How to insert an image with rounded borders in Kivy while cropping it to keep its ratio?

I have a square image and I want to display it as background of a rounded rectangle. I want to avoid this image to be deformed. Here is a visual explanation of what I mean.

Explanation_Image

I tried to create a canvas and to set my image as the source of a RoundedRectangle to obtain the desired visual. Nevertheless the image is distorted to fit into the rectangle, which changes the ratio of the image. This is not what I want to obtain; is there a way to crop the image automatically? I tried to use the property fit_mode of Kivy 2.2 but it doesn't apply to RoundedRectangle.

Here is my Python file:

from kivy.app import Appfrom kivy.lang import Builderfrom kivy.uix.relativelayout import RelativeLayoutclass CustomImage(RelativeLayout):    passclass MyScreen(RelativeLayout):    passkv = Builder.load_file("draft_question.kv")class MyApp(App):    def build(self):        return kvif __name__ == "__main__":    MyApp().run()

Here is my .kv file:

#:kivy 2.2.1<CustomImage>:    canvas.before:        RoundedRectangle:            pos: (0,0)            size: self.size            source: "image.jpg"            radius: [40,]            #fit_mode: "cover"MyScreen:    CustomImage:        size_hint: (0.6, 0.2)        pos_hint: {"center_x": 0.5, "center_y": 0.5}

Thank you in advance for your help!