Multiprocessing-friendly animations

# Multiprocessed Animations

## Summary

This library helps building movies in python from images. Specifically, it intends to allow multiple threads to produce images which are stitched together into movies with ffmpeg.

## Use-cases

This can be used to multithread the image creation process when using matplotlib animations. FFMpeg will use multithreading to encode the images into a video, however producing the images themselves is a bottleneck if you have many cores available. Hence, this will vastly reduce the time it takes to generate matplotlib animations.

This can also be used for generating scenes with PIL.

This library's goal is to make generating videos as simple as possible first, then to go as fast as possible within those simple techniques. This gives fast enough performance for many projects, and has the enormous benefit that you can throw more hardware at the problem. Using ffmpeg directly without multithreading image generation will not scale to more hardware.

## Performance

With ideal settings, the images should be generated at a rate that just barely does not fill the ffmpeg process input pipe. This will ensure that images are being generated as quickly as they can be encoded.

By default, this library will attempt to find the settings that accomplish this task. This takes a bit of time to accomplish, so the final settings are exposed and it can be helpful to use those when re-running roughly the same task. The correct settings will depend on how long it takes to generate images and how long it takes to encode them which varies based on the image statistics.

## Installation

pip install pympanim

## Dependencies

This depends on ffmpeg being installed. It can be installed here. Other python dependencies will be automatically installed by pip.

## Usage - Acts

### Motivation

Many times you have something which is capable of rendering some underlying state in a consistent way, and the video you want to produce is made up of parts that manipulate the state that is sent to the renderer. This use case is handled specifically by pympanim/acts.py.

### Summary

Define the state that completely describes how to render things in a class that subclasses pympanim.acts.ActState. Then create the thing which can render the given state to an image (described via rgba bytes or a Pillow Image).

With these ready, create (one or more) Scenes, which are things that manipulate the state you just created. A scene has some duration, and must be able to set the state to correspond to a particular time within the scene.

This library will provide common manipulations of scenes - nonlinear time manipulations, cropping, reversing, and sequencing. To make the most use of these manipulations, Scenes should be as simple as possible.

Multiple Acts can be combined in much the same way. See Usage - Frame Generators for details below.

To produce the video, use pympanim.worker.produce, creating a frame generator out of the scenes using pympanim.acts.Act

### Boilerplate

import PIL.Image
import pympanim.worker as pmaw
import pympanim.acts as acts
import pympanim.frame_gen as fg
import os

class MyActState(acts.ActState):
pass

class MyActRenderer(acts.ActRenderer):
@property
def frame_size(self):
return (1920, 1080) # in pixels

def render(self, act_state: MyActState) -> bytes:
return fg.img_to_bytes(self.render_pil(act_state))

def render_pil(self, act_state: MyActState) -> PIL.Image:
# By default, render_pil delegates to render. It is shown reversed
# here for completeness.
return PIL.Image.new('RGBA', self.frame_size, 'white')

class Scene1(acts.Scene):
@property
def duration(self):
return 1 # 1 millisecond; allows you to treat time_ms as % progress

def apply(self, act_state: MyActState, time_ms: float, dbg: bool = False):
# dbg is passed across the scene heirarchy and will be false when
# rendering the video. you may find it helpful to print some debug
# information when it is true
pass

def _scene():
scene = Scene1()
return (acts.FluentScene(scene)
.dilate(lambda x: x**2) # any easing works here. see
# pympanim.easing and pytweening
.time_rescale(1 / 10000) # 10s of real time corresponds to
# 1ms of scene time
.build()
)

def _main():
os.makedirs('out', exist_ok=True)
act_state = MyActState()
act_renderer = MyActRenderer()

pmaw.produce(
acts.Act(act_state, renderer, [_scene()]),
fps=60,
dpi=100,
bitrate=-1,
outfile='out/video.mp4'
)

if __name__ == '__main__':
_main()


For method-level documentation, use the built-in help command, i.e.,

>python3
>>> import pympanim.acts
>>> help(pympanim.acts)
Help on module pympanim.acts in pympanim:
.. (omitted for readme brevity) ..


## Usage - Frame Generators

### Motivation

The most novel part of this library is the boilerplate to generate videos where the image generation itself is multithreaded. This is exposed in as raw a manner as possible using pympanim/frame_gen.py which is merely wrapped by pympanim/acts.py. This section discusses how to use this library with minimal abstraction.

### Summary

Create a subclass of pympanim.frame_gen.FrameGenerator, which requires defining a duration, frame size in pixels, and a generate_at function which can generate an image given just the time within the video.

This library provides common manipulations of vidoes that you can wrap your video with, such as cropping, time dilations, reversing time, and combinations of frame generators.

To produce the video, use pympanim.worker.produce. Everything else is handled for you, including reasonable guesses for performance settings and runtime performance tuning.

### Boilerplate

import PIL.Image
import pympanim.frame_gen as fg
import pympanim.worker as pmaw
import os

class MyFrameGenerator(fg.FrameGenerator):
@property
def duration(self):
return 1 # 1 ms, allows you to treat time_ms as % progress

@property
def frame_size(self):
return (1920, 1080) # in pixels

def generate_at(self, time_ms):
# by default generate_at_pil delegates to generate_at. we show the
# reverse for completeness
return fg.img_to_bytes(self.generate_at_pil(time_ms))

def generate_at_pil(self, time_ms):
# this from white to red, you can do whatever
return PIL.Image.new('RGBA', self.frame_size, f'#{int(time_ms*255):02x}0000')

def _fg():
base = MyFrameGenerator()

# random example of the stuff you can do
return (fg.FluentFG(base)
.time_rescale(1 / 10000)         # 10s long
.then(                           # after current
fg.FluentFG(base)            # play again
.time_rescale(1 / 10000) # also 10s long
.reverse()               # but this time in reverse
.build()
)
.build())

def _main():
os.makedirs('out', exist_ok=True)

pmaw.produce(
_fg(),
fps=60,
dps=100,
bitrate=-1,
outfile='out/video.mp4'
)

if __name__ == '__main__':
_main()


## Examples

The examples/ folder has the sourcecode for the following examples:

python3 -m examples.redsquare

python3 -m examples.mpl_line


## Known Issues

If processes are forked instead of spawned, text will appear spastic in matplotlib. In general, matplotlib does not handle forked processes well. The worker attempts to force spawning mode instead of fork-mode, but this will fail if the process is already forked. If you are experiencing weird or glitchy videos, include this before any other imports which might be spawning processes (e.g. torch or torchvision).

import multiprocessing as mp

try:
mp.set_start_method('spawn')
except RuntimeError:
pass


## Project details

Uploaded source