autoscale_label.py

from kivy.app import App
from kivy.lang import Builder
from kivy.factory import Factory


KV = '''
<AutoScaleLabel@Label>:
    scale: 1
    step: self.font_size // 2
    interpolate: True
    _params: divmod(self.scale, self.step or 1)
    font_size: self._params[0] * (self.step or 1)
    _scale_factor: (1 + self._params[1] / (self.font_size or 1)) if self.interpolate else 1

    canvas.before:
        PushMatrix:
        Scale:
            origin: self.center
            xyz: self._scale_factor or 1, self._scale_factor or 1, 1

    canvas.after:
        PopMatrix:


FloatLayout:
    BoxLayout:
        orientation: 'vertical'
        AutoScaleLabel:
            text: 'test'
            scale: slider.value
            interpolate: interpolate.active

        GridLayout:
            cols: 2
            Label:
                text: 'font_size'

            Slider:
                id: slider
                min: .1
                max: 500

            Label:
                text: 'interpolate?'
            CheckBox:
                id: interpolate

'''

class Application(App):
    def build(self):
        return Builder.load_string(KV)


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