Add scale_stroke boolean parameter to VMobject.scale() to allow scaling stroke_width along with the VMobject (#3965)

* Add option to scale object outlines with objects

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* Move outline scaling into VMobject.scale()

* Add unit tests for stroke width

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* Fix docstring for VMobject.scale

Co-authored-by: Aarush Deshpande <[email protected]>

* Remove tautology from VMobject.scale

Co-authored-by: Aarush Deshpande <[email protected]>

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Aarush Deshpande <[email protected]>
Co-authored-by: Francisco Manríquez Novoa <[email protected]>

Author: 4 people

manim/mobject/types/vectorized_mobject.py

@@ -481,6 +481,64 @@ def set_opacity(self, opacity: float, family: bool = True) -> Self:
         self.set_stroke(opacity=opacity, family=family, background=True)
         return self
 
+    def scale(self, scale_factor: float, scale_stroke: bool = False, **kwargs) -> Self:
+        r"""Scale the size by a factor.
+
+        Default behavior is to scale about the center of the vmobject.
+
+        Parameters
+        ----------
+        scale_factor
+            The scaling factor :math:`\alpha`. If :math:`0 < |\alpha| < 1`, the mobject
+            will shrink, and for :math:`|\alpha| > 1` it will grow. Furthermore,
+            if :math:`\alpha < 0`, the mobject is also flipped.
+        scale_stroke
+            Boolean determining if the object's outline is scaled when the object is scaled.
+            If enabled, and object with 2px outline is scaled by a factor of .5, it will have an outline of 1px.
+        kwargs
+            Additional keyword arguments passed to
+            :meth:`~.Mobject.scale`.
+
+        Returns
+        -------
+        :class:`VMobject`
+            ``self``
+
+        Examples
+        --------
+
+        .. manim:: MobjectScaleExample
+            :save_last_frame:
+
+            class MobjectScaleExample(Scene):
+                def construct(self):
+                    c1 = Circle(1, RED).set_x(-1)
+                    c2 = Circle(1, GREEN).set_x(1)
+
+                    vg = VGroup(c1, c2)
+                    vg.set_stroke(width=50)
+                    self.add(vg)
+
+                    self.play(
+                        c1.animate.scale(.25),
+                        c2.animate.scale(.25,
+                            scale_stroke=True)
+                    )
+
+        See also
+        --------
+        :meth:`move_to`
+
+        """
+        if scale_stroke:
+            self.set_stroke(width=abs(scale_factor) * self.get_stroke_width())
+            self.set_stroke(
+                width=abs(scale_factor) * self.get_stroke_width(background=True),
+                background=True,
+            )
+        super().scale(scale_factor, **kwargs)
+        return self
+
     def fade(self, darkness: float = 0.5, family: bool = True) -> Self:
         factor = 1.0 - darkness
         self.set_fill(opacity=factor * self.get_fill_opacity(), family=False)

tests/module/mobject/types/vectorized_mobject/test_stroke.py

@@ -39,3 +39,25 @@ def test_streamline_attributes_for_single_color():
     )
     assert vector_field[0].stroke_width == 1.0
     assert vector_field[0].stroke_opacity == 0.2
+
+
+def test_stroke_scale():
+    a = VMobject()
+    b = VMobject()
+    a.set_stroke(width=50)
+    b.set_stroke(width=50)
+    a.scale(0.5)
+    b.scale(0.5, scale_stroke=True)
+    assert a.get_stroke_width() == 50
+    assert b.get_stroke_width() == 25
+
+
+def test_background_stroke_scale():
+    a = VMobject()
+    b = VMobject()
+    a.set_stroke(width=50, background=True)
+    b.set_stroke(width=50, background=True)
+    a.scale(0.5)
+    b.scale(0.5, scale_stroke=True)
+    assert a.get_stroke_width(background=True) == 50
+    assert b.get_stroke_width(background=True) == 25