Kivy摄像头不能在android手机上保存图片

<uses-permission android:name="android.permission.CAMERA" />
<uses-feature android:name="android.hardware.camera" android:required="true" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.camera import Camera
from kivy.clock import Clock
from kivy.logger import Logger

class CameraApp(App):
    def build(self):
        layout = BoxLayout(orientation='vertical')
        self.camera = Camera(resolution=(720, 1280), play=False)
        layout.add_widget(self.camera)
        self.button = Button(text='拍照', size_hint=(1, 0.2))
        layout.add_widget(self.button)
        self.button.bind(on_press=self.capture_image)
        return layout

    def capture_image(self, instance):
        Logger.info('CameraApp: Capturing image')
        self.camera.export_to_png('image.png')

if __name__ == '__main__':
    CameraApp().run()

adb pull /sdcard/image.png