以随机运动分离的 Manim 对象

Question

I am trying to animate a particle and a vector that is attached to its center as a random motion is applied to the particle. The particle behaves as intended, but the vector always has a offset from the particle. I tried to set the random seed of each scene, but it didn't work.

from manim import *
import numpy as np 

class Particles(ThreeDScene):
    def construct(self):

        self.camera.background_color = WHITE
        coordinate = np.array([ (0,0,0) ])
        angle = 90 
        radius = 1.25

        #Polar coordinates
        def polar2cart(theta_degrees, rho):
            theta = theta_degrees*(np.pi/180)
            x = rho * np.cos(theta) 
            y = rho * np.sin(theta) 
            return(x, y, 0)

        start_arrow = np.array(coordinate) - np.array( [polar2cart(angle,radius )] )
        end_arrow =  np.array(coordinate) + np.array( [polar2cart(angle,radius )] )
        arrow = Arrow( start= start_arrow, end = end_arrow, color = '#ff0000')  
        dot = Dot(point=coordinate, radius=0.2, color = '#000000')
        
        #Display
        self.add(arrow, dot)

        ##############################################################################################################################
        #Random motion
        valor = ValueTracker(0)
        dot.add_updater(lambda m: m.move_to( m.get_center() + (np.random.normal(0,0.05),np.random.normal(0,0.05),np.random.normal(0,0.05)) ) )
        arrow.add_updater(lambda m: m.move_to( dot.get_center() + (np.random.normal(0,0.05),np.random.normal(0,0.05),np.random.normal(0,0.05)) ) )

        self.play(valor.animate.set_value(10),rate_func=smooth, run_time=5)
        self.wait()

Particle and vector

How can I make that the vector and the particle remain fixed in all frames?

Answer 1

Here is some things that you need to know:

• The order in which updaters attached to mobjects are executed is the order in which their mobjects have been added to the scene. In your code, the updater attached to arrow runs before the updater attached to dot (which will make the arrow always lag behind).
• Even if you fix the seed, subsequent calls to np.random.norm will not yield the same numbers (fortunately, otherwise your dot would move in a straight line out of the scene). If you want to fix the arrow to the center of the dot, then there is no need to add the random offset to that as well.

Here are possible solutions:

• Add the objects in the other way around, self.add(dot, arrow) , and remove the random offset from the updater attached to the arrow, arrow.add_updater(lambda m: m.move_to(dot.get_center())) . If the order of mobjects on the screen is important to you, you can also run dot.set_z_index(1) .
• Alternatively, you could also simply create a group of your two objects and attach an updater to that; particle_group = VGroup(arrow, dot) followed by particle_group.add_updater(lambda m: m.shift(...)) .

I'd personally go for the group, but it really depends on what else you intend to do in this scene.