Learning by Example

You create videos in manim by writing Scene instances. example_scenes.py contains a few simple ones that we can use to learn about manim. For instance, take SquareToCircle.

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class SquareToCircle(Scene):
    def construct(self):
        circle = Circle()
        square = Square()
        square.flip(RIGHT)
        square.rotate(-3 * TAU / 8)
        circle.set_fill(PINK, opacity=0.5)

        self.play(ShowCreation(square))
        self.play(Transform(square, circle))
        self.play(FadeOut(square))

construct() specifies what is displayed on the screen when the Scene is rendered to video. You can render a Scene by running extract_scene.py. Run python extract_scene.py -h to see how it’s used.

> python extract_scene.py -h
usage: extract_scene.py [-h] [-p] [-w] [-s] [-l] [-m] [-g] [-f] [-t] [-q] [-a]
                [-o OUTPUT_NAME] [-n START_AT_ANIMATION_NUMBER]
                [-r RESOLUTION] [-c COLOR] [-d OUTPUT_DIRECTORY]
                file [scene_name]

positional arguments:
  file                  path to file holding the python code for the scene
  scene_name            Name of the Scene class you want to see

optional arguments:
  -h, --help            show this help message and exit
  -p, --preview
  -w, --write_to_movie
  -s, --show_last_frame
  -l, --low_quality
  -m, --medium_quality
  -g, --save_pngs
  -f, --show_file_in_finder
  -t, --transparent
  -q, --quiet
  -a, --write_all
  -o OUTPUT_NAME, --output_name OUTPUT_NAME
  -n START_AT_ANIMATION_NUMBER, --start_at_animation_number START_AT_ANIMATION_NUMBER
  -r RESOLUTION, --resolution RESOLUTION
  -c COLOR, --color COLOR
  -d OUTPUT_DIRECTORY, --output_directory OUTPUT_DIRECTORY

The most common flags are -p, to automatically play the generated video, -l, to render in lower quality in favor of speed, and -s, to show the last frame of the Scene for faster development. Run python extract_scene.py example_scenes.py SquareToCircle -pl to produce a file called SquareToCircle.mp4 in the media directory that you have configured, and automatically play it.

Let’s step through each line of the Scene. Lines 3 and 4 instantiate a Circle and Square, respectively. Both of these subclass Mobject, the base class for objects in manim. Note that instantiating a Mobject does not add it to the Scene, so you wouldn’t see anything if you were to render the Scene at this point.

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circle = Circle()
square = Square()

Lines 5, 6, and 7 apply various modifications to the mobjects before animating them. The call to flip() on line 5 flips the Square across the RIGHT vector. This is equivalent to a refection across the x-axis. Then the call to rotate() on line 6 rotates the Square 3/8ths of a full rotation counterclockwise. Finally, the call to set_fill() on line 7 sets the fill color for the Circle to pink, and its opacity to 0.5.

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square.flip(RIGHT)
square.rotate(-3 * TAU / 8)
circle.set_fill(PINK, opacity=0.5)

Line 9 is the first to generate video. ShowCreation, Transform, and FadeOut are Animation instances. Each Animation takes one or more Mobject instances as arguments, which it animates when passed to play(). This is how video is typically created in manim. Mobject instances are automatically added to the Scene when they are animated. You can add a Mobject to the Scene manually by passing it as an argument to add().

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self.play(ShowCreation(square))
self.play(Transform(square, circle))
self.play(FadeOut(square))

ShowCreation draws a Mobject to the screen, Transform morphs one Mobject into another, and FadeOut fades a Mobject out of the Scene. Note that only the first argument to Transform is modified, and the second is not added to the Scene. After line 10 is executed square is a Square instance with the shape of a Circle.