how to add a label to a line in manim

Daniel Parker

3 min read

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05-02-2025

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Manim, the powerful animation engine created by Grant Sanderson (3Blue1Brown), allows for the creation of stunning mathematical animations. One common task is adding labels to lines to clarify their meaning or properties. This guide will show you how to effectively label lines in your Manim animations. We'll cover several methods, from simple text labels to more sophisticated approaches.

Understanding Manim's Object-Oriented Structure

Before diving into labeling lines, it's crucial to understand Manim's object-oriented nature. Every element in a Manim animation, including lines, is an object with its own properties and methods. This means you interact with and modify these elements programmatically.

Method 1: Using Line and MathTex

This is the most straightforward approach. We create a line using Manim's Line class and then add a MathTex object for the label. We'll strategically position the label using Manim's coordinate system.

from manim import *

class LineLabelExample(Scene):
    def construct(self):
        line = Line(start=LEFT*2, end=RIGHT*2)
        label = MathTex("My Line")
        label.next_to(line, UP) # Position the label above the line

        self.add(line, label)

This code creates a horizontal line and places the label "My Line" directly above it. The next_to() method is invaluable for precise label placement. You can experiment with other positioning methods like align_to(), shift(), etc., to fine-tune the label's location.

Method 2: Utilizing label Parameter (where applicable)

Some Manim objects, when created, offer a label parameter directly. If you're using a class that supports this, it simplifies the labeling process. Check the documentation for the specific object you're using to see if this parameter exists.

For example, there might be a function to create an arrow which directly takes a label argument for convenience. However, this isn't available for a plain Line.

Method 3: More Complex Label Placement with Anchors

For more precise control, especially with angled lines, we can utilize the line's midpoint and rotate the label accordingly.

from manim import *

class AdvancedLineLabel(Scene):
    def construct(self):
        line = Line(start=LEFT*2 + DOWN, end=RIGHT*2 + UP) # angled line
        label = MathTex("Angled Line")

        midpoint = line.get_center()
        label.move_to(midpoint) # Move label to midpoint
        label.rotate(line.get_angle()) # Rotate to match line angle

        self.add(line, label)

This method first finds the line's midpoint using get_center(). Then, it aligns the label with this midpoint using move_to(). Finally, it rotates the label using rotate() to match the line's angle (get_angle()), ensuring the label remains parallel to the line.

Method 4: Dynamic Labels with Updating Positions

In scenarios where the line's position changes during the animation, you will need to update the label's position dynamically within the animation.

from manim import *

class DynamicLineLabel(Scene):
    def construct(self):
        line = Line(start=LEFT*2, end=RIGHT*2)
        label = MathTex("Moving Line")
        label.next_to(line, UP)

        self.add(line, label)
        self.play(line.animate.shift(UP*2)) # Animate line movement

        #Here's the crucial part: we need to update the label position after moving the line.
        label.next_to(line, UP)
        self.wait(1)

This example demonstrates how to keep the label positioned correctly even after animating the line's movement by updating its position using next_to() after the animation is complete. For smoother animations, you'd typically handle this within an animate block.

Choosing the Right Method

The best method depends on your specific needs:

• Method 1: Simplest for basic horizontal or vertical lines.
• Method 2: Most convenient if the Manim object you're using supports a label parameter.
• Method 3: Provides precise control for angled lines.
• Method 4: Necessary for dynamic line positions during animations.

Remember to consult the Manim documentation for the most up-to-date information and advanced techniques. Experiment with different approaches to find what best suits your animation style and complexity. Happy animating!