My first time 3D graphics on Kivy

main.py

from kivy.base import EventLoop, runTouchApp
from kivy.factory import Factory
from kivy.lang import Builder
from kivy.graphics import RenderContext
from kivy.properties import NumericProperty
from kivy.graphics.transformation import Matrix
from kivy.clock import Clock


EventLoop.ensure_window()


class CustomizedMesh(Factory.Mesh):
    def __init__(self, *args, **kwargs):
        kwargs['fmt'] = [(b'v_pos', 3, 'float'), ]
        super().__init__(*args, **kwargs)


Factory.register('CustomizedMesh', cls=CustomizedMesh)

Builder.load_string(r'''
<Renderer>:
    canvas:
        PushMatrix:
        Translate:
            z: -2
        Rotate:
            angle: root.angle
            axis: 0.5, 0.7, 0
        CustomizedMesh:
            vertices:
                [  # 立方体
                .5, .5, .5,
                .5, .5, -0.5,
                -0.5, .5, -0.5,
                -0.5, .5, .5,
                .5, -0.5, .5,
                .5, -0.5, -0.5,
                -0.5, -0.5, -0.5,
                -0.5, -0.5, .5,
                ]
            indices:
                [
                0, 1, 1, 2, 2, 3, 3, 0,
                4, 5, 5, 6, 6, 7, 7, 4,
                0, 4, 1, 5, 2, 6, 3, 7,
                ]
            mode: 'lines'
        PopMatrix:
''')


class Renderer(Factory.Widget):

    angle = NumericProperty()

    def __init__(self, **kwargs):
        self.canvas = RenderContext()
        self.canvas.shader.source = './simple.glsl'
        super().__init__(**kwargs)
        Clock.schedule_interval(self._update_angle, 0)

    def _update_angle(self, dt):
        self.angle += dt * 20

    def on_size(self, __, size):
        aspect_ratio = self.width / float(self.height)
        projection_mat = Matrix().view_clip(
            -aspect_ratio, aspect_ratio, -1, 1, 1, 100, 1)
        self.canvas['projection_mat'] = projection_mat


root = Renderer()
runTouchApp(root)

# root = Builder.load_string(r'''
# FloatLayout:
#     RelativeLayout:
#         pos: 100, 100
# ''')
# renderer = Renderer()
# root.children[0].add_widget(renderer)
# runTouchApp(root)

simple.glsl

---VERTEX SHADER---
#ifdef GL_ES
    precision highp float;
#endif

attribute vec3  v_pos;

uniform mat4 modelview_mat;
uniform mat4 projection_mat;

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

---FRAGMENT SHADER---
#ifdef GL_ES
    precision highp float;
#endif

void main(void)
{
    gl_FragColor = vec4(1.0);
}