app.kv

<VideoPreviewWidget>:
    Image:
        id: img_left
        source: "img/bad_camera.png"
        allow_stretch: True
        size_hint: None, None
        size: 640, 480
        animation_offset: .1
    BoxLayout:
        orientation: 'vertical'
        size_hint_x: None
        width: 160
        spacing: 40
        padding: 20, 0, 20, 0
        Widget:
        Button:
            text: 'START'
            on_release: root.on_btn_start()
            size_hint_y: None
            height: 50
        Button:
            text: 'STOP'
            on_release: root.on_btn_stop()
            size_hint_y: None
            height: 50
        Button:
            text: 'EXIT'
            on_release: app.stop()
            size_hint_y: None
            height: 50
        Widget: