Slight offset in MoveAlongPath

[jeremysalwen]

Description of bug / unexpected behavior

When using MoveAlongPath with a rotated object, a slight offset is introduced

Expected behavior

The object follows the line exactly, not with an offset.

How to reproduce the issue

Minimum Reproducible Code

%%manim -qm MowerAnimation

class MowerAnimation(Scene):
    def construct(self):
        # Create the mower as a small triangle
        mower = Triangle(color=GREEN, fill_opacity=1).scale(0.2)

        start_pos = np.array([-4, -3, 0])
        end_pos = np.array([-1, 4, 0])
        mower.move_to(start_pos)

        delta = end_pos - start_pos
        angle = np.arctan2(delta[0], delta[1])
        mower.rotate(-angle)

        path = Line(start_pos, end_pos)

        dashed_line = DashedLine(start_pos, end_pos)

        self.add(dashed_line)

        self.play(
            MoveAlongPath(mower, path, rate_func=linear),
            run_time=5
        )
        self.wait()

Additional media files

Logs

Terminal output

[09/19/24 21:12:51] DEBUG    Hashing ...                                                             [hashing.py](file:///usr/local/lib/python3.11/site-packages/manim/utils/hashing.py):[352](file:///usr/local/lib/python3.11/site-packages/manim/utils/hashing.py#352)

                    DEBUG    Hashing done in 0.132026 s.                                             [hashing.py](file:///usr/local/lib/python3.11/site-packages/manim/utils/hashing.py):[364](file:///usr/local/lib/python3.11/site-packages/manim/utils/hashing.py#364)

                    DEBUG    Hash generated :  2016333726_3528055951_4110290056                      [hashing.py](file:///usr/local/lib/python3.11/site-packages/manim/utils/hashing.py):[367](file:///usr/local/lib/python3.11/site-packages/manim/utils/hashing.py#367)

                    INFO     Animation 0 : Using cached data (hash :                           [cairo_renderer.py](file:///usr/local/lib/python3.11/site-packages/manim/renderer/cairo_renderer.py):[88](file:///usr/local/lib/python3.11/site-packages/manim/renderer/cairo_renderer.py#88)
                             2016333726_3528055951_4110290056)                                                     

                    DEBUG    List of the first few animation hashes of the scene:              [cairo_renderer.py](file:///usr/local/lib/python3.11/site-packages/manim/renderer/cairo_renderer.py):[97](file:///usr/local/lib/python3.11/site-packages/manim/renderer/cairo_renderer.py#97)
                             ['2016333726_3528055951_4110290056']                                                  

                                                                           

                    DEBUG    Animation with empty mobject                                          [animation.py](file:///usr/local/lib/python3.11/site-packages/manim/animation/animation.py):[175](file:///usr/local/lib/python3.11/site-packages/manim/animation/animation.py#175)

                    DEBUG    Hashing ...                                                             [hashing.py](file:///usr/local/lib/python3.11/site-packages/manim/utils/hashing.py):[352](file:///usr/local/lib/python3.11/site-packages/manim/utils/hashing.py#352)

                    DEBUG    Hashing done in 0.163328 s.                                             [hashing.py](file:///usr/local/lib/python3.11/site-packages/manim/utils/hashing.py):[364](file:///usr/local/lib/python3.11/site-packages/manim/utils/hashing.py#364)

                    DEBUG    Hash generated :  543634251_1704852926_3284192126                       [hashing.py](file:///usr/local/lib/python3.11/site-packages/manim/utils/hashing.py):[367](file:///usr/local/lib/python3.11/site-packages/manim/utils/hashing.py#367)

                    INFO     Animation 1 : Using cached data (hash :                           [cairo_renderer.py](file:///usr/local/lib/python3.11/site-packages/manim/renderer/cairo_renderer.py):[88](file:///usr/local/lib/python3.11/site-packages/manim/renderer/cairo_renderer.py#88)
                             543634251_1704852926_3284192126)                                                      

                    DEBUG    List of the first few animation hashes of the scene:              [cairo_renderer.py](file:///usr/local/lib/python3.11/site-packages/manim/renderer/cairo_renderer.py):[97](file:///usr/local/lib/python3.11/site-packages/manim/renderer/cairo_renderer.py#97)
                             ['2016333726_3528055951_4110290056',                                                  
                             '543634251_1704852926_3284192126']                                                    

                    INFO     Combining to Movie file.                                      [scene_file_writer.py](file:///usr/local/lib/python3.11/site-packages/manim/scene/scene_file_writer.py):[617](file:///usr/local/lib/python3.11/site-packages/manim/scene/scene_file_writer.py#617)

