manim/docs/source/getting_started/learning_by_example.rst

Learning by Example
===================

You create videos in manim by writing :class:`~scene.scene.Scene` instances.
`example_scenes.py <https://github.com/3b1b/manim/blob/master/example_scenes.py>`_. contains a few simple ones that we can use to learn about
manim. For instance, take ``SquareToCircle``. Copy the code given below to a new file (say test_manim.py) and run it with ``$ manim test_manim.py SquareToCircle -p``.

.. code-block:: python
   :linenos:

   from manimlib.imports import *

   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))


:meth:`~scene.scene.Scene.construct` specifies what is displayed on the screen
when the :class:`~scene.scene.Scene` is rendered to video. The flag ``-p`` automatically plays the rendered video.
Other frequently used flags are ``-l`` for rendering video in lower resolution (in favour of higher speed) and ``-s`` to show the last frame of the video.
Run ``manim -h`` all the available flags

.. code-block:: none

   > manim -h
   Media will be stored in ./media/. You can change this behavior by writing a different directory to media_dir.txt.
    usage: manim [-h] [-p] [-w] [-s] [-l] [-m] [-g] [-f] [-t] [-q] [-a]
                 [-o FILE_NAME] [-n START_AT_ANIMATION_NUMBER] [-r RESOLUTION]
                 [-c COLOR] [--sound] [--leave_progress_bars] [--livestream]
                 [--to-twitch] [--with-key TWITCH_KEY]
                 [file] [scene_names [scene_names ...]]

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

    optional arguments:
      -h, --help            show this help message and exit
      -p, --preview         Automatically open the saved file once its done
      -w, --write_to_movie  Render the scene as a movie file
      -s, --save_last_frame
                            Save the last frame
      -l, --low_quality     Render at a low quality (for faster rendering)
      -m, --medium_quality  Render at a medium quality
      -g, --save_pngs       Save each frame as a png
      -f, --show_file_in_finder
                            Show the output file in finder
      -t, --transparent     Render to a movie file with an alpha channel
      -q, --quiet
      -a, --write_all       Write all the scenes from a file
      -o FILE_NAME, --file_name FILE_NAME
                            Specify the name of the output file, ifit should be
                            different from the scene class name
      -n START_AT_ANIMATION_NUMBER, --start_at_animation_number START_AT_ANIMATION_NUMBER
                            Start rendering not from the first animation, butfrom
                            another, specified by its index. If you passin two
                            comma separated values, e.g. "3,6", it will endthe
                            rendering at the second value
      -r RESOLUTION, --resolution RESOLUTION
                            Resolution, passed as "height,width"
      -c COLOR, --color COLOR
                            Background color
      --sound               Play a success/failure sound
      --leave_progress_bars
                            Leave progress bars displayed in terminal
      --livestream          Run in streaming mode
      --to-twitch           Stream to twitch
      --with-key TWITCH_KEY
                            Stream key for twitch





    <video width="560" height="315" controls>
        <source src="../_static/SquareToCircle.mp4" type="video/mp4">
    </video>

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

.. code-block:: python
   :linenos:
   :lineno-start: 3

   circle = Circle()
   square = Square()

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

.. code-block:: python
   :linenos:
   :lineno-start: 5

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

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

.. code-block:: python
   :linenos:
   :lineno-start: 9

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

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