MainScreen.kv

<MainScreen>:
    name: "main"

    GridLayout:
        size_hint: None, None
        size: root.size
        rows: 2

        GridLayout:
            size_hint: None, None
            height: 480
            cols: 2

            # Just an empty widget to reserve space for camera preview
            Widget:
                width: 640
                size_hint: None, None
                height: self.parent.height

            Label:
                size_hint: None, None
                width: 384
                height: self.parent.height
                padding: [16, 16]
                font_color: [1, 1, 1, 1]
                font_size: "20sp"
                text: root.decoded_text
                text_size: self.size
                halign: "left"
                valign: "top"

        Button:
            on_release: root.quit_app()
            text: "Quit"
            font_size: "28sp"