                    DEBUG    Partial movie files to combine (2 files):                     [scene_file_writer.py](file:///usr/local/lib/python3.11/site-packages/manim/scene/scene_file_writer.py):[561](file:///usr/local/lib/python3.11/site-packages/manim/scene/scene_file_writer.py#561)
                             ['/manim/media/videos/manim/720p30/partial_movie_files/MowerA                         
                             nimation/2016333726_3528055951_4110290056.mp4',                                       
                             '/manim/media/videos/manim/720p30/partial_movie_files/MowerAn                         
                             imation/543634251_1704852926_3284192126.mp4']                                         

                    INFO                                                                   [scene_file_writer.py](file:///usr/local/lib/python3.11/site-packages/manim/scene/scene_file_writer.py):[737](file:///usr/local/lib/python3.11/site-packages/manim/scene/scene_file_writer.py#737)
                             File ready at                                                                         
                             '/manim/media/videos/manim/720p30/MowerAnimation.mp4'                                 
                                                                                                                   

                    INFO     Rendered MowerAnimation                                                   [scene.py](file:///usr/local/lib/python3.11/site-packages/manim/scene/scene.py):[247](file:///usr/local/lib/python3.11/site-packages/manim/scene/scene.py#247)
                             Played 2 animations

System specifications

System Details

``` https://notebooks.gesis.org/binder/jupyter/user/manimcommunity-jupyter_examples-xz4gvrcr/notebooks/First%20Steps%20with%20Manim.ipynb ```

[uwezi]

It's not a bug, it's a feature.
In Manim an object's position is generally identified by the center point of a rectangular bounding box around the object. This center point does not coincide with the mid-point of your rotated triangle though.

class MowerAnimation(Scene):
    def construct(self):
        # Create the mower as a small triangle
        mower = Triangle(color=GREEN, fill_opacity=0.5).scale(0.2).scale(3)

        start_pos = np.array([-4, -3, 0])
        end_pos = np.array([-1, 4, 0])
        mower.move_to(start_pos)

        delta = end_pos - start_pos
        angle = np.arctan2(delta[0], delta[1])
        mower.rotate(-angle)

        bbox = always_redraw(lambda:
            VGroup(
                Polygon(*[mower.get_critical_point(p) for p in (UR,UL,DL,DR)],stroke_width=1,stroke_color=YELLOW),
                Line(mower.get_critical_point(DOWN),mower.get_critical_point(UP),stroke_width=1,color=YELLOW),
                Line(mower.get_critical_point(LEFT),mower.get_critical_point(RIGHT),stroke_width=1,color=YELLOW),
            )
        )
        self.add(bbox)

        path = Line(start_pos, end_pos)

        dashed_line = DashedLine(start_pos, end_pos)

        self.add(dashed_line)

        self.play(
            MoveAlongPath(mower, path, rate_func=linear),
            run_time=5
        )
        self.wait()

MowerAnimation.mp4

How to change this?

• use always_redraw or updater functions to run along the line rather than MoveAlongPath
• or calculate the actual offset between the two centers, add an invisible, shifted path which you use for the MoveAlongPath
• or create a VGroup which contains a larger object (e.g. a square or circle) than your triangle, place your triangle in the VGroup so that the mid-point of the triangle coincides with the bounding box of the complete VGroup

Somehow I was most curious to try out my third idea there:

class MowerAnimation(Scene):
    def construct(self):
        # Create the mower as a small triangle
        mower = VGroup(
            Square(stroke_opacity=0.2, fill_opacity=0)
        )
        tri = Triangle(color=GREEN, fill_opacity=0.5).scale(0.2)

        start_pos = np.array([-4, -3, 0])
        end_pos = np.array([-1, 4, 0])
        mower.move_to(start_pos)

        delta = end_pos - start_pos
        angle = np.arctan2(delta[0], delta[1])
        tri.rotate(-angle).shift(mower.get_center()-tri.get_center_of_mass())
        mower += tri

        path = Line(start_pos, end_pos)

        dashed_line = DashedLine(start_pos, end_pos)

        self.add(dashed_line)

        self.play(
            MoveAlongPath(mower, path, rate_func=linear),
            run_time=5
        )
        self.wait()

MowerAnimation.mp4

[uwezi]

In case you intend to rotate your triangle more often in this script, then you should use tri.rotate(angle, about_point=tri.get_center_of_mass())
This will keep the center of mass of the triangle fixed inside the square and thus the VGroup and only rotate the triangle. Don't rotate the mower in this script.

[jeremysalwen]

Thank you, this is very helpful!

[uwezi]

Thank you, this is very helpful!

you are welcome. Come over to Discord for more tips about Manim and the details about its workings:
https://docs.manim.community/en/stable/faq/general.html?highlight=discord#where-can-i-find-more-resources-for-learning-manim