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Slight offset in MoveAlongPath
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 ```
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()
https://github.com/user-attachments/assets/c0641b87-c57a-4911-8bc4-2d1a1e49acf0
How to change this?
- use
always_redrawor updater functions to run along the line rather thanMoveAlongPath - or calculate the actual offset between the two centers, add an invisible, shifted path which you use for the
MoveAlongPath - or create a
VGroupwhich 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()
https://github.com/user-attachments/assets/bdacd608-a30f-4dd6-99a8-3cf8c38b338e
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.
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