How to get position of face crop window

I can obtain the size of the cropped window (481x478) with face crop texture as follows:
//@input Asset.Texture faceCropTexture
//@input Asset.Texture deviceCameraTexture

var cropProvider = script.faceCropTexture.control;
cropProvider.inputTexture = script.deviceCameraTexture;
cropProvider.faceIndex = 0;
cropProvider.faceCenterMouthWeight = 0.0;


However, I couldn't find the position of face crop window.
How can I get position of face crop window?