Wrong result of Mobject.length_over_dim()

[gvauvert]

Description of bug / unexpected behavior

In some conditions, the result of Mobject.length_over_dim(self, dim: int) -> float is wrong.

Given:
a VGroup containing:

• an empty VGroup (no submobject)
• any MObject with some position, like Square
  and with only positive [resp. negative] coordinates,
  Then:
  the result of length_over_dim is the max [resp. the min] coordinate of the square

Expected behavior

The result of length_over_dim should be the size of the square.

How to reproduce the issue

Code for reproducing the problem

from manim import *


class LengthOverDimBug(Scene):
    # create the simplest case that reproduce the issue
    vgroup = VGroup()
    sub_vgroup = VGroup()
    square = Square(side_length=2.0)
    vgroup.add(sub_vgroup)
    vgroup.add(square)
   # move the vgroup to have all values positive (resp. negative)
    vgroup.shift(UP * 10)  # makes the min wrong, and then the height 
    # vgroup.shift(UP * 10) # makes the max wrong, and then the height
    vgroup.arrange_in_grid(rows=2, cols=1, flow_order="ur")
    print(f"vgroup: {vgroup.__class__.__name__}")
    print(f"vgroup.height={vgroup.height}")
    print(f"vgroup.reduce_across_dimension(min, 1)={vgroup.reduce_across_dimension(min, 1)}")
    print(f"vgroup.reduce_across_dimension(max, 1)={vgroup.reduce_across_dimension(max, 1)}")
    for submobject in vgroup.submobjects:
        print(f"submobject: {submobject.__class__.__name__}")
        print(f"submobject.height={submobject.height}")
        print(f"submobject.reduce_across_dimension(min, 1)={submobject.reduce_across_dimension(min, 1)}")
        print(f"submobject.reduce_across_dimension(max, 1)={submobject.reduce_across_dimension(max, 1)}")


if __name__ == '__main__':
    with tempconfig({'quality': 'low_quality', 'preview': True}):
        scene = LengthOverDimBug()
        scene.render()

Additional media files

USELESS

Logs

Terminal output

Manim Community v0.18.1

vgroup: VGroup
vgroup.height=11.0
vgroup.reduce_across_dimension(min, 1)=0
vgroup.reduce_across_dimension(max, 1)=11.0
submobject: VGroup
submobject.height=0
submobject.reduce_across_dimension(min, 1)=0
submobject.reduce_across_dimension(max, 1)=0
submobject: Square
submobject.height=2.0
submobject.reduce_across_dimension(min, 1)=9.0
submobject.reduce_across_dimension(max, 1)=11.0

System specifications

System Details

• OS (with version, e.g., Windows 10 v2004 or macOS 10.15 (Catalina)): Ubuntu 22.04.5 LTS
• RAM: 32GB
• Python version (python/py/python3 --version): 3.10.12
• Installed modules (provide output from pip list):

Package           Version
----------------- -----------
click             8.1.7
cloup             3.0.5
decorator         5.1.1
distlib           0.3.9
filelock          3.16.1
glcontext         3.0.0
isosurfaces       0.1.2
manim             0.18.1
ManimPango        0.6.0
mapbox_earcut     1.0.2
markdown-it-py    3.0.0
mdurl             0.1.2
moderngl          5.12.0
moderngl-window   3.0.3
networkx          3.4.2
numpy             2.2.0
packaging         24.2
pbr               6.1.0
pillow            11.0.0
pip               24.3.1
pipdeptree        2.24.0
platformdirs      4.3.6
pycairo           1.27.0
pydub             0.25.1
pyglet            2.0.20
PyGLM             2.7.3
Pygments          2.18.0
rich              13.9.4
scipy             1.14.1
screeninfo        0.8.1
setuptools        59.6.0
skia-pathops      0.8.0.post2
srt               3.5.3
stevedore         5.4.0
svgelements       1.9.6
tqdm              4.67.1
typing_extensions 4.12.2
virtualenv        20.28.0
virtualenv-clone  0.5.7
virtualenvwrapper 6.1.1
watchdog          6.0.0

LaTeX details

UNUSED

Additional comments

Impacts:

• arrange_in_grid is not working well, because it uses length_over_dim.

Cause:

• length_over_dim is based on self.reduce_across_dimension(max, dim) and self.reduce_across_dimension(min, dim)
• self.reduce_across_dimension computes the min/max recursively
• when an object has no submobject, it returns 0 (center), what is the case for our empty VGroup
• but for an empty VGroup, that is wrong, it should be None (as this MObject should not be taken into account), and None should be managed in both reduce_across_dimension and length_over_dim
• I am going to propose a PR to solve this issue
  • Fix wrong value of MObject width/height with empty VGroup #4088

[gvauvert]

See fix in #4088