使用Manim，我可以在现有对象的背景中绘制新对象吗？

Question

我已经说明了由两个向量跨越的平行四边形，并希望在该平行四边形的区域中进行着色，我尝试这样做：

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 ) )

然而，这个平行四边形的颜色覆盖了我已经绘制的箭头，如下所示：

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我想让它出现在背景中。有没有一种方法可以在场景中引入一个新的对象，使其在逻辑上位于任何现有对象的后面，就像我先绘制了它一样，这样它看起来就像：

​

​

Answer 1

可以使用set_z_index方法将平行四边形的z_index属性设置为小于箭头的值。

在这里，我将它设置为比v1更低的值

poly.set_z_index(v1.z_index - 1)

或者，您可以直接操作z_index属性：

poly.z_index = v1.z_index - 1

使用set_z_index方法将是更干净的解决方案。

完整代码：

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 ) )