如何在Kivy中访问Modelview矩阵？

Question

我正在使用Kivy编写python应用程序以渲染一些图形。我想了解如何在kivy中访问当前的modelview矩阵，该矩阵与顶点着色器中的矩阵相同。

这是我的示例代码（Python 2.7，Kivy 1.9）：

#!/usr/bin/env python
from kivy.app import App
from kivy.clock import Clock
from kivy.core.window import Window
from kivy.graphics import *
from kivy.graphics.transformation import Matrix
from kivy.resources import resource_find
from kivy.uix.widget import Widget

vertices = [0.0, 0.0, 0.0, 0.0, 0.0, 1.0,
            2.0, 0.0, 0.0, 0.0, 0.0, 1.0,
            0.0, 2.0, 0.0, 0.0, 0.0, 1.0,
            2.0, 2.0, 0.0, 0.0, 0.0, 1.0]
indices = [0, 1, 2, 1, 2, 3]
vformat = [(b'v_pos', 3, 'float'), (b'v_normal', 3, 'float')]

class Renderer(Widget):
    def __init__(self, **kwargs):
        super(Renderer, self).__init__(**kwargs)
        self.canvas = RenderContext(compute_normal_mat=True)
        self.canvas.shader.source = resource_find('simple.glsl')
        with self.canvas:
            PushMatrix()
            Translate(0, -1, -3)
            self.rot = Rotate(1, 0, 1, 0)
            UpdateNormalMatrix()
            self.mesh = Mesh(vertices=vertices, indices=indices,
                             fmt=vformat, mode='triangles')
            PopMatrix()
        Clock.schedule_interval(self.update_glsl, 1 / 60.)

    def update_glsl(self, delta):
        asp = self.width / float(self.height)
        proj = Matrix().view_clip(-asp, asp, -1, 1, 1, 100, 1)
        self.canvas['projection_mat'] = proj
        self.rot.angle += delta * 100

        print self.canvas['modelview_mat']

class RendererApp(App):
    def build(self):
        return Renderer()

if __name__ == "__main__":
    RendererApp().run()

和着色器文件 simple.glsl ：

---VERTEX SHADER-----------------------------------

attribute vec3  v_pos;
attribute vec3  v_normal;
uniform mat4 modelview_mat;
uniform mat4 projection_mat;
varying vec4 normal_vec;

void main (void) {
    normal_vec = vec4(v_normal, 0.0);
    gl_Position = projection_mat * modelview_mat * vec4(v_pos, 1.0);
}


---FRAGMENT SHADER---------------------------------

uniform mat4 normal_mat;
varying vec4 normal_vec;

void main (void){
    vec4 v_normal = normalize( normal_mat * normal_vec ) ;
    float theta = clamp(abs(dot(v_normal, vec4(0.0, 0.0, 1.0, 1.0))), 0.0, 1.0);
    gl_FragColor = vec4(theta, theta, theta, 1.0);
}

在函数update_glsl中，我需要当前的Modelview矩阵进行一些计算。我尝试过self.canvas['modelview_mat']，但这总是给我身份矩阵。