Using Manim, can I draw a new object in the background of existing ones? - manim

Question

I have illustrated the parallelogram spanned by two vectors, and would like to shade in the area of that parallelogram, which I tried to do like so:
from manim import *
import numpy as np
class DrawParallelogram( Scene ):
def construct( self ):
o = np.array( [ 0, -2, 0 ] )
p1 = np.array( [ 3, 1, 0 ] ) # u
p2 = np.array( [ 1, 3, 0 ] ) # v
op1 = o + p1
op2 = o + p2
op3 = o + p1 + p2
v1 = Arrow( start = o, end = op1, buff = 0, color = RED ) # u
v2 = Arrow( start = o, end = op2, buff = 0, color = YELLOW ) # v
v1p = Arrow( start = op2, end = op3, buff = 0, color = RED ) # u'
v2p = Arrow( start = op1, end = op3, buff = 0, color = YELLOW ) # v'
parallelogram = [ o, op1, op3, op2 ]
poly = Polygon( *parallelogram, color = PURPLE, fill_opacity = 0.5 )
self.play( AnimationGroup( Write( v1 ), Write( v2 ), Write( v1p ), Write( v2p ) ) )
self.wait( )
self.play( Write( poly ) )
However, this parallelogram colors over the arrows that I have already drawn, like so:
and I'd like it to be in the background. Is there a way to introduce a new object into the scene so that it is logically behind any of the existing ones, as if I had drawn it first, so that it would look like:

Answer 1

You can use the set_z_index method to set the z_index property of the parallelogram to a value less than that of the arrows.
Here I have set it to a lower value than that of v1:
poly.set_z_index(v1.z_index - 1)
Alternatively you can manipulate the z_index property directly:
poly.z_index = v1.z_index - 1
Using the set_z_index method would be the cleaner solution.
Complete code:
from manim import *
import numpy as np
class DrawParallelogram( Scene ):
def construct( self ):
o = np.array( [ 0, -2, 0 ] )
p1 = np.array( [ 3, 1, 0 ] ) # u
p2 = np.array( [ 1, 3, 0 ] ) # v
op1 = o + p1
op2 = o + p2
op3 = o + p1 + p2
v1 = Arrow( start = o, end = op1, buff = 0, color = RED ) # u
v2 = Arrow( start = o, end = op2, buff = 0, color = YELLOW ) # v
v1p = Arrow( start = op2, end = op3, buff = 0, color = RED ) # u'
v2p = Arrow( start = op1, end = op3, buff = 0, color = YELLOW ) # v'
parallelogram = [ o, op1, op3, op2 ]
poly = Polygon( *parallelogram, color = PURPLE, fill_opacity = 0.5 )
# Set the z-index
poly.set_z_index(v1.z_index - 1)
self.play( AnimationGroup( Write( v1 ), Write( v2 ), Write( v1p ), Write( v2p ) ) )
self.wait( )
self.play( Write( poly ) )