True)

        # Prepare GIF conversion command using the generated palette
        gif_command = [
            'ffmpeg', '-i', combined_mp4,
            '-i', palette_file,
            '-lavfi', f'fps={frame_rate},scale={target_width}:{target_height}:flags=lanczos[x];[x][1:v]paletteuse',
            '-y', output_filename
        ]

        subprocess.run(gif_command, check=True)

        # Check file size and adjust if necessary (e.g., by reducing frame rate or dimensions)
        while os.path.getsize(output_filename) > max_size_kb * 1024:
            print(f"GIF size exceeds {max_size_kb} KB. Adjusting...")
            frame_rate = max(1, frame_rate - 1)  # Reduce frame rate by 1 fps
            
            gif_command = [
                'ffmpeg', '-i', combined_mp4,
                '-i', palette_file,
                '-lavfi', f'fps={frame_rate},scale={target_width}:{target_height}:flags=lanczos[x];[x][1:v]paletteuse',
                '-y', output_filename
            ]
            
            subprocess.run(gif_command, check=True)

        print(f"Combined GIF saved as {output_filename}")
        
    except subprocess.CalledProcessError as e:
        print(f"Error combining MP4s or converting to GIF: {e}")

    # Clean up temporary files
    os.remove('input_files.txt')
    os.remove(combined_mp4)
    os.remove(palette_file)


def clean_up_mp4(base_name):
    for file in os.listdir():
        if file.startswith(base_name) and file.endswith('.mp4'):
            os.remove(file)

start_frame = 0
end_frame = 2000
base_name = "archimedes_spiral"

fps = 30
gif_format = "{base_name}_part_{part_number:03d}.gif"
plot_interval = 0.1

total_frames = end_frame - start_frame
frames_per_part = 500
create_all_partial(total_frames, frames_per_part, base_name, fps, gif_format, plot_interval)
combine_mp4s_and_convert_to_gif(
    base_name + "_", 
    output_name + ".gif", 
    max_size_kb=10000, 
    target_width=320, 
    target_height=320, frame_rate=10)
clean_up_mp4(base_name)

実行のためのDockerfileはこちら

Dockerfile

# Use an official Python runtime as a parent image
FROM python:3.9-slim

# Set the working directory in the container
WORKDIR /app

# Copy the current directory contents into the container at /app
COPY . /app

# Install FFmpeg and any needed packages specified in requirements.txt
RUN apt-get update && apt-get install -y ffmpeg && \
    pip install --no-cache-dir matplotlib numpy pillow

出力されるgif

(表示に時間がかかるかも)