Decorations and Visual Polish
Decorations are mobjects that annotate, highlight, or frame other mobjects. They turn a technically correct animation into a visually polished one.
SurroundingRectangle
Draws a rectangle around any mobject. The go-to for highlighting:
`python
highlight = SurroundingRectangle(
equation[2], # the term to highlight
color=YELLOW,
buff=0.15, # padding between content and border
corner_radius=0.1, # rounded corners
stroke_width=2
)
self.play(Create(highlight))
self.wait(1)
self.play(FadeOut(highlight))
`
Around part of an equation
`python
eq = MathTex(r"E", r"=", r"m", r"c^2")
box = SurroundingRectangle(eq[2:], color=YELLOW, buff=0.1) # highlight "mc²"
label = Text("mass-energy", font_size=18, font="Menlo", color=YELLOW)
label.next_to(box, DOWN, buff=0.2)
self.play(Create(box), FadeIn(label))
`
BackgroundRectangle
Semi-transparent background behind text for readability over complex scenes:
`python
bg = BackgroundRectangle(equation, fill_opacity=0.7, buff=0.2, color=BLACK)
self.play(FadeIn(bg), Write(equation))
Or using set_stroke for a "backdrop" effect on the text itself:
label.set_stroke(BLACK, width=5, background=True)
`
The set_stroke(background=True) approach is cleaner for text labels over graphs/diagrams.
Brace and BraceLabel
Curly braces that annotate sections of a diagram or equation:
`python
brace = Brace(equation[2:4], DOWN, color=YELLOW)
brace_label = brace.get_text("these terms", font_size=20)
self.play(GrowFromCenter(brace), FadeIn(brace_label))
Between two specific points
brace = BraceBetweenPoints(point_a, point_b, direction=UP)
`
Brace placement
`python
Below a group
Brace(group, DOWN)
Above a group
Brace(group, UP)
Left of a group
Brace(group, LEFT)
Right of a group
Brace(group, RIGHT)
`
Arrows for Annotation
Straight arrows pointing to mobjects
`python
arrow = Arrow(
start=label.get_bottom(),
end=target.get_top(),
color=YELLOW,
stroke_width=2,
buff=0.1, # gap between arrow tip and target
max_tip_length_to_length_ratio=0.15 # small arrowhead
)
self.play(GrowArrow(arrow), FadeIn(label))
`
Curved arrows
`python
arrow = CurvedArrow(
start_point=source.get_right(),
end_point=target.get_left(),
angle=PI/4, # curve angle
color=PRIMARY
)
`
Labeling with arrows
`python
LabeledArrow: arrow with built-in text label
arr = LabeledArrow(
Text("gradient", font_size=16, font="Menlo"),
start=point_a, end=point_b, color=RED
)
`
DashedLine and DashedVMobject
`python
Dashed line (for asymptotes, construction lines, implied connections)
asymptote = DashedLine(
axes.c2p(2, -3), axes.c2p(2, 3),
color=YELLOW, dash_length=0.15
)
Make any VMobject dashed
dashed_circle = DashedVMobject(Circle(radius=2, color=BLUE), num_dashes=30)
`
Angle and RightAngle Markers
`python
line1 = Line(ORIGIN, RIGHT * 2)
line2 = Line(ORIGIN, UP * 2 + RIGHT)
Angle arc between two lines
angle = Angle(line1, line2, radius=0.5, color=YELLOW)
angle_value = angle.get_value() # radians
Right angle marker (the small square)
right_angle = RightAngle(line1, Line(ORIGIN, UP * 2), length=0.3, color=WHITE)
`
Cross (strikethrough)
Mark something as wrong or deprecated:
`python
cross = Cross(old_equation, color=RED, stroke_width=4)
self.play(Create(cross))
Then show the correct version
`
Underline
`python
underline = Underline(important_text, color=ACCENT, stroke_width=3)
self.play(Create(underline))
`
Color Highlighting Workflow
Method 1: At creation with t2c
`python
text = Text("The gradient is negative here", t2c={"gradient": BLUE, "negative": RED})
`
Method 2: set_color_by_tex after creation
`python
eq = MathTex(r"\nabla L = -\frac{\partial L}{\partial w}")
eq.set_color_by_tex(r"\nabla", BLUE)
eq.set_color_by_tex(r"\partial", RED)
`
Method 3: Index into submobjects
`python
eq = MathTex(r"a", r"+", r"b", r"=", r"c")
eq[0].set_color(RED) # "a"
eq[2].set_color(BLUE) # "b"
eq[4].set_color(GREEN) # "c"
`
Combining Annotations
Layer multiple annotations for emphasis:
`python
Highlight a term, add a brace, and an arrow — in sequence
box = SurroundingRectangle(eq[2], color=YELLOW, buff=0.1)
brace = Brace(eq[2], DOWN, color=YELLOW)
label = brace.get_text("learning rate", font_size=18)
self.play(Create(box))
self.wait(0.5)
self.play(FadeOut(box), GrowFromCenter(brace), FadeIn(label))
self.wait(1.5)
self.play(FadeOut(brace), FadeOut(label))
`
The annotation lifecycle
Annotations should follow a rhythm:
1. Appear — draw attention (Create, GrowFromCenter)
2. Hold — viewer reads and understands (self.wait)
3. Disappear — clear the stage for the next thing (FadeOut)
Never leave annotations on screen indefinitely — they become visual noise once their purpose is served.